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The Space Environment of Io and Europa 1

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Fran Bagenal & Vincent Dols 3

Laboratory for Atmospheric and Space Physics 4

University of Colorado, Boulder CO 5

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Abstract 7

The Galilean moons play major roles in the giant magnetosphere of Jupiter. At the same 8 time, the magnetospheric particles and fields affect the moons. The impact of magnetospheric 9 ions on the moons’ atmospheres supplies clouds of escaping neutral atoms that populate a 10 substantial fraction of their orbits. At the same time, ionization of atoms in the neutral cloud is 11 the primary source of magnetospheric plasma. The stability of this feedback loop depends on the 12 plasma / moon-atmosphere interaction. The purpose of this review is to describe the physical 13 processes that shape the space environment around the two innermost Galilean moons – Io and 14 Europa – and to show their impact from the planet Jupiter out into interplanetary space. 15 16

1. Introduction 17

In the 60s and 70s ground-based observations indicated Io was peculiar: the moon 18 triggered radio emissions, appeared to brighten on emerging from eclipse, and optical emissions 19 indicated clouds of sodium atoms and sulfur ions around Io. Further hints of Io’s peculiarity 20 (specifically, its ionosphere) were indicated by Pioneers in 1973-4. Such strange behavior became 21 understandable when Voyager 1 and 2 flybys of Jupiter in 1979 revealed Io’s remarkable 22 volcanism. The Voyagers also detected strong UV emissions from a torus of sulfur and oxygen 23 ions surrounding Jupiter at Io’s orbit, and measured the torus plasma in situ. When Voyager 1 24 passed close to Io, perturbations in the plasma and magnetic field showed Io generating Alfven 25 waves propagating away from the moon, carrying million-Ampere electrical currents along the 26 magnetic field towards Jupiter. The Voyagers characterized a plethora of radio and plasma waves, 27 several associated with Io or the Io plasma torus. Neither of the Voyager spacecraft passed close 28 to Europa but distant images hinted at an intriguingly smooth, icy surface. 29

When the Galileo spacecraft went into orbit around Jupiter in 1995, close-up pictures of 30 cracks, and a mysterious brown material on the surface made Europa a major focus of attention. 31 Moreover, magnetic perturbations measured near Europa indicated an electrically-conducting 32 liquid ocean under the ice. On several passes, the Galileo spacecraft also measured magnetic and 33 particle perturbations near Io– as well as discovering that Ganymede has an internal magnetic 34 field. In late 2000, Cassini spacecraft got a gravity kick from Jupiter to accelerate its journey to 35 Saturn. The UVIS instrument provided months of high-quality observations with of the torus UV 36 emissions, revealing the plasma composition and how the torus changed after a volcanic eruption 37 on Io. 38

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The aim of the 2004 monograph Jupiter: The Planet, Satellites and Magnetosphere 39 (Bagenal et al. 2004) was to summarize the post-Galileo view of the jovian system. Chapters by 40 Kivelson et al. (2004) and Saur et al. (2004) reviewed the current understanding of the 41 electrodynamics of the plasma-moon interactions, and Thomas et al. (2004) reviewed Io’s neutral 42 clouds and plasma torus, describing how ~1 ton/s of sulfur and oxygen escapes Io, becomes 43 ionized and trapped in the magnetic field, and moves out to fill Jupiter’s vast magnetosphere. 44

In the meantime, while HST and ground-based telescopes continue to observe the 45 system, new measurements and models have emerged. In spring 2007 the New Horizons 46 spacecraft got a gravity assist from Jupiter to speed its journey to Pluto, observing Io’s volcanism 47 including an eruption of Tvashtar (Spencer et al. 2007; Rathbun et al. 2014; Tsang et al. 2014) and 48 aurora on Io’s flanks (Retherford et al. 2007). The Japanese space agency (JAXA) put the Hisaki 49 satellite into orbit around Earth in September 2013 (Yoshioka et al. 2013; Yoshikawa et al. 2014, 50 2016; Kimura et al. 2019). Hisaki’s UV spectrometer has been observing emissions from the Io 51 plasma torus, determining composition (Yoshioka et al. 2014, 2017), mapping the oxygen neutral 52 cloud (Koga et al. 2018a,b), and measuring temporal variations (Tsuchiya et al. 2015, 2018, 2019; 53 Yoshikawa et al. 2017; Kimura et al. 2018; Koga et al. 2019). The Juno spacecraft went into orbit 54 around Jupiter in July 2016 (Bolton et al. 2017). While the prime Juno mission does not bring the 55 spacecraft inside Europa’s orbit, the particles and fields instruments are measuring the 56 consequences of the Io-Europa space environment both out in the middle magnetosphere and 57 close over the poles (Bagenal et al. 2017b). Future missions to Europa further emphasize the 58 need to consider the possible impacts that the changing environment near Io might have on the 59 Europa system. 60

In past reviews the separate components of the Io-Europa system are generally discussed 61 in isolation. To understand physical processes and the interconnections of the components, we 62 need to look at the whole system. The goal of this review is to summarize the current 63 understanding of this multi-component system and to present the outstanding questions. The 64 remainder of Section 1 provides an overview of the system and a brief summary of what we know 65 about the moons Io and Europa. In Section 2 we compare the atmospheres of these moons. In 66 Section 3 we look at the interaction between these atmospheres and the plasma in which they 67 are embedded. Section 4 describes the extended clouds of neutral atoms that come the plasma-68 atmosphere interactions. The space environment between Io and Europa, the plasma torus, is 69 reviewed in Section 5. Section 6 presents the evidence that these space environments spread 70 their influence out into interplanetary space and into the atmosphere of Jupiter. Finally, we 71 present a summary in Section 7, listing outstanding questions for future research. We have 72 attempted to provide extensive references but also provide introductory and summarizing 73 paragraphs without references to present a readable review that is accessible to a broad 74 audience. 75

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1.1 – Ten components of the Io-Europa Space Environment 77

Figure 1 shows the 10 main components of the system. Central to this picture are the 78 moons Io and Europa. The interactions of the magnetospheric plasma with these moons’ 79 atmospheres produce clouds of escaping neutrals that extend for a substantial fraction of Io’s 80

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and Europa’s orbits around Jupiter. Electron impact ionization of these neutral clouds produces 81 the plasma that is trapped in Jupiter’s strong magnetic field. It is the intriguing feed-back systems 82 between the moons and the magnetospheric plasma that begs understanding. The 83 magnetospheric plasma primarily corotates with the planet’s 10-hour spin period, but also moves 84 radially outwards over several weeks. About 10% of the torus material moves inwards from Io’s 85 orbit. The flows inward of Io are about a factor 50 slower than the outward flow. As the majority 86 of iogenic plasma moves out through the giant magnetosphere, it is heated to 10s-100s keV 87 energies by an unknown process. Thus, while the lower-energy plasma moves outward and is 88 heated, energetic electrons and ions (protons, sulfur and oxygen ions) move inward. 89

Charge exchange reactions between the corotating plasma and the neutral clouds 90 produces fast neutral atoms (FNAs with energies from a few hundred eV to a few keV) that 91 escape Jupiter to form a neutral disk that extends 100s of RJ around Jupiter. Recent observations 92 of heavy ions in Jupiter’s upper ionosphere by the Juno spacecraft suggest that some of the FNAs 93 are also ejected inward and eventually hit the planet’s equatorial atmosphere to form a source 94 of cold (ionospheric) heavy ions close to the planet. 95

When the energetic (10s keV – MeV) ions move inward through the neutral cloud around 96 Europa’s orbit, they charge exchange, making very energetic neutral atoms (ENAs with energies 97 from a few 10s of keVs to MeVs)). These ENAs have high speeds in all directions and likely spread 98 into a huge sphere around the Jupiter system. Juno also detected new equatorial belt of energetic 99 radiation close to the planet produced when the ENAs bombard Jupiter’s atmosphere. 100

This system of ten coupled components (listed in Figure 1) comprises a wide range of 101 physics and extends from 10s of kilometers to Astronomical Unit scales. 102

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1.2 –Io and Europa: Bulk Properties 104

Io and Europa are similar in size to Earth’s Moon (Figure 2). Io, Europa and Ganymede are 105 locked in a tight orbital resonance (with orbital periods increasing with distance from Jupiter by 106 a factor of two) which drives tidal interactions between the moons (see review by Schubert et al. 107 2004). Tidal forces produce orbit-phase lock with Jupiter, so that when a moon is viewed from 108 Earth, the surface longitude facing the observer and local time along the orbit are correlated and 109 we do not see much of the night side of the moon. The strong dependence of tidal heating on 110 radial distance, R, from the planet (1/R6) means Io is the most volcanically active object and the 111 densest moon in the solar system. Io emits ~1014 W internal heat, about 1/3 the solar heating 112 (McEwen et al. 2004). Having lost any water it may have had when formed, Io is covered with 113 over 425 volcanic features (Figure 3), of which 50-100 are actively erupting at any one time, 114 resurfacing the moon at a rate of ~1 cm/year, equivalent to the whole moon turning over ~40 115 times in 4.5 Ga (McEwen et al. 2004; Davies 2007; Lopes & Spencer 2007; Williams et al. 2011). 116 A recent study of long term variations by de Kleer et al. (2019b) suggest Io’s volcanism may be 117 modulated by periodic (~1.25 year) variations in orbital parameters. 118

Europa’s bulk density of 2989 ± 46 kg m-3 is intermediate between that of Io (3528 ± 3 kg 119 m-3) and Ganymede (1942 ± 5 g cm-3), suggesting it has lost much but not all of its primordial 120 water, resulting in a mantle of ~80:20% rock:water composition (Anderson et al. 1996; Schubert 121

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et al. 2004). The magnetic field perturbations measured by Galileo on multiple flybys of Europa 122 determined the presence of the water ocean (Khurana et al. 1998), supported by the presence 123 of tidally-driven cycloidal fractures (Hoppa et al. 1999). The net tidal heating is less well known 124 for Europa, ranging between 1012-1013 W (Greenberg et al. 2002). Europa’s outer layer of water 125 is equivalent to about twice the mass of Earth’s ocean, forming a 100-130 km layer capped by a 126 5-30 km ice layer (Cammarano et al. 2006; Vance et al. 2018). The depth:diameter ratio of the 127 largest multi-ringed craters suggests penetration of the impactor through 19-25 km of ice (Schenk 128 2002). Geological features such as ridged plains, chaotic terrain, spreading bands, pits and domes 129 suggest convection of ductile ice under a cold brittle surface (e.g., Schmidt et al. 2011). The major 130 question that is driving future exploration of Europa (Pappalardo et al. 2013) is whether there 131 are regions where the ice is thin or broken – spurred on by the detection of water plumes (Roth 132 et al. 2014a,b; Sparks et al. 2016, 2017) – that might be places to search for evidence of life (Hand 133 & Chyba, 2007). 134

As discussed in the next couple sections, little of the surrounding energetic particles 135 penetrate through Io’s atmosphere to reach the surface. But Europa’s thin atmosphere does little 136 to protect Europa where the chemistry of the exposed icy surfaces are likely altered by space 137 weathering – some combination of delivering exogenic O and S and radiolysis of surface ices (as 138 reviewed by Paranicas et al. 2009, 2018) 139

Despite very different surface compositions (lava, sulfur, SO2 vs. ice and sulfates) , Io and 140 Europa have similar albedos (0.62 and 0.64 respectively) and similar average surface temperature 141 of ~110 Kelvin (Rathbun et al. 2004, 2010a,b; Spencer et al. 1999). But the differences only 142 increase as we look above the surface to their atmospheres. 143

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2. Io and Europa Atmospheres 145

Io and Europa both have significant atmospheres but, like their surfaces, they are very 146 different in character (see Figure 4, Table 1, and references in sections 2.1, 2.2). McGrath et 147 al.2004’s review and references therein summarize the observations and modeling efforts 148 available at the time. Io’s atmosphere is primarily due to sublimation of SO2 frost with some 149 limited contribution of volcanic plumes. Europa’s atmosphere comes from charged particle 150 sputtering of the icy surface producing H2O, O2 and H2. The water quickly freezes back onto the 151 surface and hydrogen escapes, leaving Europa with an oxygen atmosphere. Io’s atmosphere is 152 confined to low- to mid-latitudes with the polar pressures about 2% of equatorial pressures. 153 Europa’s atmosphere is more uniform, with pressures comparable to the poles of Io. 154 Remembering that Io and Europa are spin-phase locked and that the atmospheres are strongly 155 affected by the plasma flowing from the trailing (upstream) side around to the leading 156 (downstream) sides of the moons, one might expect variations with longitude. The sub-/anti-157 jovian longitudes (System III West) are 0°/180° while the leading/trailing longitudes are 90°/270°. 158 Io has at least 16 active plumes (Lopes & Williams 2015). Europa occasionally has an active plume. 159 The roles of these eruptions on each atmosphere is poorly known – but keenly pursued research 160 topics. 161

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The total mass of Io’s atmosphere is about a hundred times that of Europa. This may not 162 seem a huge difference, but, as we shall see in subsequent sections, the material escaping from 163 Io’s atmosphere totally dominates the surrounding space environment. 164

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2.1 Io’s Atmosphere and corona 166

Io’s atmosphere is still surprisingly poorly known. Some atmospheric properties (density, 167 composition, asymmetries, equatorial extension) have been observed via telescopes on the 168 ground, Earth orbit or several spacecraft at Jupiter. Some atmospheric properties have also been 169 deduced from in-situ plasma observation and numerical modeling of the plasma-atmosphere 170 interaction. From these observations, it is concluded that Io has a collisionally-thick SO2 171 atmosphere with a surface pressure of 1-10 nbar, concentrated ±40° of the equator (see reviews 172 by McGrath et al. 2004; Lellouch et al. 2007). 173

Earth-based telescopes provide important information about the dayside atmosphere 174 from UV to millimeter wavelengths. These remote observations consist of brightness integrated 175 along the line of sight on the sunlit hemisphere of Io. They provide an estimate of the column 176 density on the dayside atmosphere but they give limited information on its radial extension or 177 variations with time of day. Io’s surface pressure is primarily controlled by the sublimation of SO2 178 frost (Tsang et al. 2012, 2013a,b, 2016; Lellouch et al. 2015), which is supported by the fact that 179 the atmospheric column density drops by a factor of 5 ± 2 after 40 minutes of eclipse (Tsang et 180 al. 2016; Retherford et al. 2007, 2019). The atmospheric density above the poles just ~2% of the 181 equatorial density (Feaga et al. 2009, Strobel and Wolven 2001; Lellouch et al. 2015). This 182 distribution is likely primarily caused by the equatorial distribution of the frost on the surface but 183 also by the location of some of the main volcanoes at low latitude. The vertical column density 184 at the equator ranges from 1.5 x 1016 cm-2 at sub-jovian longitudes to 15 x 1016 cm-2 at anti-jovian 185 longitudes (Feaga et al. 2009; Jessup et al. 2004; Spencer et al. 2005; Lellouch et al. 2015). The 186 anti-jovian atmosphere is not just denser but also latitudinally more extended than the sub-jovian 187 side (Feaga et al. 2009). 188

Radio occultations of Io’ s ionosphere both with Pioneer10 and Galileo supported the 189 existence of a substantial atmosphere. Pioneer 10 radio occultations revealed a dense 190 ionosphere with a peak density ~ 60,000 cm-3 at ~ 100 km altitude on the dayside (Hinson et al. 191 1974; 1976), comparable to ionospheres of Mars and Venus. The night side ionosphere appeared 192 less dense. Six radio occultations were performed by Galileo in 1997 (Hinson et al. 1998). The 193 ionosphere appears asymmetrical with the plasma density depending strongly on Io’s longitude. 194 The interpretation of the observations assumes that the increased plasma density is distributed 195 in a spherically symmetrical bound ionosphere with a dense downstream wake. Depending on 196 location, peak densities of at least~ 50,000 cm-3 were found, reaching a maximum of ~ 250,000 197 cm-3 in one of the occultations. The asymmetries of the ionospheric densities are not surprising 198 considering the many parameters that influence the ionization process (electron deflection by 199 electromagnetic interaction, electron cooling by interaction with molecules, electron beam 200 ionization in the wake, etc;). It is notable that numerical simulations of the plasma/atmosphere 201 interaction do not usually reproduce electron density as high as ~ 100,000 cm-3 (Saur et al. 1999; 202 Dols et al. 2011) but interpretation of the observations presents many uncertainties (accurate 203

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spacecraft trajectory, presence of active hotspots, etc;) and limitations (entry and exit points of 204 the occultations are always at the terminator, assumption of a spherical symmetry of the bound 205 ionosphere, etc;). 206

The sublimation/condensation response of the atmosphere to day/night/eclipse 207 variations was modeled by Moore et al. (2009) and Walker et al. (2010, 2012) who showed that, 208 on the dayside, most of the atmospheric column is probably contained in the first 200 km with 209 a steep density profile under 20-30 km. The night side atmosphere of Io is poorly constrained. 210 The low temperatures at night would suggest SO2 condenses onto the surface and the 211 composition of the atmosphere changes drastically. Walker et al. (2010) results show a drop of 212 column density from the peak density on the sunlit hemisphere to the night atmosphere by a 213 factor 100-1000 (in their base case), but these simulations depends on many parameters that are 214 still poorly constrained. The possibility of non-consensible components (e.g. O2) on the nightside 215 of Io was first suggested by Kumar (1982). Modeling by Wong and Johnson (1996) indicate that 216 the night-side atmosphere may be dominated by non-condensable gases (O2 and possibly SO) 217 with SO2 condensing onto the surface. Moore et al. (2009) show that a buffer could be created 218 by even a small amount of non-condensable gases close to the surface, limiting the condensation 219 of SO2. On the other hand, Lellouch et al. (2015) report that the column density changes by a 220 factor of ~2 before and after noon corresponding to sublimation driven by ±2 Kelvin change in 221 temperature. Furthermore, the factor 5 ± 2 collapse (40 min into the 2-hour eclipse) observed by 222 Tsang et al. (2016) indicates that the effect of non-condensable gases is small. The data are 223 currently very limited but it is reasonable to assume that the atmosphere is significantly less 224 dense on the night-side of Io but probably does not completely condense out. SO2 comprises 90% 225 of the surface atmospheric pressure. Atmospheric SO was detected in mm-wave observations at 226 the 3-10% level (Lellouch et al. 1996; Moullet et al. 2010, 2013; deKleer et al. 2019a). But the 227 global coverage and column density of SO are uncertain. Moullet et al. (2013) report some minor 228 species (e.g. NaCl, KCl) that are likely linked to volcanic outgassing and Spencer et al. (2000) 229 reported enhancement of S2 over the Pele volcanic plume. From modeling the photochemistry 230 of the atmosphere, Moses et al. (2002a) suggest the SO/SO2 and S/O ratios may increase during 231 more highly volcanic epochs. 232

Deep in Io’s atmosphere photodissociation plays important roles (while higher up in the 233 atmosphere, electron collisions are more important). Photodissociation of molecular species 234 within ~5 hour time frame means that the dayside atmosphere will have a higher atomic 235 composition (O, S, Na) than the nightside (Moses et al. 2002a,b). Post-eclipse brightening of 236 sodium emissions suggest that photodissociation of the volcanically-produced NaCl accounts for 237 most of the atomic sodium produced by Io (Grava et al. 2014). 238

The Io atmosphere also has a remarkable high-altitude component of atomic species: S 239 and O (Wolven et al. 2001). There is a corona within Io’s Hill sphere (~5.8 RIo) as well as neutrals 240 that escape Io’s gravity and extend partly around Io’s orbit. While O is observed in both visible 241 and UV, there has only been sparse detection of S in the UV (Durrance et al. 1995; Ballester et al. 242 1987; Wolven et al. 2001). Auroral emissions of O and S are produced by electron impact 243 excitation of the neutral atmosphere (Roesler et al. 1999; Retherford et al. 2000, 2003; Geissler 244 et al. 2001, 2004). It is commonly accepted that the emissions are caused by thermal (5 eV) 245 electrons impacting O. The emissions depend on the neutral density as well as the electron 246

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density and temperature (including the supra-thermal population for UV emissions), all of which 247 are thought to vary drastically in the environment close to Io. The relative importance of each 248 electron property as well as the neutral density can only be estimated through numerical 249 simulations of the plasma/atmosphere interaction (see Section 3.1). The S and O auroral 250 emissions display several structures: Two large elongated equatorial spots on Io’s flanks (Roessler 251 et al., 1999; Retherford et al. 2000; 2003), further emission downstream along the flanks of Io at 252 low altitude (Roth et al. 2011, 2014a,b), a coronal emission that extends smoothly to ~ 10 RIo 253 from the surface (Wolven et al. 2001; Retherford et al. 2019), and polar limb brightening over 254 the pole that faces the plasma sheet during the diurnal rotation of Jupiter (Retherford et al. 255 2003). The density profiles of O and S in the atmosphere are still not well known. From modeling 256 of the interaction and its resulting O and S emissions, the O/SO2 ratio is estimated to be between 257 10-20% in the upper atmosphere (the models do not resolve the deep atmosphere) and S/SO2 ~ 258 1.5% with a scale height slightly larger than SO2 (Feaga et al. 2009; Roth et al. 2011, 2014a,b). 259

In summary, the atmosphere of Io is mostly sublimation-controlled SO2, with a vertical 260 column of SO2 ranging from 1.5 to 15 x 1016 cm-2, concentrated around the equator. Extending 261 far from Io’s surface are minor species S and O, that form a tenuous corona stretching to 262 distances of ~10 RIo. There are still major uncertainties about the radial profile, longitudinal 263 asymmetries, minor species composition and how much the atmosphere collapses at night or in 264 eclipse. 265

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2.2 Europa’s Atmosphere 267

The existence of an atmosphere at Europa is supported by the observation of UV aurora 268 in atomic oxygen lines as well as the detection of an ionosphere by radio occultation. As at Io, 269 properties of the global atmosphere are also inferred from numerical simulations of the plasma-270 atmosphere interaction, constrained by Galileo plasma observations (see Section 3.2). Europa’s 271 atmosphere is tenuous (surface pressure ~3 pbar), only collisional near the surface, and mainly 272 composed of O2 that is fairly uniformly distributed over the moon (see reviews by Johnson et al. 273 2009, 2019; McGrath et al. 2004, 2009; Burger et al. 2010; Coustenis et al. 2010; Plainaki et al. 274 2018). 275

UV auroral emission of Europa’s atmosphere in the 130.4 and 135.6 nm oxygen lines are 276 observed with the Hubble Space Telescope (HST). These UV emissions of oxygen atoms are 277 consistent with electron impact dissociation of an O2 atmosphere with vertical column density of 278 2-15 x 1014 cm-2 (Hall et al. 1995, 1998; McGrath et al. 2009; Saur et al. 2011; Roth et al. 2014a,b). 279 The UV emissions depend on the neutral density, as well as on the electron density and 280 temperature. Thus, deriving neutral densities and their distribution is difficult and requires 281 numerical simulations (Saur et al. 1998). The extensive analysis of the O aurora by Roth et al. 282 (2016) argues for a more limited range of 2-10 x 1014 cm-2. The O brightness is similar in daylight 283 or in eclipse showing that the atmosphere does not collapse at night like at Io (because O2 is non-284 condensable). Unlike Io, the main auroral emissions are not on the flanks but above the pole, and 285 specifically brighter on the polar hemisphere facing the plasma sheet. It is concluded that, in 286 contrast to Io, Europa’s atmosphere extends from pole to pole (see Figure 4). 287

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The presence of an extended atomic O corona is supported by Cassini observations during 288 its Jupiter flyby (Hansen et al. 2005) and HST observations in UV (Roth et al. 2016). The resulting 289 picture is an O2-dominanted atmosphere up to 900km (1.6 REu) where the O mixing ratio varies 290 from 6% to 30%. De Kleer & Brown (2018) measured optical emissions in Europa’s atmosphere 291 (dominated by electron impact dissociation of O2 leaving atomic products in an excited state) and 292 concluded that the global O/O2 ratio is less than 35%. 293

Erupting plumes may also contribute water vapor to the atmosphere. They are observed 294 in O lines and H-Lyman alpha lines (Roth et al. 2014a,b; Sparks et al. 2016, 2017). The effect of 295 these plumes on the Galileo plasma and magnetic field observations during low altitude flybys is 296 reported by Jia et al. (2012) and Arnold et al. (2019). Longitudinal asymmetries in the aurora 297 brightness are observed but their interpretation in terms of inhomogeneity of the atmosphere is 298 unclear. Roth et al. (2016), in a compilation of many observations at different Europa orbital 299 longitudes, shows that the dusk hemisphere is always brighter than the opposite hemisphere. 300 This asymmetry of the auroral brightness is attributed to a local denser atmosphere caused by 301 not only the potential presence of plumes, but also ice thermal inertia (Plainaki et al. 2013) or 302 effect of Europa’s varying illumination (Oza et al. 2018, 2019). Oza et al. (2018) argues that the 303 dawn-dusk asymmetry could not be reproduced with just a sputtered (trailing) source and that 304 an additional sub-solar source is needed, which Johnson et al. (2019) tentatively identified as 305 thermal desorption of O2 from the regolith. But Roth et al. (2016) suggests that some of these 306 brightness asymmetries could also result from the variation of the plasma interaction. 307

The degree of upstream/downstream asymmetry of the atmosphere is not very clear. 308 Pospieszalska and Johnson (1989) modeled the sputtering of Europa’s surface and show that the 309 sputtering flux is not uniformly distributes and peaks at the trailing hemisphere. But there is little 310 observational support for this. The oxygen auroral brightness is slightly lower on the leading 311 hemisphere compared to the trailing, which is interpreted as a global atmosphere distributed 312 upstream and downstream (Saur et al. 2011; Roth et al. 2016). The Kliore et al. (1997) non-313 detection of an ionosphere on the downstream hemisphere was interpreted as largely due to a 314 lack of ionization rather than a lack of atmosphere. 315

The source of the atmosphere is the sputtering of ions (thermal and hot) and maybe high-316 energy electrons on the icy surface of Europa. Lab experiments of ion bombardment of water ice 317 (Fama et al. 2008) suggests that H2O molecules are the dominant ejecta when magnetospheric 318 ions hit Europa’s surface. Molecules of O2 and H2 are also sputtered off the icy surface. Molecular 319 oxygen, O2, probably dominates the atmosphere because it neither freezes to the surface like 320 H2O, nor quickly escapes Europa’s gravity like H2 (Johnson et al. 1982; 2009). 321

There is no consensus on the relative contributions of thermal ions versus energetic ions 322 to the production of the O2 atmosphere (Mauk et al. 2004; Paranicas et al. 2009). Cassidy et al. 323 (2013) show that thermal heavy ions (S, O at 1-2 keV ram energy) could be responsible for 324 supplying the atmosphere. They estimate the O2 and H2 production rates by multiplying the ion 325 flux with the sputtering yields for O2. The yields are functions of the ion kinetic energy and are 326 provided by Teolis et al. (2010, 2017). The ion flux was calculated very simply, ignoring the 327 electromagnetic interaction that diverts the plasma flow around the moon and shields its surface. 328 Other processes could contribute to the atmosphere production. Ip (1996) proposed that 329

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ionization creates O2+ ions which, when accelerated by local electric fields, could cause significant 330 sputtering. But Saur et al. (1998), based on plasma-atmosphere simulations, suggested that these 331 picked-up ions are too slow for efficient sputtering. Both Dols et al. (2016) and Luchetta et al. 332 (2016) argue that charge exchange reactions between in-coming ions and the neutral 333 atmosphere are key. Dols et al. (2016) proposed that fast neutrals resulting from multiple charge 334 exchange reactions between pickup O2+ ions and atmospheric O2 provide supplemental 335 sputtering. But the sputtering rate of these fast neutrals has yet to be well quantified. 336

Six radio-occultations of the Galileo spacecraft by Europa were performed between 1996 337 to 1997 (Kliore et al. 1997). They revealed the existence of a tenuous ionosphere with an average 338 electron density ~9,000 cm-3 with an ionospheric scale height ~240 km close to the surface. This 339 scale height is consistent with later observations of the O auroral emissions (Roth et al. 2016) and 340 numerical simulations (Saur et al. 1998). These observations supported the existence of a neutral 341 atmosphere with a vertical column density ~0.7-3.6 x 1015 cm-2 and a neutral scale height ~ 120 342 km.In contrast to Io, it is remarkable that no ionosphere was detected near the middle of the 343 wake region. This absence of a plasma wake suggests an absence of ionization, rather than an 344 absence of atmosphere. 345

In summary, Europa’s tenuous, global atmosphere is still poorly constrained by 346 observations. We await better measurements of the number and frequency of the sporadic 347 plumes but their contribution to the global atmosphere seems to be limited. The source of the 348 atmosphere is primarily particle sputtering of the icy surface but a comprehensive model of the 349 atmosphere is still lacking. 350

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3. Plasma-Satellite interactions 352

Figure 5 illustrates the main components of plasma-satellite interactions, Figure 5a 353 presenting the basic geometry. Figure 5b indicates some of the chemical reactions as the plasma 354 impacts the atmosphere. Figures 5c and 5d show the UV and radio emissions excited at Jupiter 355 by streams of electrons that are generated by the plasma-satellite interactions. Below we provide 356 the historical context of indications of satellite-driven processes far from the moons. But the 357 primary focus of this review is what is happening at Io and Europa. 358

Before the 1958 Explorer 1 discovery of Earth’s Van Allen radiation belts, bursts of radio 359 emission revealed Jupiter has a magnetic field that traps electrons (Burke & Franklin 1955). These 360 high frequency (GHz or decimetric wavelength) radio emissions come from the MeV electrons 361 trapped close (<2.5 RJ) to Jupiter (reviewed by Bolton et al. 2004). The bursts of emission at 362 decametric wavelengths (few-40 MHz) showed modulations with Jupiter’s spin period and, to 363 great surprise, the orbital phase of Io (Bigg 1964). The Io modulation of radio bursts triggered 364 ideas of an electrically-conducting Io acting like a generator as it moved relative to Jupiter’s 365 magnetic field (a “unipolar inductor”), driving electrical currents that complete a circuit between 366 the moon and the planet’s ionosphere (Marshall & Libby 1967; Piddington & Drake 1968; 367 Goldreich & Lynden-Bell 1969). The radio emissions indicate processes close to the planet, 368 involving electrons radiating as they move along the magnetic field away from Jupiter, and say 369 little about the generation mechanism at Io. On 6 March 1979 Voyager 1 flew past Io where the 370

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instruments detected plasma and magnetic field perturbations consistent with a packet of Alfvén 371 waves forming an Alfvén wing propagating from Io (Belcher et al. 1981; Acuna et al. 1981). The 372 Voyager observations provoked theoretical work by Goertz (1980), Neubauer (1980), and 373 Southwood et al. (1980). Gurnett & Goertz (1981) suggested that multiple ionospheric reflections 374 of such Alfvénic disturbances between north and south hemispheres of Jupiter (Figure 5d) could 375 produce the pattern of arcs in the frequency-time spectrograms of the Voyager Planetary Radio 376 Astronomy instrument (Warwick et al. 1979). As Earth-based telescopes improved, auroral 377 emissions in Jupiter’s atmosphere – IR, UV, – revealed features at first associated with Io 378 (Connerney et al. 1993; Clarke et al. 1996; Prange 1996), and then not just Io but also Ganymede, 379 Europa (Clarke et al. 2002; Grodent et al. 2006; Bonfond et al. 2017a,b) and, recently, Callisto 380 (Bhattacharyya et al. 2018). These auroral emissions (Figure 5c) indicate that the plasma-satellite 381 interactions all involve electrodynamic perturbations, which generate Alfven waves propagating 382 from the moon, carrying electric currents parallel to the magnetic field, accelerating electrons 383 that bombard Jupiter’s atmosphere and generate auroral emissions. The Juno mission is revealing 384 new details about these auroral emissions and flying through the polar end of the fluxtubes that 385 couple to the moons (Mura et al. 2018; Szalay et al. 2018, 2019). 386

Close to the moons, the interactions of the jovian plasma with moon atmospheres 387 produce large perturbations of the magnetic field and local plasma properties (flow velocity, 388 density, temperature and composition). Over 33 orbits of Jupiter, the Galileo spacecraft made 389 several close passes of Io and Europa where the particles and fields instruments measured the 390 local perturbations (summarized by Kivelson et al. 2004). Table 2 summarizes the range of plasma 391 conditions upstream of Io and of Europa. 392

Saur et al. (1998) propose an estimate of the relative strength of the interaction at the 393 moons as the ratio of the ionospheric electric field to the corotation electric field (called 𝛼 ). 394 This ratio is based on the calculation of the Alfven, Pedersen and Hall conductances of the 395 moons’ ionosphere (see also Southwood et al. 1980). It is a quantitative estimate of the 396 divergence and slowing of the flow around the moon’s ionosphere: 𝛼 =0 corresponds to the 397 case of no electrodynamical interaction (plasma interaction with an insulating moon) where the 398 plasma impinges, undiverted, on the moon’s surface, and 𝛼 =1 correspond to the strongest 399 interaction (the moon as a perfect conducting body) where the plasma is fully diverted around 400 the moon by the electrodynamic interaction and does not reach the surface. The interaction at 401 Io is stronger than at Europa, with 𝛼 in the center of the torus ~ 0.8 for Europa and ~0.96 for Io 402 (Saur et al. 2013) 403

When looking at the plasma conditions listed in Table 2 and the strength of the 404 interaction (𝛼 ), we note that the atmosphere/plasma interaction is intrinsically time-variable for 405 two main reasons. The first is the inclination of the torus centrifugal equator by 7° relative to Io’s 406 orbital plane (further discussed in section 5). During a jovian synodic period (13 hours at Io and 407 11 hours at Europa), the moons experience variable upstream plasma densities and thus, a 408 variable strength of the interaction. Secondly, the moons’ atmospheres are also variable 409 depending on the atmospheric sources. At Io, the SO2 atmosphere is primarily sublimation-410 supported, varying with illumination and when there are particularly strong volcanic eruptions. 411

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For Europa, the atmosphere results from sputtering of the surface by thermal and energetic ions 412 but the O2 desorption from the icy surface seems to depend on illumination as well (Johnson et 413 al. 2018, 2019). Thus, the atmospheric column and the interaction’s strength depend on the 414 illumination of the hemisphere impacted by the torus plasma. 415

In Figure 5b, the impact of thermal electrons on the neutral atmosphere is the main local 416 source of new ions (called “mass-loading”), which are mostly molecular (SO2+ at Io and O2+ at 417 Europa). Photoionization represents only ~10-15% of the total ionization rate (Saur et al. 1998, 418 1999). A molecular atmosphere is very efficient at cooling the impinging thermal electrons (Saur 419 et al. 1998, 1999; Dols et al. 2008, 2012, 2016) via ionization and dissociation of molecular 420 neutrals and also by exciting molecular electronic, vibrational and rotational levels. These cooling 421 processes are so efficient that the primary electrons would rapidly become too cold to provide 422 any further ionization if they were not replenished by the content of the flux tube above and 423 below the moon impacting the moon’s atmosphere. 424

Ion charge exchange processes (asymmetrical and resonant) do not provide new charge 425 density ions but they can change the ion composition and constitute an important sink of 426 momentum for the upstream plasma (called “momentum-loading”). After an ionization or a 427 charge exchange, the new ion is initially at rest in the frame of the moon. It is then “picked up” 428 by the background bulk plasma flow and also starts a gyro-motion at the local flow velocity 429 (further discussed in Section 5.1 below). The pickup process also affects the average ion 430 temperature of the plasma. Depending on the location of this pickup, the local flow velocity is 431 larger than the upstream velocity (on the flanks) or smaller (in the deep atmosphere) and the 432 pickup process is either a gain or a loss of energy that affects the average ion temperature. For 433 instance, an SO2+ ion picked up by a 60 km/s flow at Io will gain 1080 eV, which represent a net 434 heating of the upstream plasma (with a typical temperature of ~100 eV). In Figure 5b, the 435 ion/neutral process called “atmospheric sputtering” refers to ion/neutral elastic collisions 436 without exchange of charge and after multiple collisions, a neutral is finally ejected from the 437 atmosphere (McGrath & Johnson 1987). The resulting neutral clouds are discussed in section 4. 438

The pickup (of both newly ionized and charge-exchange products) and collision processes 439 slow the plasma flow close to the moon and divert some of the plasma around the flanks as 440 illustrated in Figure 5. To first approximation, the magnetic field is frozen to the plasma flow. 441 Thus, any flow perturbation also produces large magnetic field perturbations that propagate 442 away from the moon in the three classical MHD wave modes (fast, slow and Alfven). In particular, 443 the magnetic perturbation propagates along field lines as Alfven waves towards the ionosphere 444 of Jupiter (Acuna et al. 1981) and forms a stationary structure in the reference frame of Io called 445 an Alfven wing (Goertz et al. 1980; Neubauer 1980; Belcher 1987). A large current is carried along 446 this Alfven wing (~ 3 MAmp for Io, according to Acuna et al. 1981). The plasma flow is diverted 447 not only around the solid body of the moon but also around the entire Alfven wing extending 448 from Io to the ionosphere of Jupiter (see Fig 21.2 in Kivelson et al. 2004). 449

Approaches to modeling these complex plasma-moon interactions range from focusing 450 on the electrodynamics with limited chemistry to assuming simple electrodynamics and focusing 451 on the chemistry. Figure 6 illustrates an example of the latter approach for the Io case. A similar 452 approach can be taken for Europa, with appropriate modification of the atmosphere and 453

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upstream plasma. Numerical simulations of this interaction are constrained by in-situ 454 observations of the plasma properties close to the moons (Voyager and Galileo) and by remote 455 observations of the moons’ auroral emissions. Beyond matching the observations, the modeling 456 approach allows an exploration of the main features of the interaction. Numerical simulations by 457 Linker et al. (1998) and Combi et al. (1998) illustrate the generation and propagation of the three 458 MHD wave modes. Numerical simulations (Saur et al. 1998, 1999, 2002, 2003; Dols et al. 2008, 459 2012, 2016; Blocker et al. 2016, 2018), reveal the relative importance of each plasma process 460 (ionization, charge-exchange, collision), constrain the neutral atmosphere distribution radially 461 and longitudinally, estimate the local plasma production and neutral loss and explore the 462 presence of an induced magnetic field either in Io’s asthenosphere (Khurana et al. 2011) and/or 463 core (Roth et al. 2017b), or in a subsurface ocean for Europa ( Zimmer et al. 2000; Schilling et al. 464 2007), plus test evidence for plumes (Blocker et al. 2016; Jia et al. 2018; Arnold et al. 2019). 465

While the physical processes (Figures 5, 6) are similar at Io and Europa, there are major 466 differences. Below we describe each interaction separately and then make a comparison in 467 section 3.3. 468

469

3.1 – Plasma-Io interaction 470

The Galileo spacecraft made 5 flybys of Io between 1995 and 2001 revealing very strong 471 plasma and field perturbations. The relative velocity of the torus plasma in Io’s frame is ~ 60km/s. 472 The plasma is mainly composed of S++ and O+ ions at temperatures of ~ 100 eV. The gyroradius 473 of the thermal ions ~ 2 km and the gyrofrequency ~ 1.5 Hz (see Table 2). When Io is in the 474 centrifugal plane of the torus, the flow is quasi-stagnated at the point where the plasma impinges 475 on the atmosphere, with 95% of the plasma diverted around the flanks. The electron flow close 476 to Io is strongly twisted towards Jupiter because of the Hall conductivity of the ionosphere (Saur 477 et al. 1999). The magnetic perturbation reaches ~ 700 nT in a background field ~ 1800 nT (Kivelson 478 et al. 1996a). Downstream of Io, Galileo detected a wake of plasma that was very dense (~ 30,000 479 cm-3), very slow (flow speed <1 km s-1) and very cold (Ti ~ 10 eV) (Frank et al. 1996; Bagenal 1997). 480 Bi-directional parallel electron beams above the poles and in Io’s wake were also detected with 481 an energy ranging from ~140 eV to several 10s keV (Frank and Paterson 1999; Williams et al. 482 1996, 2003; Mauk et al. 2001). Finally, Electro-Magnetic Ion Cyclotron (EMIC) waves were 483 observed far downstream of Io at the SO2+ and SO+ ion gyro-frequencies, suggesting a pickup 484 process far (>10 RIo) from Io (Warnecke et al. 1997; Russell & Kivelson 2000; 2001). 485

The local auroral emissions provide additional constraints on the atmospheric 486 distribution, composition and plasma interaction, including during eclipse. Saur et al. (2000) and 487 Roth et al. 2011 have modeled the plasma atmosphere interaction at Io, constrained by the 488 oxygen auroral emissions. Roth et al. (2014c) propose a phenomenological model of the 489 structures of the auroral emissions, based on observations gathered from 1997 to 2001. 490

They conclude that during this 4-year period, the auroral emissions did not vary beyond 491 the changes of the periodically varying local environment caused by the latitudinal motion of Io 492 in the torus. This stability is remarkable considering the potential variation of Io’s atmospheric 493 content caused by sporadic major volcanic eruptions during such a long period. Furthermore, 494

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Roth et al. (2017b) argue that the rocking of Io’s auroral spots is consistent with induction in the 495 deep iron core rather than shallow asthenosphere as proposed by Khurana et al. (2011). Blocker 496 et al. (2018) supported Roth’s idea by claiming that most of the magnetic field perturbation 497 observed during Galileo flybys were caused by an asymmetric atmosphere and not by a strong 498 induction signal. 499

In Figures 7 and 8, we illustrate the plasma-neutral interaction at Io using the multi-500 species chemistry approach sketched in Figure 6. Although the model results vary with the 501 assumptions of the simulation, they provide reasonable estimates of the contribution of each 502 process and the resulting plasma properties. The simulations presented here are 2D simulations 503 in the equatorial plane of Io similar to Dols et al. (2008). They are a mixed of material already 504 published and new unpublished simulations. Here, the SO2 atmospheric radial profile is based 505 on a later publication in Dols et al. (2012). 506

The main assumptions and results are summarized below. The plasma flow is prescribed 507 as an incompressible flow around a conducting obstacle. The simulation shown in Figure 7 does 508 not include the parallel electron beams detected by Galileo. These parallel electron beams 509 probably provide much of the ionization in the wake, particularly when a dense atmosphere is 510 present (Saur et al. 2002; Dols et al. 2008, 2012). Figure 7 shows the plasma properties in the 511 equatorial plane of Io. The SO2 distribution is assumed cylindrically symmetrical and thus does 512 not include any day-night asymmetry resulting from a collapse of the SO2 atmosphere at night. 513 The radial distribution is exponential with a scale height ~500 km and a denser component with 514 a scale height of ~30 km where most of the SO2 column resides (Dols et al. 2012). The resulting 515 vertical column density of 5 x 1016 cm-2 is consistent with dayside observations. To account for 516 the equatorial distribution of the SO2 atmosphere (Strobel and Wolven 2011) in this 2D approach, 517 we assume that the SO2 atmosphere extends 1 RIo perpendicular to the equatorial plane. The S 518 and O atmosphere is assumed to be spherically symmetric based on the UV emission radial 519 profiles of Wolven et al. (2001). 520

The plasma flow is slowed upstream and downstream and accelerated on the flanks. 521 Following Dols et al. (2008), we prescribe a radial slowing of the flow consistent with the ion 522 temperature observed along the Galileo J0 (first) flyby. The ion temperature increases on the 523 flanks because of the pickup of SO2+ ions in the local fast flow. An increased electron and SO2+ 524 density is carried along the flow while the electron temperature decreases because of the 525 efficient cooling process provided by the molecular atmosphere. The SO2 electron-impact 526 ionization and dissociation are located mainly on the upstream hemisphere because of the 527 efficient electron cooling processes. The SO2 resonant charge-exchange rate (SO2 + SO2+ => SO2+ 528 + SO2) is larger on the flanks. In the wake of Io, along the Galileo J0 flyby, the flux tubes are 529 emptied of the upstream S and O ions because of charge-exchange reactions with SO2 and SO2+ 530 becomes the dominant ion. 531

In Figure 8 we show the volume-integrated rates of each plasma-SO2 process. These 532 results are computed using a 3D MHD simulation of the plasma flow around Io published in Dols 533 et al. (2012). The MHD flow allows for a better description of the slowing of the flow in Io’s 534 atmosphere, which affects the reaction rates and the results are thus somewhat different in 535 detail to the 2D simulations shown in Figure 7. 536

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Figure 8 provides a reasonable estimate of the relative contribution of each process. The 537 dominant SO2 loss process is the electron-impact dissociation, which provides slow atomic 538 neutrals that potentially feed on an extended corona. Another significant process is a cascade of 539 resonant charge exchange reactions, which provides slow and fast SO2 molecules, depending on 540 the local flow speed where the charge exchange reaction occurs. This cascade of charge exchange 541 and dissociation reactions potentially feeds neutral clouds of S, O atoms and SO2 molecules 542 similar to the observed Na structures (Figure 11), which extend along Io’ s orbit or through the 543 whole magnetosphere (discussed in section 4). 544

Such modeling efforts require a consistent and robust set of observational constraints, 545 mainly provided by Galileo instruments (PLS, MAG, EPD, PWS). The plasma measurements are 546 essential to infer the plasma flow perturbation, the ion temperature, density and composition. 547 Unfortunately, the PLS sensitivity evolved during the Galileo mission and PLS plasma densities 548 are often not consistent with those inferred from the PWS measurements (Dols et al. 2012). This 549 lack of reliable plasma observations limits the reach of the numerical modeling of the local 550 interaction at Io. The Galileo flybys were also relatively far from the denser part of the 551 atmosphere (~200–900 km) and thus mostly constrain the tenuous corona. Moreover, no Galileo 552 flybys were achieved in eclipse and on the night side. Thus, the spatial and temporal variabilities 553 of the plasma-atmosphere interaction remain poorly constrained. A new mission at Io is needed 554 to provide further major breakthrough in our understanding of its local interaction. 555

556

3.2 – Plasma- Europa Interaction 557

Galileo made 9 flybys of Europa between 1996 and 2000 on the upstream hemisphere, 558 the flanks and > 3 REu downstream. These flybys were far (~200-3500 km) from the deep part of 559 the atmosphere and, unlike Io, no flybys were achieved above the poles. 560

The relative velocity of the upstream plasma in Europa’s reference frame is ~ 100 km/s 561 (Bagenal et al. 2015) and the background magnetic field ~450 nT (Kivelson et al. 2004). The 562 electron thermal population (Tel~20 eV) is warmer than Io’s with a 5% non-thermal population at 563 ~250 eV (Sittler & Strobel 1987; Bagenal et al. 2015). The gyroradius of the average thermal ion 564 is ~6 km. The Europa interaction is similar to Io’s, but weaker (as described by 𝛼 in the 565 introduction of section 3) because of a less dense upstream plasma (ne~160 cm-3). The magnetic 566 perturbation along the E4 flyby of ~50 nT is described by Kivelson et al. (1999) and Saur et al. 567 (1998) estimate that ~80% of the upstream plasma flow is diverted around the moon. 568

The most important Galileo discovery at Europa is the confirmation of the existence of a 569 salty ocean under the icy crust, based on the magnetic perturbations measured along several 570 flybys. Consequently, a major focus of analyzing and modeling the Galileo observations is the 571 derivation of the induction signal to sound the conductivity and depth of the sub-ice ocean 572 (Khurana et al. 1998, 2002; Kivelson et al. 1999, 2000; Zimmer et al. 2000; Schilling et al. 2004). 573

More recently, water plumes were discovered with the UV cameras onboard HST (Roth 574 et al. 2014a,b; Sparks et al. 2016), and indirect evidence of the plume existence were inferred 575 from Galileo magnetometer observations along the E26 flyby at low (~400km) altitude upstream 576 of Europa (Blocker et al. 2016; Arnold et al. 2019) and along the E12 flyby (Jia et al. 2018), and 577

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from Galileo observations of Europa’s ionosphere (McGrath & Sparks 2017). These plumes are 578 intermittent (Roth et al. 2014a,b, 2017a) but if they are to be sampled by future missions at 579 Europa, they might provide direct information about the composition of the sub-surface ocean. 580

The morphology of the Europa oxygen aurora is remarkably different from Io’s. It is 581 comprehensively studied by Roth et al. (2016) using the STIS UV image-spectrometer onboard 582 HST. These emissions are not located along the equatorial flanks like Io, but mainly above the 583 poles and they are almost systematically brighter on the polar hemisphere that faces the centrifugal 584 equator. These polar emissions are reminiscent of similar observations of the auroral polar limb at 585 Io. Contrary to Io, Europa’s atmosphere is not localized around the equator and the vertical column 586 is much lower than Io’s. Two-fluid simulations (similar to Saur et al. 1998) of the auroral emissions 587 were presented in McGrath et al. (2004). These simulations, based on specific assumptions about 588 the atmosphere distribution, assume no magnetic field induction and Europa’s location in the 589 center of the torus. The results show bright limb emissions all around Europa’s disk and do not 590 seem to explain the morphology observed in Roth et al. (2016) who state: “A comparison of the 591 Saur et al. (1998) simulation results with the first images of the aurora morphology on the trailing 592 hemisphere taken by STIS reveals significant differences of the observed and simulated emission 593 pattern (McGrath et al. 2004, Figure 19.10). In contrast to the rather symmetric global limb glow 594 in the model results, the observed morphology appears asymmetric and irregular, (...) and the 595 brightest emissions are often found on the disk rather than right at or near the limb. Moreover, 596 the area near the tangent points of the magnetic field lines at the flanks of Europa is fainter than 597 the magnetic poles (...). Such a decrease of aurora excitation in this area is not expected for the 598 symmetric interaction scenario as assumed in the model of Saur et al. (1998)”. Roth et al. (2016) 599 proposed that the inductive magnetic field in Europa’s ocean might perturb the flow around 600 Europa and suppress the formation of equatorial auroral spots seen at Io but this hypothesis has 601 yet to be tested with numerical simulations. 602

The various numerical simulations of the Europa plasma-atmosphere interaction are 603 similar to Io’s: Two-fluid (electron and single O2+ ion in a constant uniform magnetic field, Saur 604 et al. 1998), MHD (Schilling et al. 2007, 2008 ), multi-fluid MHD (Rubin et al. 2015) , hybrid 605 (Lipatov et al. 2010, 2013) and multi-chemistry (Dols et al. 2016) with the goal of matching the 606 Galileo plasma observations, understanding the main processes driving the interaction, 607 constraining the global atmosphere, estimating the neutral loss rates and constraining the 608 subsurface ocean. As the Galileo flybys did not probe the denser atmosphere, the simulations 609 usually assume a large scale global O2 atmosphere with a scale-height ranging from 100-200 km 610 and some upstream-downstream asymmetries due to the thermal sputtering source upstream. 611

Saur et al. (1998) focused on the plasma interaction and the formation of an O2 612 atmosphere. They inferred an equilibrium atmospheric column by balancing the sputtering 613 sources and the atmospheric losses, constrained by the oxygen auroral emissions observed by 614 HST. Using a relatively low upstream plasma density ~ 40 cm-3, they compute an equilibrium 615 atmosphere with a vertical column of 5 x 1014 cm-2, a 145 km scale height and ~ 70 km exobase. 616 The resulting atmospheric loss by ionization is estimated at ~ 6 kg s-1 and the atmospheric 617 sputtering loss by charge exchange and elastic collisions above the exobase ~ 40 kg s-1. 618

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Dols et al. (2016) focused on the multi-species chemistry, using a simplified 2D plasma 619 flow described as an incompressible flow around a perfectly conducting body of radius 1.0 REu. 620 Figures 9 and 10 show simulation results under a similar assumption of a perfectly conducting 621 obstacle but we assume that the obstacle extends to 1.26 REu to provide results similar to Io’s 622 shown on Figures 7 and 8. The assumed O2 atmosphere is spherically symmetrical, with a scale-623 height of 150 km and a vertical column = 5 x 1014 cm-2, consistent with observations. As Europa’s 624 atmosphere is more tenuous than Io’s, the plasma variations close to the moon are smaller. After 625 comparing the general properties of the plasma variations around Europa, we now turn to the 626 reaction rates integrated on the whole simulation volume. As for Io in Figure 8, we now use a 627 MHD flow that is not completely diverted around the moon so that 20 % of the flow reaches the 628 surface (𝛼 =0.8), consistent with Saur et al. (1998). Although these MHD flow results have not yet 629 been published, we adopt this flow description to make the results comparable to Io’s, shown in 630 Figure 8. The Europa volume-integrated rates for each plasma-O2 process are presented in Figure 631 10 and should be understood as an illustration of the relative magnitude of each process. 632

Similar to the Io case, a cascade of resonant charge-exchange reactions is an important 633 neutral loss process at Europa but, in general, all rates are much smaller than at Io. The H2 634 chemistry is included in the simulations but the H2 atmospheric loss rates due to plasma-neutral 635 reactions in the atmosphere are insignificant. The escaping flux of H2 (~2 x 1027 s-1) is probably 636 lost by thermal escape after surface sputtering of the icy surface. H2 is probably the source of the 637 extended neutral cloud detected around Europa (see Section 4) as the O2 rates calculated here 638 (~0.5 x 1027 s-1) are not sufficient to feed a dense cloud of neutrals. 639

Blocker et al. (2016) use MHD simulations to study the effect of localized plumes on the 640 global interaction. They show that each plume forms an Alfven winglet embedded in the main 641 Alfven wing caused by the interaction with the global atmosphere. The plasma production or 642 neutral loss resulting from the direct interaction at the plume is probably very small compared 643 to the effect of the global atmosphere. 644

645

3.3 Io-Europa Comparison 646

Properties of the plasma interactions with Io and with Europa are summarized in Table 3 647 while Figure 11 shows the plasma densities and ion composition downstream of the interactions 648 with Io and Europa. In the Io case we did not include electron beams in this simulation so the 649 cold, dense wake is missing. In each case the inflowing atomic ions are replaced downstream 650 with molecular ions. 651

While our main conclusion is that the neutral loss at Europa is much smaller than at Io, 652 for both Io and Europa, comprehensive self-consistent models of the full, coupled 653 plasma/atmosphere/neutral cloud system are still needed. The atmospheric part of such models 654 could be built on Saur et al. (1998)’s concept of source and sink balance. The atmospheric sources 655 would include the detailed sputtering rates by thermal torus ions, pickup ions, hot ions, and 656 maybe fast neutrals and hot electrons. The atmospheric loss processes would include the 657 ionization, elastic collisions, charge-exchange and molecular recombination. These loss rates 658 would be used as input to a neutral cloud model similar to Smith et al. (2019). Such a model could 659

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also assess the significance of Europa’s atmosphere as a neutral and plasma source for the Jovian 660 magnetosphere. 661

A remarkable difference between the interactions at Io and Europa is the absence of 662 parallel electron beams in the wake of Europa. It is thought that at the foot of Io’s Alfven wing, 663 electrons are accelerated toward Jupiter to produce the footprint aurorae and also in the other 664 direction to produce the parallel electron beams detected at Io (Bonfond et al. 2008). As footprint 665 auroras are sometimes detected at the foot of the Europa’s Alfven wing, it was expected that 666 parallel electron beams would be present at Europa as well. Moreover, Io’s dense wake is thought 667 to be caused by electron beams ionization in the downstream hemisphere and Galileo 668 measurements in Europa’s wake did not reveal a dense plasma comparable to Io’s (Kurth et al. 669 2000; Paterson et al. 1999). One possible explanation of the non-detection of parallel electron 670 beams along the Galileo flybys is that, because of Europa’s weaker interaction, the beams are 671 located further downstream and so Galileo missed them. An alternative scenario is that the local 672 interaction at Europa does not systematically produce footprint auroral emissions and 673 concurrent electron beams. 674

In summary, the strong plasma-atmosphere interaction produces locally at Io only ~200 675 kg/s of ions but up to 2000 kg/s of neutral dissociation products are distributed around Io’s orbit. 676 Meanwhile at Europa the interaction is much weaker. The interaction produces at best 20 kg/s 677 of O2

+ locally and is probably a minor source of plasma to the magnetosphere of Jupiter. 678 Nonetheless, extensive neutral clouds (probably hydrogen) have been detected along the orbit 679 of Europa, as we discuss in the next section. 680

681

4. Neutral Clouds 682

While radio astronomers were still scratching their heads about Io triggering bursts of 683 radio emission, in 1973 optical astronomers detected indications of atmospheric escape from Io. 684 Emissions from sodium and potassium were observed to emanate from a cloud around Io. 685 Sodium is extremely efficient at scattering visible sunlight. Other neutral species are much harder 686 to detect, particularly molecular species, so that our picture of the iogenic neutral clouds are 687 largely based on the behavior of sodium and on models (Figures 12, 13). Table 4 lists the sources 688 of neutrals and plasma at Io and Europa. 689

Models of neutral clouds take a flux of particles from the moon’s exobase (where 690 atmospheric collisions become negligible) and follow the motions of the particles under the 691 gravity first of the moon and then, farther away, Jupiter. The neutral particles interact with the 692 surrounding plasma being eventually ionized or charge exchanged. Photoionization also plays a 693 role in lower density regions of the torus. Figure 13 shows a model recently published by Smith 694 et al. (2019) that takes typical escape fluxes of different species Io (Smyth & Marconi 2003) and 695 from Europa (Smyth & Marconi 2006) and a very simplified SO2 escape flux from Io’s exobase and 696 tracks particles under Jupiter’s gravity using a background plasma model to predict when along 697 its trajectory a neutral particle of a particular species will be changed (via dissociation, ionization 698 or charge exchange). 699

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Figure 13 shows that molecular species are more easily dissociated and ionized, confining 700 the molecular neutral clouds to the vicinity of the moons. Atoms with longer lifetimes (O, H) lead 701 to atomic clouds that spread around the moon’s orbit. The color scale is the same for all five of 702 the contour plots and shows that both Io and Europa neutral clouds are of remarkably 703 comparable density. As shown in Figures 8 and 10, Io is a more prolific neutral source (~250-3000 704 kg/s) than Europa (~25-70 kg/s) but the plasma near Io is denser and the neutrals are quickly 705 removed, especially outside Io’s orbit. 706

707

4.1 – Io neutral cloud 708

The first observational evidence of neutral atoms escaping Io was the detection of optical 709 sodium D-line emission from a cloud in the vicinity of Io by Brown (1974). The efficient resonant 710 scattering of sunlight by sodium produced the bright emission (Trafton et al. 1974; Bergstrahl et 711 al. 1975; Brown and Yung 1976). Many images of the sodium cloud have been acquired (see 712 Figure 12 and reviews by Thomas et al. 2004; Schneider & Bagenal 2007). Voyager 1 observations 713 revealed Io’s S02 atmosphere (Pearl et al. 1979) and a sulfur-dominated surface (Sagan 1979), 714 indicating that Na was just a trace element in a neutral cloud dominated by sulfur and oxygen 715 atoms and their compounds. While sodium is a trace component of the atmosphere (~few %), its 716 large cross section for scattering visible sunlight (30 times brighter than any other emissions) 717 makes it the most easily observed species in the neutral clouds. 718

Early theoretical studies suggested that various neutral species could be removed from 719 Io’s atmosphere by many different processes (with variable efficiency) to feed the neutral clouds: 720 Jeans escape, charged particle sputtering of its surface, atmospheric sputtering, charge-721 exchange, direct single collisional ejection (Matson et al. 1974; Haff et al. 1981; Kumar 1982; Ip 722 1982; Johnson and Strobel 1982; Sieveka and Johnson 1984; McGrath and Johnson 1987). More 723 recently electron impact molecular dissociation is added to the list (Dols et al. 2008). 724

Models of the neutral clouds showed that electron impact ionization and charge exchange 725 with torus plasma are significant loss processes on timescales of a few hours (e.g., Smyth and 726 Combi 1988; Smyth 1992). Hence, the observed distributions of the neutral clouds are dependent 727 upon both the neutral source and the spatial distribution of plasma properties in the torus. 728 Sodium, potassium and sulfur have relatively short (2-5 hours) lifetimes against electron impact 729 ionization in the densest regions of the torus. The rate coefficient for ionization of oxygen, 730 however, is at least an order of magnitude lower than for sulfur so that charge-exchange loss of 731 O with torus ions dominates over ionization. The minimum lifetime for O is around 20 hours (e.g., 732 Thomas 1992) resulting in significant densities of neutrals farther from Io and an O neutral cloud 733 that extends all the way around Jupiter. 734

After the initial detection by Brown (1981) of atomic oxygen emissions from Io’s neutral 735 cloud at the wavelength of 630.0 nm leading to an estimate of a ~ 30 cm-3 oxygen atoms, further 736 observations of these optical emissions from the Io cloud by Thomas (1992) and close to Io by 737 Oliversen et al. (2001) showed substantial variations with longitude, local time and Io phase. 738 Meanwhile, atomic emissions were also detected in the UV. Atomic sulfur emissions at 142.9 nm 739 and atomic oxygen emissions at 130.4 nm were discovered with a rocket-borne telescope by 740

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Durrance et al. (1983). The first detections of neutral O and S were around 180° in Io phase away 741 from Io itself and analyzed by Skinner and Durrance (1986) to infer densities of neutral O and S 742 of 29 ± 16 cm-3 and 6 ± 3 cm-3 respectively. 743

The presence of dense neutral clouds along the orbit of Io is also confirmed by Lagg et al. 744 (1998) who note losses of energetic sulfur and oxygen ions (~ 10s keV/nucleon) with a 90o pitch 745 angle measured by the Galileo EPD detector. These ions are thought to be removed by charge 746 exchange with extended neutral clouds with a density ~35 cm-3, consistent with previous 747 estimates of Brown (1981) and Skinner and Durrance (1986). 748

The IUE satellite detected UV emissions from O and S near Io (Ballester et al. 1987). Using 749 30 observations by the HST-STIS, Wolven et al. (2001) mapped out O emission at 135.6 nm in the 750 UV to 10 RIo. They also demonstrated that these neutral emissions downstream of Io were 751 brighter than those upstream, that there seemed to be a weak, erratic System III longitude effect, 752 and that the local time (dawn versus dusk) asymmetry depends on the Io phase angle. 753

The Hisaki UV observatory in Earth orbit has provided new oxygen observations, mapping 754 out Io’s neutral oxygen cloud to show it comprises a leading cloud inside Io’s orbit and an 755 azimuthally uniform region extending to 7.6 RJ (Koga et al. 2018a,b, 2019). The peak number 756 density of oxygen atoms is estimated at 80 cm-3, spreading ~1.2 RJ vertically. The Hiskaki team 757 estimate the source rate of oxygen ions at 410 kg/s, roughly consistent with previous studies 758 (Smyth & Marconi 2003; Delamere and Bagenal 2003; Yoshioka et al. 2018). When Io exhibited a 759 volcanic eruption in 2015, lasting ~90 days, Koga et al. (2019) used Hisaki observations of the 760 time variations of O and O+ to show volcanism shortens the lifetime of O+ and that Io’s neutral 761 oxygen cloud spreads outward from Jupiter during the 2015 volcanic event. The number density 762 of O in the neutral cloud at least doubles during the active period. 763

From 1977 to 2011 Bill Smyth and colleagues published ~25 papers applying a neutral 764 cloud model to study satellite atmospheres and neutral clouds (Smyth & MacElroy 1977; Smyth 765 et al. 2011). The Smyth neutral cloud model involved solving the kinetic equations for a 766 population of neutral particles embedded in prescribed plasma torus. For each small time-step, 767 the state (location, velocity, ionization state) of the different particle species is updated following 768 the consequences of the effects of gravity and collisions (elastic, inelastic, and chemically 769 reactive), which are calculated for the average conditions in each model bin. They applied their 770 model, including various refinements, to study Io’s Na, K, O, S, SO2 and SO corona, plus the plasma 771 torus properties (Smyth & Marconi 2003, 2005; Smyth et al. 2011). The recent Smith et al. (2019) 772 model shown in Figure 13 is conceptually similar to the Smyth models, with updated reaction 773 cross-sections, updated neutral fluxes from the exobase at Io and Europa and incorporating 774 current data on the variability of conditions at orbit of Io. 775

Detailed measurements of Io’s sodium cloud require models to explain the main “banana-776 shaped” cloud, a “directional feature” or jet, as well as a “stream” of fast neutral atoms (FNAs: 777 100 eV to keV) that spread out into a nebula extending for 100s of RJ (Figure 12). The main sodium 778 cloud is consistent with a roughly uniform corona of sodium atoms (a little above escape speed) 779 around Io (Schneider et al. 1991; Burger et al. 1999, 2001) producing a uniform source of an 780 extended cloud that is shaped by interaction with the surrounding plasma (Smyth & Combi 1997). 781 Jets of fast sodium neutrals require a process that includes acceleration, probably as an atomic 782

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or molecular ion, followed by a neutralizing charge exchange (or dissociative recombination) 783 reaction that sends out a fast (~100 km/s) neutral (Wilson & Schneider 1994; 1999). We return 784 to the extended sodium nebula in section 6. 785

The main goal of Smith et al. (2019) was to show that Io’s neutral material, whatever its 786 composition, would not significantly reach Europa’s orbit. The Smith et al. (2019) model 787 illustrated in Figure 13 assumes that only SO2 escapes directly from Io with a canonical rate of 1 788 ton/s and a prescribed low velocity distribution. They produce iogenic neutral clouds that are 789 consistent with observations of O and S emissions (Skinner & Durrance 1986; Koga et al. 2018a). 790 But there are no observations of the neutral SO2 cloud around Io. Future models of the neutral 791 clouds need to include sources of SO2, S and O based on quantitative results from models of the 792 plasma-atmosphere interaction (section 3.1) 793

Smyth and Marconi (2003] and Smith et al. (2019) study the formation of extended 794 neutral structures but to this day, a self-consistent approach of their formation, from the 795 processes in the atmosphere of Io, to the neutral cloud and finally to the torus ion supply, has 796 not yet been carried out. Such extended neutral structures of S, O and SO2 are notably difficult 797 to observe (Brown & Ip 1981; Durrance et al. 1983; Koga et al. 2018a,b), beyond the minor Na 798 species and we hope that in a near future, new creative observational methods will help to 799 constrain quantitatively these extended neutral structures and improve our understanding of 800 their atmospheric sources. 801

802

4.2 – Europa Neutral Cloud 803

Strong indications of an extended source of neutrals escaping from Europa were implied 804 by measurements of (a) depletion of energetic protons at about the orbital distance of Europa 805 (Lagg et al. 2003; Kollman et al. 2016), and (b) energetic neutral atoms (ENAs: 10s keV per 806 nucleon) observed by the MIMI instrument on Cassini as it flew past Jupiter (Mauk et al. 2003, 807 2004). A stringent limit of ~8 cm-3 on the density of atomic oxygen from Cassini UVIS 808 measurements (Hansen et al. 2005) meant that the neutral cloud, estimated to have a density of 809 20-50 cm-3 to produce the observed ENAs, must be mostly hydrogen (atomic or molecular) rather 810 than oxygen (consistent with simulations of Shematovich et al. 2005; Smyth & Marconi 2006; 811 Smith et al. 2019). The fate of the ENAs is discussed in section 6. 812

The low mass of hydrogen allows it to easily escape Europa so that hydrogen has a much 813 higher (at least x10) escape rate than oxygen. Roth et al. (2017a) detected an extended corona 814 of atomic H around Europa, consistent with electron impact dissociation of molecular H2, as 815 modeled by Smyth & Marconi (2006), but the net contribution of escaping atomic H is relatively 816 minor (~5%). The Smith et al. (2019) model is similar to Smyth & Marconi (2006) but applies 817 additional particle interaction processes, updated reaction cross-sections and updated neutral 818 fluxes from the exobase. The Europa neutral clouds are presented on the right side of Figure 13, 819 showing that the dominant species is H2 that extends all the way around Europa’s orbit with 820 oxygen (particularly in molecular form) being more limited to the Europa environment. Smith et 821 al. (2019) also conclude “Source rate changes take longer to impact the Europa neutral cloud than 822 the Io neutral cloud. The dominant processes at Europa’s orbit have lifetimes from at least 2–3 823

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days up to longer than a week, while at Io, the neutral particles start interacting (with the plasma) 824 in 8–13 hr. Additionally, the range in observable ambient plasma environments can vary particle 825 interaction rates by more than an order of magnitude.” 826

Recently, Nenon & Andre (2019) note the removal of energetic (~MeV) sulfur ions with 827 ~90° pitch angle near Europa’s magnetic flux-shell and argue that inward-transported heavy ions 828 interact with the extended hydrogen cloud while protons (which have smaller cross-section for 829 charge-exchange reactions) interact with the tenuous, more equatorially-confined oxygen cloud. 830 Nenon & Andre (2019) point out that since the heavy energetic ions tend to be higher ionization 831 states (Clark et al. 2016), their interaction with Europa’s neutral clouds reduces their charge state 832 so that when they move inward to Io’s neutral cloud they are singly charged and therefore leave 833 the system as ENAs on charge-exchanging with Io’s neutral clouds (Mauk et al. 2004). While 834 protons are less likely to charge-exchange, when they do react with Europa’s neutral hydrogen 835 and oxygen clouds, they are lost as ENAs – as detected by the Cassini MIMI instrument (Krimigis 836 et al. 2002; Mauk et al. 2004). 837

Neutral clouds of sodium (Brown & Hill 1996) and potassium (Brown 2001) have also been 838 detected around Europa. Burger & Johnson (2004) model the distribution of sodium and found a 839 cloud shape similar to the O cloud of Smith et al. (2019) in Figure 13, extending farther on the 840 trailing (upstream) side where particles are moving away from Jupiter into lower plasma densities 841 compared with the leading (downstream) side where particles are moving towards Jupiter into 842 higher plasma densities. The big question is whether Europa’s alkalis are representative of its 843 brines and ocean salinity or is this merely iogenic material painted onto Europa’s surface (Brown 844 2001; Leblanc et al. 2002, 2005). 845

846

5. Plasma Torus 847

The Io plasma torus comprises three main regions: the outer region that has a roughly 848 circular cross-section – sometimes called the “doughnut” – that contains 90% of the mass; just 849 inside Io’s orbit there is narrow but vertically-extended region – sometimes called the “ribbon” – 850 that is bright in UV and, particularly, visible wavelengths; extending inwards from the ribbon is a 851 thin disk or “washer”. Figure 14 illustrates these three regions and their properties are listed in 852 Table 5. The warm torus extends past the orbit of Europa (section 5.4) and merges into the 853 plasma sheet. 854

Optical emissions from a toroidal cloud of S+ ions surrounding the orbit of Io were first 855 detected in ground-based observations by Kupo et al. (1976), which Brown (1976) recognized as 856 coming from a cold, dense plasma. The Voyager 1 flyby of Jupiter in 1979 revealed Io spewing 857 out SO2 from active volcanoes and provided detailed measurements of the lo plasma torus both 858 from the strong emissions in the EUV, observed remotely by the Voyager Ultraviolet 859 Spectrometer (UVS) (Broadfoot et al. 1979), as well as in situ measurements made by the Plasma 860 Science (PLS) instrument (Bridge et al. 1979) and the Planetary Radio Astronomy (PRA) 861 instrument (Warwick et al. 1979). 862

Since Voyager, the Io plasma torus has been observed remotely via visible and UV 863 emissions as (Thomas 1992; Hall et al. 1994; Gladstone et al. 1998; Feldman et al. 2001; 2004) 864

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well as in situ by the Galileo spacecraft (Gurnett et al. 1996, 2001; Frank & Paterson 2000). Each 865 of these techniques for measuring the plasma properties in the torus has its pros and cons. The 866 remote sensing techniques provide good temporal and spatial coverage but suffer from being 867 integral measurements along the line of sight as well as being dependent on calibration of the 868 instrument and accurate atomic data for interpretation of the spectra. Moreover, a very broad 869 wavelength range is needed to cover emissions from all the main ion species. The in situ plasma 870 measurements provide detailed velocity distributions but suffer from limited spatial and 871 temporal coverage as well as poor determination of parameters for individual ionic species in the 872 warm region of the torus where the spectral peaks for different species overlap. Since these two 873 data sets are complementary, they can be combined to construct a description of the plasma 874 conditions in the torus, i.e. an empirical model, that can be compared to theoretical models 875 based on the physical chemistry of the torus plasma (e.g. Bagenal 1994; Herbert & Hall 1998; 876 Herbert et al. 2003, 2008; Smyth & Marconi 2005; Smyth et al. 2011; Nerney et al. 2019). A 877 summary of observations and theoretical understanding at end of Galileo mission is provided by 878 Thomas et al. (2004). 879

The dominance of sulfur and oxygen composition of heavy ions throughout the 880 magnetosphere tell us that these products of volcanic gases fill the vast volume of Jupiter’s 881 magnetosphere. At the same time, energetic particles from the outer magnetosphere are 882 transported inwards, bringing in supra-thermal ions that charge-exchange with the neutral 883 clouds, plus electrons that ionize and excite UV emissions. In the next sections we first describe 884 the underlying physical processes in the Io-Europa space environment and then summarize 885 current understanding of the three torus regions. 886

887

5.1 – Physical Processes 888

The key physical processes controlling the plasma in the Io-Europa environment are ion 889 pick-up, centrifugal confinement to the equatorial region, radial transport and collisional 890 processes (dissociation, ionization, charge-exchange, radiation, etc; gathered under the term 891 “physical chemistry”) that drive the radial distribution of different ion species as well as the 892 energy of the plasma. 893

Ion Pick-Up: As discussed in Section 3 above, a relatively small amount of plasma is 894 directly produced in the interaction with either Io’s or Europa’s atmospheres. Most of the plasma 895 is produced via electron impact ionization of the extended neutral clouds (Section 4 above) where 896 the neutral atoms that follow Keplerian orbits (~17 and 14 km s-1 at Io and Europa respectively) 897 around Jupiter. The plasma is coupled via Jupiter’s magnetic field to planet’s electrically-898 conducting ionosphere, which corotates with the planet’s ~10 hour spin period (~75 and 118 km 899 s-1 at Io and Europa respectively). Minor deviations from rigid corotation mean that typical 900 plasma flow speeds are ~5% less than corotation at Io (71 km s-1) and ~15% sub-corotational at 901 Europa (~100 km s-1). When a slowly moving atom is ionized (either via electron impact or charge 902 exchange), the fresh ion starts with an initially high velocity relative to the corotating plasma (55 903 and 85 km s-1 at Io and Europa respectively). Each pickup ion therefore experiences an electric 904 field due to its motion relative to the local magnetic field (and the plasma) and is accelerated – 905 i.e. "picked up” – gaining a gyro-velocity (in the plane perpendicular to the local magnetic field) 906

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equal to its initial motion relative to the corotating plasma. Each fresh ion gains a gyro-energy 907 dependent on its mass: 270 eV for O+, 540 eV for S+ and 1.8 keV for SO2+ at Io's orbital distance; 908 650 eV for O+ at Europa’s orbital distance. Fresh O+ pick-up ions gyrate around the magnetic field 909 at ~2 and 0.5 Hz with a 5- and 40-km gyro-radius at Io and Europa respectively. The energy of 910 these fresh ions ultimately comes from Jupiter' s rotation, coupled to the plasma via the magnetic 911 field. If the gyro-speed were distributed into an isotropic Maxwellian distribution (e.g., via 912 Coulomb collisions) the O+ and S+ ions would have temperatures of 2/3 their initial pick-up 913 energy. The corresponding pick-up electrons gain negligible gyro-energy but are accelerated to 914 corotate with the surrounding plasma. Electrons are heated quite quickly via Coulomb collisions 915 with the ions but only in the inner torus does the plasma hang around long enough to get close 916 to full thermal equilibrium. Evidence of local pick-up comes from the Electro-Magnetic Ion 917 Cyclotron (EMIC) waves were observed far downstream of Io at the SO2+ and SO+ ion gyro-918 frequencies (Warnecke et al. 1997; Russell & Kivelson 2000; 2001). These waves are generated 919 by the unstable “ring-beam” velocity distributions of fresh pick-up ions. The presence of a 920 background population of thermalized ions suppresses the generation of EMIC waves which 921 explains why emissions are not detected at the gyro-frequencies of the dominant ion species 922 such as S+ and O+ (Crary & Bagenal 2000). 923

When a neutral particle is ionized it not only picks up gyro-motion but it also starts an ExB 924 drift at the bulk velocity of the flowing plasma. Ions are added to the torus at a rate of about 1 925 ton/s, which corresponds to ~10-3 ions cm-3 s-1 or just ~5 ions cm-3 per 10-hour Jupiter spin period. 926 When integrated around Io’s orbit, the additional radial current to pick up these fresh ions and 927 accelerate them to corotation is 0.6 megaAmp (for a torus height of 2 RJ). The electrical currents 928 close along the magnetic field, coupling the plasma to the planet’s ionosphere (that is collisionally 929 coupled to the neutral atmosphere). The corotating torus plasma continually experiences an 930 inward J x B force associated with an azimuthal current through the torus (see derivations in 931 Cravens (1997) chapter 8). This corotation current is about 10 megaAmp (for 4 RJ2 torus cross-932 section). On the other hand, keeping the plasma corotating as it moves into the plasma sheet (as 933 well as balancing pressure gradient forces) drives radial currents in the plasma sheet of several 934 10s megaAmps, closing via parallel current from/to the ionosphere (Cowley & Bunce 2001; 935 Nichols et al. 2015). 936

Centrifugal confinement. Torus plasma trapped by Jupiter 's magnetic field is confined 937 toward the equator by centrifugal forces. A corotating ion gyrates around local field lines several 938 times per second while bouncing along field lines every few hours. On field lines at Io's orbit, each 939 ion experiences about 1 g of centrifugal acceleration outward from the rotation axis. Ions in a 940 Maxwellian velocity distribution are distributed along the magnetic field line, centered around 941 the point farthest from Jupiter's rotation axis. The locus of all such positions around Jupiter is 942 called the centrifugal equator. In an approximately dipolar magnetic field tipped like Jupiter's, 943 ~10° from the rotation axis, the centrifugal equator has 2/3 of the tilt, or ~7° from Jupiter's 944 rotational equator (Hill et al. 1974). As the tilted torus corotates with Jupiter, the torus viewed 945 from Earth appears to wobble ±7°. Non-dipolar components to the field can measurably warp 946 the centrifugal equator. 947

In equilibrium, one can consider the plasma distribution along the field as a fluid in 948 balance between the effect of gradients in the thermal pressure of the plasma (nkT) and the 949

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centrifugal force, much as the vertical distribution of a planet’s atmosphere comes from a 950 balance between pressure gradient and gravitational forces. Thus, the torus vertical structure is 951 a rough indication of ion temperature. The fluid approach can incorporate further complexity 952 associated with the ambipolar electric field (~15 volts across the torus) arising from small charge 953 separation between ions (more strongly affected by the centrifugal force) and the much lighter 954 electrons, plus the addition of multiple ion species and thermal anisotropy. 955

For the approximation of a single, isotropic ion species, cold electrons (with net charge 956 neutrality) and a roughly dipole magnetic field, the north-south or vertical dimension (z-axis) 957 distribution of plasma density n(z) about the centrifugal equator is a Gaussian function 958

n(z) = n(z=0) exp -(z/H)2 959

where the scale height H is determined by the spin rate of Jupiter, W, the ion temperature, T, and 960 the mass of the ions, A: 961

H = [2/3 kT / (mp A W 2 ]1/2 = Ho [T(eV) / A(amu)]1/2 962

For H in units of Jovian radii, RJ, T is in eV and A is average ion mass in atomic mass units, we have 963 Ho = 0.64 RJ. For a more detailed description of the “diffusive equilibrium” distribution of plasma 964 along the magnetic field for a multi-species, anisotropic plasma see Bagenal (1994). 965

Radial Transport: Sulfur and oxygen ions, assumed to originate in the Io plasma torus, are 966 detected throughout the magnetosphere, implying that plasma from the Io plasma torus is 967 spread throughout the magnetosphere. For a region of a planetary magnetosphere where the 968 plasma is tightly coupled to the planet (called a “plasmaphere”) and where rotational effects are 969 small (e.g. Earth), radial transport of plasma has been described as a diffusive process whereby 970 neighboring fluxtubes randomly switch positions. For the Earth, such fluxtube interchange was 971 assumed to be driven by turbulent motions in the ionosphere (Falthammer 1966; Brice 1968). 972 The transport of plasma in a rotation-dominated magnetosphere such as Jupiter’s, is usually 973 described to be via centrifugally-driven flux tube interchange (e.g., Hill et al. 1981). The 974 interchange instability is the magnetospheric analog of the Rayleigh-Taylor instability, the same 975 instability that drives convection in a planetary atmosphere or a pot of boiling water. The 976 effective gravity, dominated by the centrifugal force of corotation, is radially outward. The 977 essence of the centrifugally-driven interchange instability is that heavily-loaded magnetic flux 978 tubes from the source region move outward and are replaced by flux tubes containing less mass 979 (the outer magnetosphere is relatively empty) moving inward, thereby reducing the (centrifugal) 980 potential energy of the overall mass distribution without significantly altering the configuration 981 of the confining magnetic field. And because Jupiter's magnetosphere is a giant centrifuge, 982 outward transport is energetically strongly favored over inward transport. 983

In the standard theory, interchange rate is regulated by the electrical conductivity in 984 Jupiter's ionosphere, at the “feet” of the magnetic flux tubes (Hill 1979). Early studies used 985 Voyager measurements of the onset of deviation from corotation (McNutt et al. 1981) to 986 constrain the value of ionospheric conductance (Hill 1980; Hill et al. 1981). But theoretical models 987 using only ionospheric conductance to limit fluxtube interchange have difficulty in matching the 988 long lifetimes (~20-80 days) for plasma in the Io torus derived from physical chemistry models 989 (discussed in the next section). A detailed discussion of possible additional mechanisms (e.g., ring 990

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current impoundment, velocity shears, etc;) to slow down radial transport from the torus is given 991 in section 23.7 of Thomas et al. (2004). While many have looked, direct in situ evidence of the 992 instability has been very rare (Thorne et al. 1997; Bolton et al. 1997; Kivelson et al. 1997; Frank 993 and Paterson 2000; Russell et al. 2005). The theoretical problem turns out to be, less an issue of 994 explaining why outward transport occurs, but more a matter of explaining why it occurs so slowly. 995 Or, put another way, why is the torus so long-lived, and therefore so massive? 996

An empirical approach describes fluxtube interchange as a diffusive process that depends 997 on the radial gradient of the content of a magnetic flux tube, NL2 where N is the number density 998 integrated over an L-shell of (dipolar) magnetic flux (Richardson et al. 1980). A diffusion 999 coefficient (usually simplified as a constant times a radial power-law) is derived to match 1000 observed radial profiles of fluxtube content. The strong preference for outward transport via 1001 centrifugally-driven fluxtube interchange means that the derived diffusion rates are about a 1002 factor of 50 faster for transport outwards from the source at Io than inwards. This is a major 1003 factor explaining why there is ~100 less mass in the inner torus (Richardson et al. 1980, 1981; 1004 Bagenal 1985; Herbert et al. 2008). 1005

One topic that theorists struggle with understanding is the scale on which centrifugally 1006 driven interchange is being driven. Richardson and McNutt (1987) looked at the currents into the 1007 Voyager PLS sensors at the highest temporal resolution (L modes) and found that on 0.24 second 1008 timescale (which translates to an effective spatial resolution of ~20 km), the plasma density does 1009 not fluctuate more than 20% (more typically <5%). These observations ruled out theories such as 1010 proposed by Pontius et al. (1986) that required interchange of full and (essentially) empty flux 1011 tubes. In a survey of Galileo data, Russell et al. (2005) showed that small-scale (~2 second 1012 duration corresponding to scales of 100-1000 km) magnetic perturbations indicative of inward-1013 moving fluxtubes (relatively empty of dense, cold plasma but could have plenty of energetic 1014 particles) occurred rarely (~0.32% of the time), which suggests these fluxtubes need to move in 1015 quite rapidly (10s km/s) compared with the slow outward motion of full fluxtubes. Hess et al. 1016 (2011a) argues that the fast, inward-moving, empty fluxtubes are analogous to the Io Alfven 1017 wing. The Alfven waves propagating along the fluxtube are responsible for acceleration of 1018 electrons close to the ionospheres of Jupiter. Some of the electrons are trapped between the 1019 mirror points of the flux tube and form the supra-thermal electron population observed by Frank 1020 & Paterson (2000) and required by physical chemistry models (described in the next section). 1021

The inward-moving fluxtubes containing energetic particles are often called “injections”. 1022 Such injections of energetic particles have been observed in the Io-Europa environment (Mauk 1023 et al. 1999; Haggerty et al. 2019) and are clearly a major element in radial transport within these 1024 regions. The structures are much broader in scale than the events observed in magnetometer 1025 data by Russell et al. (2005) and their dispersion in energy suggests such structures have aged 1026 (presumably as they travelled inwards). Isolated auroral features equatorward of the main 1027 auroral oval have been observed the Hubble Space Telescope and been associated with injection 1028 of hot electrons resulting from flux tube interchange (Mauk et al. 2002; Krupp et al. 2004; 1029 Grodent 2015; Dumont et al. 2014, 2018). Dumont et al. (2014) suggest that such injections seem 1030 to be quite common (one per 24 hours), appear at all SIII longitudes, peak close to Europa’s orbit 1031 but sometimes spread to a radial distance ~7 RJ and even to Io’s orbit (Bonfond et al. 2012). But 1032 so far, only a single clear injection event has been reported (Mauk et al. 2002) where observations 1033

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of an electron injection observed in situ by Galileo are concurrent with an auroral equatorial 1034 protrusion observed by the Hubble Space Telescope. It remains unclear, however, what the 1035 physical relationship is between injections (either the small scale empty magnetic fluxtubes 1036 occasionally observed in magnetometer data or the larger scale dispersed energetic particle 1037 structures) and radial transport processes such as fluxtube interchange. 1038

Whole fluxtubes possibly interchange in the inner magnetosphere where the field is 1039 dipolar. But as plasma moves beyond Europa’s orbit, and as the plasma is hotter at larger 1040 distances, the plasma pressure begins to dominate over the magnetic field and one expects that 1041 other more local instabilities likely dominate radial transport (Vasyliunas 1983; Krupp et al. 2004; 1042 Kivelson & Southward 2005). The topic of radial transport and energization of plasma populations 1043 is a major active topic that is being addressed by a combination of auroral emissions (from HST 1044 and Juno) and in situ particles and fields observations by the Juno mission. 1045

Physical Chemistry. In the bulk of the Io-Europa space environment the transport times 1046 are relatively long (~1 to several months) so that substantial plasma densities accumulate (1000-1047 3000 cm-3) and collisions are quite frequent (hours to days). This means that collisional reactions 1048 (excitation, ionization, dissociation, charge-exchange) are important sources and losses of both 1049 particles and energy. For each species the source and loss terms are different and depend on 1050 properties (neutral and plasma density, temperature, etc.;) that can vary in space and time, so 1051 one needs to solve a self-consistent system including both physical chemistry and diffusive 1052 transport. Early models took a homogeneous box with an input of neutrals with specified 1053 composition (e.g. 1 ton/s of O and S neutral atoms in the ratio 2:1 consistent with dissociation of 1054 SO2) and a specified timescale for transport out of the box (e.g. 40 days). Some initial plasma is 1055 included, reaction rates specified, and the system evolved to equilibrium. It was quickly realized 1056 that an additional source of relatively hot electrons needs to be added to the system in order to 1057 sustain the system (Shemansky 1988; Barbosa 1994). 1058

A major difficulty in building such models, particularly in early studies, is the fact that the 1059 atomic data for many of the various reactions are not agreed upon or even known. McGrath & 1060 Johnson (1989) calculated critical charge exchange reaction rates for typical torus conditions. 1061 Taylor et al. (1995) developed the Colorado Io Torus Emission Package (CITEP) that took atomic 1062 data provided by Don Shemansky and the Bagenal (1994) torus model to predict emissions for 1063 various species at different wavelengths and viewing geometries. In 1997 a group of UV 1064 astronomers put together a database of UV emissions in the CHIANTI database (Dere et al. 1997). 1065 This public database has been updated periodically, the most recent being CHIANTI 8.0 (Del 1066 Zanna et al. 2013; Del Zanna & Badnell 2016). For example, Nerney et al. (2017) illustrates the 1067 effects of using different data bases on the analysis of UV observations obtained by Voyager, 1068 Galileo and Cassini to derive ion composition in the Io plasma torus, finding up to 40% changes 1069 in the derived abundances of some of the species between CHIANTI versions 4 and 8. 1070

Such physical chemistry models of the Io plasma torus have been extended from a “cubic 1071 centimeter” in the center of the torus (Shemansky 1988; Lichtenburg et al. 2001; Delamere & 1072 Bagenal 2003) to radial profiles (Barbosa 1994; Delamere et al. 2005; Nerney et al. 2017; Yoshioka 1073 et al. 2014, 2017) and azimuthal variations (Steffl et al. 2008; Copper et al. 2016; Tsuchiya et al. 1074 2019) as well as temporal variations apparently driven by volcanic outbursts on Io (Delamere et 1075

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al. 2004; Kimura et al. 2018; Yoshioka et al. 2018; Tsuchiya et al. 2018). Delamere et al. (2005) 1076 considered the effect of an additional neutral source at Europa on the torus chemistry and found 1077 that the faster radial transport rate (Vr increasing from ~100 m/s at Io to few km/s at Europa) 1078 meant that the contribution was minimal. We return to the impact of Europa on the torus in 1079 Section 5.4. 1080

Finally, it must be noted that the physical chemistry models of the torus assume sources 1081 of atomic O and S and do not yet include molecular species (e.g. SO2, SO, O2) either as neutrals 1082 or ions and assume such molecules are quickly dissociated. While this is probably a reasonable 1083 assumption for most of the main, warm, outer torus, molecules probably play an important role 1084 in the ribbon and cold, inner torus (see section 5.3). 1085

1086

5.2 – Warm Io Plasma Torus 1087 The evolution of thinking about the Io plasma torus through the Voyager era illustrates 1088

the disconnect between the fields of astronomy and space physics. Initially, the two groups did 1089 not understand the other’s nomenclature. Pre-Voyager, the planetary astronomers talked of 1090 emissions from SII excited by electrons with temperatures of log10 Te = 4.4 ± 0.6 K and densities 1091 of log10 [ne] = 3.5 ± 0.6 cm-3 (Brown 1976). Meanwhile, the Pioneer space physicists talked about 1092 in situ measurements of 100 eV protons with densities of 50–100 cm−3 (Frank et al. 1976). As 1093 Voyager approached Jupiter, the UVS team said they were seeing surprisingly intense emissions 1094 from high densities of sulfur and oxygen ions. The Voyager PLS team scaled down their 1095 instrument sensitivities accordingly. A couple days later, Voyager 1 flew through the middle of 1096 the torus and measuring peak densities up to 3200 cm-3 (Bridge et al. 1979; Warwick et al. 1979) 1097 with PLS only saturated in one high resolution spectrum. 1098

The doughnut-shaped main warm torus outside Io’s orbit dominates the mass and energy 1099 of the system (Table 5). The plasma is warm (ion temperature ~60-100 eV) with a vertical scale 1100 height of ~1.0-1.5 RJ,. The plasma moves slowly outwards, the electron density dropping from 1101 ~2000-3000 cm-3 at Io’s 6 RJ orbit to 100-200 cm-3 by Europa’s 9.4 RJ orbit (Figure 15), which it 1102 reaches in ~30-80 days (Bagenal et al. 2016; 2017). While there are small blobs of cool (~20 eV) 1103 plasma in the plasma sheet (~10-30 RJ, Dougherty et al. 2017) the bulk of the plasma rises from 1104 ~60-100 eV at Io to ~few 100 eV by Europa (Bagenal et al. 2015). 1105

As the plasma expands radially outwards, in fluxtubes of increasingly weaker magnetic 1106 field, one would expect the plasma to cool. But instead of cooling on expansion, the iogenic 1107 plasma is in fact clearly heated (by an as-yet-unknown process), as it moves outwards, reaching 1108 keV temperatures in the plasma sheet on timescales of weeks, requiring something like ~0.3-1.5 1109 TW of additional energy input (Bagenal & Delamere 2011). 1110

Composition. Despite multiple ways of observing the torus, no observation uniquely 1111 determines all 5 of the dominant ion species (O+, O++, S+, S++, S+++) in the main Io torus region. The 1112 Voyager PLS instrument obtained excellent compositional measurements in the cold inner torus 1113 (<5.6 RJ) and in several cold blobs in the plasma sheet (Dougherty et al. 2017). But the analysis 1114 of both Voyager and Galileo data to obtain density, temperature and flow in the warm torus 1115

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(Bagenal et al. 2015; 2016; 2017) required making assumptions about composition derived from 1116 a combination of UV emissions and physical chemistry models. 1117

Bodisch et al. (2017) reports on minor ions detected in the torus and plasma sheet from 1118 the re-analysis of Voyager PLS data. They found protons comprise 1–20% of the plasma between 1119 5 and 30 RJ (nominally ~10% in the torus) with variable temperatures ranging by a factor of 10 1120 warmer or colder than the heavy ions. These protons, measured deep inside the magnetosphere, 1121 are consistent with a source from the ionosphere (rather than the solar wind) of ~1.5–7.5 x 1027 1122 protons s-1 (2.5–13 kg/s). Sodium ions are detected between 5 and 40 RJ at an abundance of a 1123 few percent (occasionally as much as 10%) produced by the ionization of the extended iogenic 1124 neutral cloud discussed in Section 4 above. 1125

Early discussions about the torus ion composition derived from UV emissions were 1126 plagued with poor knowledge of excitation and reaction rates, as well as debates about the role 1127 of hot electrons. The ionization state of the ions seems to increase with distance and the total 1128 emitted power (~1.5 TW) required an additional source of hot (say ~50–100 eV) electrons 1129 (Moreno et al. 1985; Smith & Strobel 1985; Smith et al. 1988; Shemansky 1988; Barbosa 1994). 1130 The Bagenal et al. (1992) study of torus composition combined analysis of in situ PLS data with 1131 Voyager UVS data analyzed by Shemansky (1987, 1988). This combined composition analysis 1132 became the basis of an empirical description of the Io plasma torus by Bagenal (1994), which was 1133 used to constrain early physical chemistry models of the torus (Schreier et al. 1998; Lichtenberg 1134 et al. 2001; Delamere & Bagenal 2003). The torus composition derived from Voyager 1 UVS 1135 emissions by Shemansky (1987) was strongly dominated by O+ ions (~40% of ne), which required 1136 neutral input to the physical chemistry model of Delamere & Bagenal (2003) with O/S~4, about 1137 twice what one would expect from the dissociation of SO2. 1138

The Cassini spacecraft flyby of Jupiter (with a primary goal of gaining a gravity assist to 1139 Saturn) provided an excellent opportunity to study UV emissions from the Io plasma torus 1140 between October 2000 and March 2001 (Steffl et al. 2004a,b). Spectral analysis of the torus 1141 emissions suggested the ion composition was very different at the Cassini epoch than that 1142 derived by Shemansky (1987) at the time of Voyager (bottom of Figure 15). Specifically, the 1143 abundance of O+ was reduced to 26% of ne, which Delamere & Bagenal (2003) could model with 1144 a neutral source of O/S~2, compatible with an SO2 origin. 1145

Meanwhile, Delamere et al. (2005) expanded their physical chemistry model to 2 1146 dimensions, assuming azimuthal symmetry. They averaged reactions over latitude and calculated 1147 radial variations in plasma properties for a fluxtube of plasma that was slowly moving outwards 1148 via centrifugally-driven fluxtube interchange. Note that the bounce periods for electrons and ions 1149 trapped in Jupiter’s magnetic field and the collisional reaction rates are relatively short compared 1150 with the transport timescale of 30-80 days. Beyond ~8 RJ the densities have dropped and the 1151 radial transport sped up so that the reactions and radiation have effectively ceased, “freezing in” 1152 the composition. The production of neutrals needed by Delamere et al. (2005) to match the 1153 Cassini UVIS data was comparable to the Smyth & Marconi (2003) neutral cloud model at Io (~7 1154 x 1026 atoms/s) but required a steeper radial gradient. 1155

Recent re-analysis by Nerney et al. (2017) of the Voyager 1 UVS data using the current 1156 atomic data for emission rates suggests an ion composition more consistent with the UV 1157

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emissions observed in the Cassini era and with models of the torus physical chemistry matched 1158 to the Cassini UV data by Delamere et al. (2005) as shown in Figure 15. Nerney et al. (2017) also 1159 looked at the Galileo UVS data (June 1996), which actually showed a depletion of O+ emission 1160 compared with Cassini. Intriguingly, Herbert et al. (2001) also reported a decrease in O+ emission 1161 observed by EUVE at about the same time. We return to the topic of temporal variations in the 1162 torus below. 1163

A measurement from the Juno mission that has important implications for the torus is the 1164 ability of the JADE instrument (McComas et al. 2017) to separate the O+ and S++ ion species that 1165 have the same mass/charge ratio (M/Q=16) with the Juno-JADE time-of-flight detector. Kim et al. 1166 (2019) report a ratio of O+/S++ ion abundances of ~0.7 (±10%) at 35 RJ in the plasma sheet. This is 1167 a bit lower than typical values of derived from UV spectroscopy and physical chemistry models 1168 (e.g. Delamere et al. 2005 found 0.95 to 1.4 at 9 RJ). Note that the Juno in situ measurements 1169 extend up to 10s keV energies and the composition may reflect species-dependent heating 1170 between the torus and the plasma sheet. 1171

JAXA’s Hisaki satellite has been in orbit around Earth since September 2013 (Yoshikawa 1172 et al. 2014). The UV spectrometer has been observing emissions (including new emission lines 1173 identified by Hikida et al. 2018) from the Io plasma torus, determining a background torus 1174 composition that is very similar to the post-Io-eruption Cassini epoch (Yoshioka et al. 2014, 2017). 1175 The greatest value of the Hisaki mission has been in monitoring spatial and temporal variations 1176 over several seasons, including a couple of periods of enhanced volcanic activity that we discuss 1177 below, after summarizing the main torus physical chemistry. 1178

Flow of Mass & Energy. Summing up the mass in the Io plasma torus comes to total of 1179 ∼2 megaton. A source of ∼1 ton/s would replenish this mass in ∼40 days. To get the total thermal 1180 energy of the torus, we multiply this total mass by a typical energy (Ti ≈ 60 eV, Te ≈ 5 eV) to obtain 1181 ∼6 x 1017 J. The torus emits (via more than 50 ion spectral lines, mostly in the EUV) a net power 1182 of ~1.5 TW. This emission is excited by electrons which, at a rate of ~1.5 TW, would drain all their 1183 energy in ∼7 hours. While pickup supplies energy to the ions, which is fed to the electrons via 1184 Coulomb collisions, the pickup energy is not sufficient to maintain the observed emissions. An 1185 additional source of energy, perhaps mediated via plasma waves and/or Birkeland currents, is 1186 needed to heat the electrons. 1187

Figure 16 shows the flow of mass and energy through the torus system from Nerney et 1188 al. (2019) taking the “cubic centimeter” physical chemistry model at 6 RJ (that matches conditions 1189 derived from the Cassini UVIS spectrum) as typical of the main torus. The model is not very 1190 sensitive to the temperature of the hot electrons (40-400 eV). Model inputs are: O/S source ratio 1191 =1.9, source of neutrals Sn=6.4 x 10-4 cm-3 s-1, transport timescale= 64 Days, fraction of hot 1192 electrons feh = 0.25%, temperature of hot electrons Teh=46 eV. This neutral production rate is 1193 equivalent to a net neutral source of 575 kg/s for a volume of 68 RJ3 . We call this the “low & 1194 slow” case. Similar results are found for a source rate of 1.8 ton/s (Sn=20 x 10-4 cm-3 s-1 over a 1195 volume of 68 RJ3 ) and correspondingly lower transport times – the “high & fast” case. Note that 1196 there is an anticorrelation of transport time and source to match the observed density (~2000 1197 cm-3) in the torus. 1198

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The “low & slow” example is shown in Figure 16a.. Sulfur is more easily ionized than 1199 oxygen so that the dominant sulfur ion becomes S++ while oxygen is quickly charge-exchanged 1200 and much (42%) of the oxygen is lost from the region as fast neutrals. Hence the abundance ratio 1201 of the dominant (M/Q=16) ions O+/S++ is about unity and we expect a denser, more extensive 1202 neutral oxygen cloud (density ~50 cm-3) than neutral sulfur cloud (density ~11 cm-3), consistent 1203 with recent modeling by Smith et al. (2019) discussed in section 4.1 above. The S+, S+++ and O++ 1204 ions, with abundances of a few percent each, are transported out of the torus with the dominant 1205 O+ and S++ ions. The supra-thermal electrons contribute to ionization, particularly for the higher 1206 ionization states, but the thermal electrons play the major role in the particle budget overall. The 1207 hot electrons become more important when considering the energy budget. 1208

Figure 16b shows the flow of energy through the system for the “low & slow” case. Here 1209 hot, supra-thermal electrons play a major role, supplying 63% of the energy. Current physical 1210 chemistry models fix the contribution of hot electrons to the total density. This is a simplistic 1211 approach and we await more sophisticated models. Nevertheless, such an approach provides 1212 useful indications of the extent of the role played by of supra-thermal electrons. The model is 1213 very sensitive to the faction of hot electrons and Nerney et al. (2019) could not find realistic 1214 physical chemistry solutions for feh > 0.5%. Additional energy sources are from ion pick up of S+ 1215 (16%) and O+ (18%) ions. Most of this energy input to the torus is coupled via Coulomb collisions 1216 to the core, thermal electrons and radiated out via mostly UV emissions (91%) excited by electron 1217 impact. 1218

The mass flow for “high & fast” case is remarkably similar with a slightly lower average 1219 ionization state since there is less time to ionize the higher source of neutrals beyond the first 1220 ionization state. The energy flow for “high & fast” case requires fewer hot electrons (~40% of the 1221 energy supply) with the higher neutral source producing more pick-up ions. The ions remain 30-1222 40eV hotter, carrying more of the energy out of the system with less (73%) being radiated 1223 through UV emissions. Note that both “low & slow” and “high & fast” cases are matching typical 1224 torus conditions and emissions. 1225

Estimates of the hot electron fraction based on UV emissions range from <1% to 15% 1226 (Steffl et al. 2004a,b; Yoshioka et al. 2014, 2017, 2018; Nerney et al. 2017; Tsuchiya et al. 2017, 1227 2019; Hikida et al. 2018) with the fraction increasing from 6 to 8 RJ. Physical chemistry models 1228 consistently suggest less than 1% (typically ~0.25%) for the hot electron fraction necessary to 1229 power the torus at 6 RJ (Delamere & Bagenal 2003; Yoshioka et al. 2011) increasing to 1.5% at 9 1230 RJ (Delamere et al. 2005). Nerney et al. (2019) shows that one can match the UV emission 1231 spectrum with a range of Feh from 0.1 to 5%, far higher values than suggested by the physical 1232 chemistry limits (<0.5% at 6 RJ). In situ measurements from Voyager PLS electron data show 1233 electron distributions that reasonably approximate a double Maxwellian for most of the torus. 1234 Beyond about 8 RJ the supra-thermal component increases, forming a tail to the thermal core. 1235 Sittler & Strobel (1987) derive values for the hot electron fraction of (0.15, 0.9, 10%) at (6, 8, 9 1236 RJ) respectively. 1237

While the need for a supply of supra-thermal electrons has been known since the mid-1238 80s, the source mechanism remains unknown. It is even debated whether they are produced 1239 locally within the fluxtubes connected to the torus (Hess et al. 2011a; Copper et al. 2016) or 1240

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injected from the outer magnetosphere (Yoshikawa et al. 2017; Tsuchiya et al. 2018, 2019; 1241 Kimura et al. 2018). The Galileo flybys of Io showed that beams of supra-thermal electrons are 1242 produced in the wake of the Io interaction (discussed in section 3.1 above) but no significant Io-1243 modulation was found in the Voyager or Cassini UV emissions from the torus (Sandel & Broadfoot 1244 1982a,b; Steffl et al. 2004a,b). In the more recent and extensive monitoring of UV emissions by 1245 Hisaki, Tsuchiya et al. (2015) reports a ~10% modulation of the UV power associated with the 1246 phase of Io’s orbit. 1247

Spatial Variations and Modulations. Over the past forty years spatial and temporal 1248 variations of the Io plasma torus have intrigued observers and puzzled theorists. For the warm 1249 torus, there are four main types of variations: (A) System III longitude variations at Jupiter’s spin 1250 period tied to the magnetic field structure ; (B) a pattern/periodicity that drifts at a few percent 1251 slower than System III, originally called System IV; (C) temporal variations associated with Io’s 1252 volcanic eruptions; (D) modulations in brightness of torus emissions and a radial shift of the peak 1253 location on the dawn vs. dusk side of Jupiter. 1254

The idea of a systematic longitudinal modulation of the magnetosphere of Jupiter was 1255 initially provoked by Pioneer observations of energetic electrons escaping the system with a 10-1256 hour periodicity (Chenette et al. 1974), which Dessler (1978) interpreted as due to an 1257 anomalously weak field at a specific longitude driving preferential outflow in this “magnetic 1258 anomaly” region (Dessler & Vasyliunas 1979; Vasyliunas & Dessler 1981). From 1980 onwards 1259 ground-based observations of optical emissions from S+ ions showed systematic longitude 1260 variations (see thorough review in Steffl et al. 2006). Voyager UV emissions showed longitudinal 1261 variations in S+ and S++ emissions (Sandel & Broadfoot 1982b; Herbert & Sandel 2000) while 1262 Lichtenberg et al. (2001) found similar variations in IR emissions from S+++ ions. The 45 days of 1263 monitoring the UV emissions from multiple ions in the torus by the Cassini UVIS instrument in 1264 late 2000 allowed Steffl et al. (2006) to analyze the temporal variability of the main Io torus. The 1265 primary effect they found was a System III longitude ~25% modulation of S+ and S+++ emissions 1266 (anti-correlated) that is consistent with a longitudinal modulation of electron density ~5% and 1267 temperature ~10%. The primary ion species, S++ and O+ showed only few percent modulation. 1268 Steffl et al. (2008) modeled these emission modulations with a “cubic centimeter” physical 1269 chemistry model, taking the fraction of hot electrons around a baseline value of 0.23% of the 1270 total electron density and superposing a 25% variation in the with longitude, peaking at 290° 1271 longitude. This small change in the hot electron fraction is enough to alter the ionization state of 1272 sulfur ions. Hess et al. (2011a) showed that such a modulation of hot electrons could be explained 1273 by Alfvenic heating of electrons close to the jovian ionospheres and a longitudinal variation (due 1274 to the non-dipole components of Jupiter’s magnetic field) of the magnetic mirror ratio – that is, 1275 the ratio of the magnetic field in Jupiter’s ionosphere to the field strength at the magnetic 1276 equator. They showed that the VIPAL magnetic field model of Hess et al. (2011b) has a strong 1277 longitudinal dependence of the magnetic mirror ratio (averaged between north and south 1278 hemispheres) with a peak at 280° System III longitude. The Juno-based JRM09 magnetic field 1279 model of Connerney et al. (2018) shows an even larger peak at the same longitude. Such a peak 1280 in magnetic mirror ratio around 280° longitude means fewer hot electrons are scattered by waves 1281 into Jupiter’s ionosphere at such longitudes. Thus the geometry of Jupiter’s magnetic field is able 1282

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to explain the small enhancement of hot electrons that is able to modulate the abundances – and 1283 hence emissions – of S+ and S+++ ions. 1284

The second longitudinal modulation of the torus emissions – the System IV variation – is 1285 more complicated. Hints of variations on a timescale a few percent longer than System III came 1286 from radio emissions (Kaiser & Desch 1980, Kaiser et al. 1996) and ground-based emissions 1287 (Roesler et al. 1984). Sandel & Dessler (1988) noted that the Voyager UV emissions had a 1288 periodicity at 10.22 hours, a few percent longer than Jupiter’s 9.925 hour System III spin rate, 1289 which they labelled System IV. Brown (1995) found a similar (10.214 hour) periodicity in ground-1290 based S+ emissions, but noted that the phase of this System IV sometimes shifted (also noted by 1291 Reiner et al. 1993; Woodward et al. 1994, 1997). Steffl et al. (2006) found a periodicity in the 1292 Cassini UVIS data of 10.07 hours, just 1.5% longer than the System III. Steffl et al. (2008) were 1293 able to model the 45 days of emission modulation as a compositional wave propagating 1294 azimuthally around the torus, driven by a combination of two sinusoidal variations in hot electron 1295 fraction about an average value (0.235%): a 30% modulation with a peak at 290° System III 1296 longitude, plus a 43% modulation drifting 12.5°/day (System IV). The beating of these two 1297 modulations produced peak amplitude with a 29-day beat frequency. The Steffl et al. (2006, 1298 2008) analysis of the UVIS data provides an empirical description of the System III/IV modulations 1299 but does not explain them. Copper et al. (2016) adapted the Delamere et al. (2005) physical 1300 chemistry model of the torus allowing variation in two-dimensions: radial distance and azimuth. 1301 They simulated the UVIS variations in S+/S+++ abundance ratio by applying a System III variation 1302 in hot electron fraction (consistent with the Hess et al. (2011a) Alfvenic heating plus magnetic 1303 field asymmetry enhancing trapping of hot electrons at 280°) for all radial distances and then 1304 imposed subcorotation of 1 km/s (~12°/day) at 6 RJ, increasing to 4 km/s at 7 RJ and then 1305 decreasing to full corotation at 10 RJ (approximating the radial profile of subcorotation observed 1306 by Brown 1994). The model simulated the System III/IV beat structure found by Steffl et al. (2006) 1307 for the densest (and brightest) part of the torus (6-7 RJ) with the System IV effect fading out 1308 beyond ~7 RJ. This behavior provides compelling evidence that the physics of magnetosphere-1309 ionosphere coupling is key to the System IV variations. The short-term changes in the System IV 1310 period (on timescales of months to years) mean it is unlikely that this erratic modulation stems 1311 from changes in the intrinsic planetary magnetic field as originally suggested by Sandel & Dessler 1312 (1988), analogous to the changing solar magnetic field. Instead, variations in 1313 thermospheric/ionospheric properties and/or variations in plasma torus properties driven by 1314 iogenic input likely play an important role. 1315

Temporal variations in the torus emissions suggest Io’s volcanic eruptions can drive 1316 changes in the plasma torus. There are multiple observations of changes in the amount of neutral 1317 gas and plasma escaping from Io (Brown & Bouchez 1997; Bouchez et al. 2000; Mendillo et al. 1318 2004; Nozawa et al. 2004, 2005, 2006; Steffl et al. 2004b; Delamere et al. 2005; Yoneda et al. 1319 2010, 2015; de Kleer & de Pater 2016; Koga et al. 2018a,b,2019; Morgenthaler et al. 2019; 1320 Schmidt et al. 2019). But it is still difficult to predict the timing, duration, and spatial extent of 1321 the activity. The transport of plasma from the Io plasma torus has been estimated to take tens of 1322 days (Delamere & Bagenal 2003; Bagenal & Delamere 2011). To investigate the magnetosphere 1323 response to changes in the plasma supply rate from Io, it is essential to monitor both changes in 1324 the neutral cloud around Io as well as the plasma conditions in the torus. As Cassini approached 1325

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Jupiter, the Galileo dust instrument reported a 1000-fold increase in dust production, assumed 1326 to be caused by volcanic eruptions on Io (Kruger et al. 2003). The dust outburst began in July 1327 2000, peaked in September and settled back to normal by December 2000, overlapping with 1328 reports of activity from Io’s Tvashtar volcano. The Cassini UVIS instrument started observing UV 1329 emissions from the Io torus in October 2000 and Steffl et al. (2004b) reported high intensities 1330 that dropped to normal values by the end of the year. Delamere et al. (2004) matched the 1331 changing UV emissions from the torus reported by Steffl et al. (2004b) with a factor ~3 increase 1332 (significantly less than the dust increase) in source of iogenic atomic neutrals lasting about 3 1333 weeks, peaking at about August 2000. By January 2001, the torus seemed to have relaxed back 1334 to more typical conditions. Why the gas production in the torus was only a factor of ~3 while the 1335 dust production increased by three orders of magnitude remains a mystery. 1336

The JAXA Hisaki satellite has produced quasi-continuous EUV spectral images of planetary 1337 exospheres since its launch in 2013 (Yoshikawa et al. 2014). Emissions from the warm outer torus 1338 show similar features to those of Cassini UVIS: dawn-dusk asymmetry a System III longitude 1339 modulation by hot electrons, plus a strong modulation by the phase of Io (Yoshioka et al. 2014; 1340 Tsuchiya et al. 2015). In addition to continually monitoring the Io plasma torus emissions the 1341 Hisaki instrument has made a spectacular measurement of the neutral atomic oxygen cloud 1342 spreading all around Io’s orbit (Koga et al. 2018a,b), which is discussed further in section 4.1 1343 above. Conveniently, Io had a major volcanic event in mid-2015 (perhaps in part associated with 1344 the eruption of Kurdalagon Patera observed in the NIR by de Kleer & de Pater 2016, 2017) and 1345 Hisaki was able to measure the response in the emissions of different ion species in the Io plasma 1346 torus (Tsuchiya et al. 2015, 2019; Yoshikawa et al. 2017; Kimura et al. 2018; Yoshioka et al. 2018). 1347 In particular, Yoshikoka et al. (2018) compared the Hisaki torus emissions at the peak of the 1348 enhancement (DOY 50 2015) with quiet-time (late 2013) to which they applied a physical 1349 chemistry model to derive a factor 4.2 increase in neutral source, a ~2-4 times faster radial 1350 transport rate, and a minor increase in hot electron fraction (from 0.4% to 0.7%). Yoshioka et al. 1351 (2018) also found a reduction in the O/S ratio of the neutral source (from 2.4 to 1.7), consistent 1352 with the analyses of an active period in the Galileo-era by Nerney et al. (2017) and by Herbert et 1353 al. (2001). Copper et al. (2016) explored the effects of increased mass-loading on their 2-1354 dimensional model, comparing with the mid-2015 Hisaki event. They found they could model the 1355 observed general changes in plasma conditions (enhanced emissions, adjustments of 1356 composition) with an enhanced (x2.4) plasma source, increased hot electron fraction and 1357 decreased radial transport time (43 to 34 days). Tsuchiya et al. (2019) collected Hisaki 1358 measurements of S+++ and S+ emissions over three seasons (the first halves of 2014, 2015, 2016) 1359 to derive S+++/S+ emission ratio variations with System III and IV. They show a shortening of the 1360 System IV period after periods of volcanic activity, with an exception of an anomalous change in 1361 February 2014 that was associated with a change in Io’s volcanic activity. 1362

Suzuki et al. (2018) made a time-series analysis of the total EUV emission from the torus 1363 observed by Hisaki to explore the lifetime of brightenings that occur on top of System III and IV 1364 modulations. They applied the technique of Volwerk et al. (1997) who derived a radiative 1365 timescale for torus emissions of <5 hours from Voyager UVS data by comparing emissions 1366 between dawn and dusk as the plasma torus rotated around Jupiter every 10 hours. Suzuki et al. 1367 (2018) applied the Coulomb collision rate for hot electrons heating the thermal electron 1368

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population to derive the temperature of a fixed 0.4% fraction of hot electrons. Out of 42 1369 brightening events in 240 days of observation over December 2013 to May 2015, they found that 1370 the majority (83%) did not persist for 5 hours suggesting that the hot electrons were colder than 1371 150 eV. The remaining 7 events (17%) persisted for longer than 5 hours, consistent with supra-1372 thermal electron energies of 270-530 eV. 1373

The fourth variation of the Io plasma torus is an apparent radial shift and intensity 1374 modulation with local time. Ground-based observers measuring optical torus emissions 1375 (primarily from S+ ions) noticed that the peak of the plasma torus emission appeared shifted by 1376 about 0.2 RJ towards the dawn side of Jupiter (i.e. to the east in the night sky as observed from 1377 Earth) compared to the dusk side (i.e. west in the sky) (Morgan 1985a,b; Oliversen et al. 1991; 1378 Schneider & Trauger 1995). Observers measuring UV emissions noticed that the plasma torus 1379 emission intensity was usually stronger on the dusk side of Jupiter than the dawn side by as much 1380 as a factor of 2 (Sandel & Broadfoot 1982b). This dawn-dusk asymmetry was explained by plasma 1381 flow down the magnetotail imposing a dawn-dusk electric field across the magnetosphere (Ip & 1382 Goertz 1983; Barbosa & Kivelson 1983). As the torus plasma moves around Jupiter, the electric 1383 field causes the plasma to move a few percent closer to (farther from) Jupiter on the dusk (dawn) 1384 side. Just a small shift produces sufficient compression (enhancing density) and heating on the 1385 dusk side, to double the UV emission. The Voyager UVS instrument resolved the spatial structure 1386 of the inner edge of the warm torus, indicating that 80-85% of the emission came from the 1387 narrow (0.2 RJ wide) ribbon region that showed this strong dawn-dusk asymmetry (Sandel & 1388 Broadfoot 1982b; Volwerk et al. 1997). The Cassini UVIS measurements showed a similar 1389 dusk/dawn emission ratio (1.3 ± 0.25) but did not resolve the narrow ribbon (Steffl et al. 2004a,b). 1390 The 3 years of Hisaki emissions show a similar dusk/dawn emission ratio but also reveal a 1391 modulation by the phase of Io in its orbit around Jupiter (Tsuchiya et al. 2015, 2019; Murakami 1392 et al. 2016), suggesting that local heating of electrons in Io’s wake is involved in the local time 1393 modulation. 1394

Thus, the four types of variations in the warm plasma torus (6-8 RJ) are indications of the 1395 processes that modulate the flow of mass and energy through the system. The bulk (~90%) of 1396 the plasma that comes from Io fills the warm plasma torus, and spreads out into the vast 1397 magnetosphere of Jupiter. We consider the 10% diffusing inward and the narrow spatial region 1398 around Io’s orbit and in the next section. 1399

1400

5.3 – Inner Io Plasma Torus: Ribbon & Washer 1401

The inner boundary to the warm torus is a narrow (width~0.2 RJ), often bright region 1402 called the “ribbon”, the location of the peak emission varying with local time between 5.6 and 1403 5.9 RJ. This ribbon has a similar vertical extension as the warm torus, suggesting it may just be 1404 the inner boundary of the main torus (Figure 14, Table 5). Inward of the ribbon, the plasma is 1405 much colder (Te and Ti both dropping to <1 eV), forming a thin (H~0.2 RJ), washer-shaped disk 1406 between 5.6 and 4.7 RJ that is thought to be relatively stable over time. 1407

The thin, bright ribbon is most distinct in optical emissions of S+ ions, as illustrated in 1408 Figure 17. The brightness reflects a combination of high density (~3000 cm-3) and high abundance 1409

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of S+ ions. Farther out in the warm torus the presence of warmer electrons raises the sulfur 1410 ionization state with S++ ions becoming dominant. Both in situ measurements and high-resolution 1411 optical emission spectra show the electron and ion temperatures plummet inside ~5.7 RJ and the 1412 dominant S+ and O+ ions become tightly confined to the centrifugal equator. The greatly reduced 1413 temperatures inside ~6 RJ complicates observations of the inner torus. The colder temperatures 1414 cut off the UV emissions while the optical emissions from the tenuous inner regions are hard to 1415 observe along a line of sight through the dense ribbon region. 1416

Just as in the main, warm plasma torus, the ribbon is modulated by System III, System IV, 1417 local time and volcanic activity. But the tight radial confinement of the ribbon means that of 1418 particular importance is the dawn-dusk shift of the plasma by ~0.2-0.3 RJ due to the global electric 1419 field, moving the center of the ribbon from 5.84 ± 0.06 RJ on the dawn side to 5.56 ± 0.07 RJ on 1420 the dusk side (Morgan 1985a,b; Schneider & Trauger 1995; Herbert et al. 2008; Smyth et al. 2011; 1421 Schmidt et al. 2018). This shift is comparable to the width of the ribbon and moves this region of 1422 peak emission relative to Io’s orbit (5.90 ± 0.024 RJ where 1 RJ=71,492 km). The ribbon occupies 1423 a volume that overlaps the cloud of neutrals from Io, the source of the plasma torus. Thus, these 1424 spatial and temporal variations of the ribbon may be influencing – or influenced by – the plasma 1425 source. Furthermore, the ribbon is associated with a 5% dip in the azimuthal flow of the plasma 1426 relative to corotation (Brown 1994; Thomas et al. 2001). The dip is lowest at ~6-7 RJ (indicating 1427 the peak plasma source region), rising back up to corotation by Europa. Inside 5.4 RJ the plasma 1428 was sufficiently cold for the Voyager PLS instrument to resolve separate ion peaks and determine 1429 that the plasma flow in the cold torus is within 1% of corotation. 1430

Post-Voyager era the plasma properties inside Io’s orbit could be characterized (Bagenal 1431 1985) by three regimes: (1) the ribbon (6.0 – 5.6 RJ) of high density plasma population where the 1432 ion distributions have substantial non-Maxwellian tails implying the presence of pick-up ions, a 1433 bulk speed with ~5% lag behind corotation, electrons a factor ~5 times colder than the ions; (2) 1434 a “gap” or “dip”(5.6 -5.4 RJ) where there are sharp drops in both density and temperature 1435 (illustrated in panel (d) of Figure 17), shown by Schmidt et al. (2018) to vary with longitude; (3) 1436 the cold, inner torus (< 5.4 RJ) where the bulk flow is close to corotation, the ion and electron 1437 species are in close thermodynamic equilibrium with dwindling supra-thermal components 1438 (panel (b) of Figure 17). The difference in plasma conditions in these regions and the sharp 1439 boundaries suggest that the structure of the inner torus cannot be explained in terms of simple 1440 models of steady radial diffusion. 1441

Little progress was made in modeling the inner torus until Herbert et al. (2008) re-1442 analyzed Nick Schneider’s 1991 observations of S+ emission and derived a quantitative 1443 description of the ribbon plus cold inner torus, including variations with local time and System III 1444 (further quantified using Galileo data by Smyth et al. 2011). Herbert et al. (2008) reported the 1445 scale height of the ribbon (proportional to the S+ temperature parallel to the magnetic field, 1446 between 20-100 eV) varies over longitude and time. They also report a vertical offset from the 1447 centrifugal equator with longitude that is probably due to the fact that with the very low 1448 temperatures and small-scale height (<0.2 RJ) the location of the centrifugal equator is sensitive 1449 to high-order structure of magnetic field (as noted by Bagenal 1994). The high-order structure of 1450 Jupiter’s magnetic field derived from Juno’s close flybys of the planet (Connerney et al. 2018) 1451 may allow accurate modeling of such fine-scale features. The red line in Figure 17 shows latest 1452

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magnetic field model is close but does not yet match on the fine spatial scale of the cold inner 1453 torus. 1454

Richardson & Siscoe (1981, 1983) first tried to address the issue of the flow of mass and 1455 energy inside Io’s orbit with a diffusive transport model and concluded that (i) the steeper 1456 gradient in fluxtube content inside the ribbon meant that the plasma diffuses inwards ~50 times 1457 slower than outwards; (ii) additional sources of both plasma and heat were needed in the cold 1458 torus. These early studies were hampered by lack of information about the neutrals, limited 1459 physical chemistry data, as well as the fact that the first reports of ion temperature from Voyager 1460 PLS were over estimated by a factor of 2 (Bagenal et al. 1985). Subsequent models (Moreno et 1461 al. 1986; Barbosa & Moreno 1988; Herbert et al. 2001) still needed an additional local source of 1462 energy. 1463

One source of mass and energy in the inner torus could be molecular SO2 coming from 1464 the Io interaction either as neutrals or ions (see section 3.1). The molecular ion SO2+ has been 1465 observed directly (Figure 17b) by Voyager PLS in the cold torus (Bagenal et al. 1985; Dougherty 1466 et al. 2017). The molecular ion has not been detected in the warm torus, partly because it is likely 1467 quickly dissociated and/or recombined but also because the heavy, molecular ion species is likely 1468 swamped by the supra-thermal tail of the major atomic ion species in the PLS spectrum. Further 1469 evidence of molecular ion species came from magnetic signatures measured on multiple flybys 1470 of Io by Galileo. To quote Russell (2005): “The existence of ion cyclotron waves at the SO2+ and 1471 SO+ gyrofrequencies (Kivelson et al. 1996b; Russell & Kivelson 2000) was not so much a surprise 1472 in the Galileo data as were their amplitudes and spatial extent”. Analysis of such waves by 1473 Huddleston et al. (1997, 1998) and Blanco-Cano et al. (2001) showed these molecular species 1474 extend for many Io radii and correspond to a source of 8 x 1026 SO2+ ions/s. Models by Smyth & 1475 Marconi (1998) and by Cowee et al. (2003, 2005) showed that SO2+ could contribute to the source 1476 of plasma needed in the inner torus. Nevertheless, the lack of remote sensing of these molecular 1477 species means that the spatial distribution of such molecular species is poorly constrained. 1478

While the relatively small volume and mass of material residing inside Io’s orbit plays a 1479 limited role in the total magnetosphere, there are major unresolved issues about spatial and 1480 temporal variability of these fine-scaled structures that could tell us about the nature of the 1481 source of neutrals and plasma from Io. 1482

1483

5.4 Influence of Europa on Plasma 1484

Figure 15 shows the UV emissions from the plasma torus indicate rising electron 1485 temperatures and increasing amount of ions with higher ionization states as the plasma moves 1486 out from Io towards Europa’s orbit. But there does not seem to be an enhanced plasma density 1487 nor change in ion composition associated with Europa. This is perhaps not surprising given the 1488 relatively small (~25-70 kg/s) escape rate of neutrals (mostly H2) from Europa (see sections 3.2 1489 and 4.2). Delamere et al. (2005) applied such a source to their physical chemistry model to 1490 explore the impact of such a source around Europa’s orbit and found that very modest (few 1491 percent) changes in the ion abundances (increasing O+, decreasing Sn+). The most significant 1492 impact of a Europa source is the production of pick-up ions (650 eV for locally produced O+ ions) 1493

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that enhances the ion temperature. Unfortunately, it is not easy to differentiate between heating 1494 due to ion pick-up around Europa’s orbit with the general heating (via as-yet known process) that 1495 seems to be operating on the plasma as it moves outwards through the magnetosphere. 1496

The most significant impact of Europa on the magnetosphere of Jupiter is likely the effect 1497 of the neutral clouds on the inwardly diffusing energetic particle populations, as discussed in the 1498 next section. 1499

1500

5.5 Energetic Particle Fluxes 1501

While we focus in this paper on the thermal plasma populations (that dominate the 1502 density of charged particles), the supra-thermal particles (electrons, protons, heavy ions) play 1503 important roles as they interact with the moons, the neutral clouds and thermal plasma. Being 1504 light, electrons are relatively easy to accelerate to high energies via various wave-particle 1505 interactions (see review by Thorne 1983). Accelerating ions, particularly heavy O and S ions, is 1506 more of a challenge. To quote Mauk et al. (2004): “Heavy ions are thought to begin life as cool 1507 ions in the inner magnetosphere, diffuse outward to the middle magnetosphere, become 1508 energized there through nonadiabatic, invariant-violating processes, and then diffuse back into 1509 the inner magnetosphere, gaining additional energy through conservation of adiabatic invariants. 1510 The two important characteristics of Jupiter’s middle magnetosphere that promote particle 1511 energization are its fast rotational flows, and associated radial electric field, and its neutral sheet 1512 (i.e. current sheet) geometry.” 1513

Figure 18 shows profiles between 5 and 15 RJ of thermal plasma density (top panel, based 1514 on Voyager), fluxes of ~100 keV ions (H+, Sn+, On+) and electrons (middle panel), and then (bottom 1515 panel) compares fluxes of On+ ions at different energies (measured by the Galileo EPD 1516 instrument). The population of energetic electrons monotonically increases as they move 1517 inwards through the Io-Europa region (Jun et al. 2005; Nenon et al. 2017). Note that all ion 1518 species in Galileo EPD data show a sharp drop as they move inwards past Europa’s orbit (Lagg et 1519 al. 1998, 2003; Kollmann et al. 2016; Nenon et al. 2018; Nenon & Andre 2019) . This structure is 1520 primarily due to charge-exchange reactions of the inward-moving energetic ions with the neutral 1521 clouds of Europa and Io (illustrated in Figure 13 from the modeling of Smith et al. 2019). Energetic 1522 particles are particularly sensitive to neutral gas clouds because their bounce motion carries 1523 them frequently through the cloud, while lower energy ions pass and interact less often. Inside 1524 Europa’s orbit oxygen ions are preferentially removed because (a) they are primarily singly-1525 charged so that a CHEX reaction produces an ENA that is lost from the system, and (b) there is 1526 more neutral oxygen (Figure 13) for resonant CHEX reactions. Mauk et al. (2004) suggest that the 1527 H+ profile flattens around 7 RJ due to local wave heating of protons, but eventually charge 1528 exchange with Io the Io neutral cloud causes the protons to plummet towards 6 RJ. The 1529 prevalence of higher ionization states of sulfur means that most incoming energetic S ions need 1530 to undergo more than one CHEX reactions before being lost as ENAs (Clark et al. 2016; Allen et 1531 al. 2019). 1532

The bottom panel of Figure 18 demonstrates the energy dependence of ion loss via charge 1533 exchange. Energetic oxygen is efficiently lost via charge exchange with neutral atomic oxygen 1534

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clouds at energies near or below 100 keV. The charge exchange cross section drops steeply above 1535 100 keV. At 100 keV the fluxes drop inside of Europa’s orbit, while 1.5 MeV oxygen ions are 1536 transported farther inward before being lost closer to Io. 1537

Lagg et al. (1998) used ion fluxes to measure Io plasma torus densities from energetic 1538 particle pitch-angle distributions while the factor 100 drop in ion intensities at Io is from 1539 scattering by local electromagnetic ion cyclotron waves, perhaps excited by local ion pick-up 1540 (Nenon et al. 2018). Mauk et al. (2004) points out that the fluxes of energetic ion fluxes between 1541 Europa and Io measured by Galileo (specifically 1995-1999) were lower than measured by 1542 Voyager in 1979 and suggests that this may be consistent with enhanced neutral cloud densities 1543 for the Galileo period observed by Wilson et al. (2002) causing enhanced energetic particle losses 1544 via charge-exchange. 1545 1546 6. Jovian Neutral Nebulae 1547

Since the early (pre-Voyager) detection of the sodium cloud near Io, there has been the 1548 realization that charge exchange of Io torus ions could generate a wind of neutrals extending out 1549 into the magnetosphere and might be a source of plasma (Eviatar et al. 1976). The Voyager 1550 measurements of energetic sulfur and oxygen ions in the middle magnetosphere fed the idea 1551 that re-ionization of such iogenic neutrals could produce pick-up ions at 10s of keV energies, 1552 gaining further energy as they diffused inwards (Cheng 1980; Eviatar & Barbosa 1984; Barbosa & 1553 Eviatar 1986). It was also realized that such a wind of neutrals might supply heavy ions upstream 1554 of Jupiter (Krimigis et al. 1980; Kirsh et al. 1981; Baker et al. 1984). As it skims over Jupiter’s 1555 ionosphere, the Juno spacecraft is now showing heavy ions at both low (Valek et al. 2019a,b) and 1556 high (100s keV in Kollmann et al. 2017) energies. Perhaps these ions come from sprays of Io 1557 and/or Europa neutrals impacting the jovian atmosphere. 1558

Below we briefly summarize the limited knowledge we have about these neutral nebulae 1559 and the roles they may play. 1560

1561

6.1 Mendillosphere – Io FNAs – Fast Neutral Atoms 1562

By the late 80s telescopic capabilities reached a level of sensitivity to detect sodium 1563 emission far from Io. Mendillo et al. (1990) reported emission extending >400 RJ from Jupiter in 1564 a disk (Figure 12). They proposed that sodium ions in the Io plasma torus charge exchange with 1565 neutrals around Io’s orbit. This means that when neutralized via charge-exchange the atoms 1566 spray mostly outwards in a disk of neutral atoms with energies of ~400 eV. To honor the 1567 discoverer, this is sometimes called the “Mendillosphere” or “Mendillodisk”. Flynn et al. (1994) 1568 modeled two-years of observations of the extended sodium nebula and found that the flaring 1569 angle of the disk (20-27°) anti-correlated with the source production rate (as indicated by Io’s 1570 surface IR output or brightness of near-Io emissions). Studies of Io’s extended neutral clouds 1571 showed variations with local time and Io’s orbital phase (Mendillo et al. 1992, 2004; Flynn et al. 1572 1994; Yoneda et al. 2015) and a source of 1-4 x 1026 atoms per second. Mendillo et al. (2004; 1573 2007) show a variation in brightness and shape with Io’s volcanic activity – the disk having angular 1574 edges (like an anulus) when Io is particularly active compared with an oblate spheroid during 1575 quiet times. The suggested explanation is that under quiet times the sodium FNAs are produced 1576

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(from neutralized Na+ in the torus) as a stream (Figure 12). When Io is more active, production 1577 (e.g. via dissociative recombination of molecular sodium ions such as NaCl+) as jets local to Io 1578 increases (Mendillo et al. 2007). 1579

Similar extended disks of escaping S and O atoms, as well as SO2 and SO molecules also 1580 probably exist. Physical chemistry models of the torus (e.g. Delamere et al. 2005) predict charge 1581 exchange processes will produce a flux of escaping neutralized atoms of 1-5 x 1028 atoms per 1582 second, mostly oxygen (Figure 16a). While this is ~100 times greater than the sodium flux, the 1583 radiation efficiency of other species is so much weaker that any neutral disk would be very hard 1584 to detect. Direct in situ detection of these escaping neutrals would require an instrument that 1585 measures Fast Neutral Atoms (FNAs) with energies less than ~300 eV (e.g., <20 eV/nucleon for 1586 oxygen atoms moving at 70 km/s). 1587

While corotating torus ions that are neutralized via charge exchange with Io’s neutral 1588 clouds will come off as an outward stream, charge exchange close to Io could come off as a jet or 1589 spray (Figure 12). For example, a recently-picked-up ion produced in the plasma-atmosphere 1590 interaction that charge exchange could be directed towards Jupiter where re-ionization on 1591 impacting the planet’s atmosphere would be a source of cold, ionospheric heavy ions near the 1592 planet (Valek et al. 2019b). 1593 1594

6.2 Neutral Nebula Sphere – Europa ENAs 1595

On route to Saturn, Cassini flew past Jupiter for a gravity assist in late 2000. Cassini carried 1596 an instrument that detected ENAs, initially assumed to be hydrogen atoms, in the range of 50-80 1597 keV emanating from what appeared to be Europa’s orbit (Krimigis et al. 2002). Note the 1598 difference between these (very energetic) 10s keV/nucleon atoms from the iogenic fast neutral 1599 atoms (FNAs) with ~200 eV – few keV. Mauk et al. (2003, 2004) modeled these ENAs as products 1600 of charge exchange reactions of inward-moving energetic (10s keV) protons with neutrals in a 1601 cloud surrounding Europa’s orbit (Figure 19). The observed flux of ENAs (1025 s-1) requires a local 1602 density of 20 to 50 cm-3 of neutrals in the Europa neutral cloud (see Section 4.2 above). Mauk et 1603 al. (2004) assumed these neutrals were atomic hydrogen (based on the model of Schreier et al. 1604 1998) but the updated Smith et al. (2019) model suggests the primary neutral species is molecular 1605 H2 (Figure 13). 1606

Note that the ions charge exchanging with the Europa neutral cloud are sufficiently 1607 energetic that their bulk motion is much less than their thermal or gyro motion (Vg >> Vco). Thus, 1608 when they become neutralized, the Europa ENAs spray pretty much equally in all directions, 1609 forming a spherical cloud (Figure 20), unlike the iogenic FNAs, which form a disk (“Mendillodisk”). 1610 1611

6.3 Re-ionization Upstream of Jupiter 1612

Krimigis et al. (2002) showed a plot (Figure 20) of ions measured by the Cassini CHEMS 1613 instrument in the solar wind upstream of Jupiter indicating the presence of energetic (55-220 1614 keV) oxygen and sulfur ions (as well as possible Na+ and SO2+ ions), which they ascribe to photo-1615 ionization of the escaping ENAs. The Cassini measurements of heavy ions upstream of Jupiter add 1616 to measurements by Voyager (Krimigis et al. 1980; Kirsh et al. 1981; Baker et al. 1981) and Ulysses 1617 (Haggerty et al. 1999), but it is not clear at this point in time how much these ions leak out of the 1618

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magnetosphere as energetic ions (Mauk et al. 2019) vs. produced locally via re-ionization of 1619 escaping neutral atoms, whether Io-FNAs or Europa-ENAs. 1620

1621

7. Oustanding Issues 1622

The peculiar role of Io in the magnetosphere of Jupiter was first noticed in 1964. A half 1623 century and a thousand or so papers later it is a good time to consider our understanding of the 1624 system and what are the key open questions. First, we consider the individual components of the 1625 system and then address coupling between them. 1626

Atmospheres. The tenuous atmospheres of Io and Europa are hard to study remotely and 1627 close-up observations are often selective in viewpoint, coverage, and gas components. From a 1628 planetary atmosphere perspective the main focus is often properties near the surface. From a 1629 space physics perspective, we are more interested in the outer regions. Specific open questions 1630 are: 1631

• What is the radial, longitudinal distribution and composition of the neutral atmospheres 1632 of Io and Europa from the surface to high altitude in daylight, in eclipse and at night? 1633

• How do the atmospheres, particularly the upper regions, vary with volcanic activity of the 1634 moon? How does the location of any active sites alter the plasma-atmosphere 1635 interaction? 1636

• Why is Io’s atmosphere sometimes very stable for many years then suddenly seems to 1637 supply large amount of neutrals, dust to the Io environment? 1638

• How much of the ionospheric density (e.g. as observed via radio occultations) is produced 1639 via photoionization vs. plasma impact on the atmosphere, and/or material convected 1640 through the interaction? Why do the ionospheres of Io and Europa appear so variable? 1641

• Where is the exobase? A specific height may be hard to define for the complex 1642 interactions at Io and Europa. Moreover, however defined, the exobase will likely vary 1643 with longitude on the moon, plasma flow direction, perhaps with solar illumination, and 1644 probably with magnetic latitude of the moon. 1645 1646 Plasma-Atmosphere Interactions. The aurora produced via electron bombardment of the 1647

atmosphere can provide useful information about the interaction of the surrounding plasma with 1648 a moon’s atmosphere. But to explore the full physics we need a combination of in situ 1649 measurements along close flybys and detailed modelling. Specific open questions are: 1650

• How much of the plasma interaction goes into heating the atmosphere? Where? What is 1651 the impact on the atmospheric distribution? 1652

• What are the roles of electron impact ionization, charge exchange, collisions, and electron 1653 beams in the plasma-atmosphere interaction? What are the net ionized products of these 1654 reactions that escape into the space beyond? 1655

• What are the composition, velocity distribution, and fluxes of the neutrals that escape the 1656 moon’s gravity and into the space beyond? 1657

• How do the plasma interactions vary around the moon (e.g. upstream vs. downstream, 1658 sub/anti-Jupiter, day/night), with magnetic latitude and longitude and with local time / 1659 phase of the moon along its orbit? 1660

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• What is the strength of the induction signal at Io? How much from the asthenosphere 1661 and/or the core? Is the induction signal at Europa purely from the conducting ocean? 1662

• How much of the iogenic material reaches the surface of Europa? 1663 • How do the plasma interactions vary (qualitatively and quantitatively) with volcanic 1664

activity of the moon? What kind and/or location of the volcanos affect the interaction? 1665 • How does the seasonal changes in distance from the Sun (that drives sublimation and 1666

photoionization rates) modify the plasma-atmosphere-ionosphere interaction? 1667 1668 Neutral Clouds. While alkali elements are clearly observed around both Io and Europa, 1669

the major species are poorly measured, except Io’s oxygen cloud that is now being mapped out 1670 by the Hisaki mission. Specific open questions are: 1671

• What are the amounts and trajectories of different neutral species that escape Io and 1672 Europa? In particular, are there molecular clouds of SO2, SO, O2, H2? What roles do they 1673 play? Is there a way to detect them? Are there any features in other atomic and molecular 1674 neutral clouds similar to the Na neutral structures such as jets, streamers and the 1675 extended Mendillosphere? 1676

• How are these neutral clouds shaped by the plasma that flows through them? At Io, how 1677 much of the neutral clouds reach inwards of Io’s orbit (as molecules and/or atoms) to 1678 provide an extended source for the ribbon and inner torus? At Europa, how do the neutral 1679 clouds change under the variable conditions in the outer torus? 1680

• What role do the neutral clouds play in controlling the influx of energetic particles 1681 (electrons, protons and heavy ions) from the middle magnetosphere through to the 1682 radiation belts within a few RJ of Jupiter? 1683 1684 Plasma Torus and Sheet. The general structure and systematic modulations (System III, 1685

IV, local time) of the three components of the torus – cold inner torus, ribbon, warm outer torus 1686 and plasma sheet – are fairly well described but underlying physical processes remain unclear. 1687 Specific open questions are: 1688

• What is (are?) the source(s?) of hot electrons? Do waves within torus accelerate local 1689 electrons? Are hot electrons injected from outside? 1690

• How is plasma heated as it moves out from the torus to the plasma sheet? How do the 1691 supra-thermal populations (both ions and electrons) evolve? 1692

• What is the specific nature of Io eruptions that cause enhanced sources of plasma? What 1693 are the physical process whereby changes in sources affect the System III/IV modulations, 1694 radial transport rate, dawn/asymmetries, etc? 1695

• How much plasma does Europa contribute to the magnetosphere? 1696 • What is the spatial distribution of the production of plasma in the three regions of the 1697

torus? Where is the separation between outward and inward transport? What is the 1698 nature and role of the torus ribbon region? 1699

• What processes drive and control the radial transport rate? How do these processes vary 1700 with radial distance and (if at all) with plasma production rate? 1701 1702

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Extended Neutral Nebulae. The only nebula that has been detected directly is that of 1703 sodium from Io. But it is clear that charge-exchange reactions must be generating clouds of Fast 1704 Neutral Atoms, Energetic Neutral Atoms – and perhaps even molecules. Specific open questions 1705 are: 1706

• What are the fluxes of different neutral species (composition, energy, direction) out of 1707 the jovian system? Are these neutrals re-ionized? Where? And what is their fate? 1708

• What are the fluxes of different neutral species (composition, energy, direction) inwards 1709 towards Jupiter and impacting the planet’s atmosphere? Are these neutrals re-ionized? 1710 Where? And what is their fate? 1711 1712 These components of the Io-Europa space environment are clearly highly coupled. While 1713

there are modulations and temporal variations, such changes seem to be limited to factors of a 1714 few (rather than wild swings of orders of magnitude). Thus, there must be both negative and 1715 positive feedback systems. Full understanding of this complex system will require both 1716 comprehensive observations as well as systematic modeling – of each component separately as 1717 well as carefully coupled – to explore what factors control (qualitatively and quantitatively) the 1718 different components. The beauty of the Io-Europa system, however, is that the timescales for 1719 the different processes make the components separable: plasma takes ~minute to pass each 1720 moon, neutrals survive ~hours orbiting Jupiter, plasma moves through the magnetosphere over 1721 many days. This separation of time scales allows us to study each component independently and 1722 then couple them together. 1723 1724 7.2 Future Observations 1725

The Juno mission is currently about 2/3 through the primary mission, mapping out the 1726 polar regions as well as outer to middle magnetosphere (for review of magnetospheric science 1727 see Bagenal et al. 2017b; for updated orbits see Bolton et al. 2017). As the line of apsides of the 1728 orbit precesses southward, the spacecraft crosses the jovigraphic equator closer to the planet. 1729 By the end of the primary mission (34 orbits, spring 2021) Juno reaches between the orbits of 1730 Ganymede and Europa. Moreover, the tilted magnetic field allows the spacecraft to cross 1731 magnetic fluxshells intersecting Europa’s and Io’s orbits and sample the torus. Should the Juno 1732 mission be continued beyond orbit 34, the spacecraft will directly pass through the Io plasma 1733 torus as many as 15 times (between November 2022 and November 2024), as well as likely 1734 intersect the Alfven wing that stretches downstream of Io (Figure 5). 1735

While Juno in situ measurements of particles and fields are very valuable, it is key that the 1736 torus emissions are also monitored from the ground (Schmidt et al. 2018; Morgenthaler et al. 1737 2019) and from the Hisaki satellite (Yoshikawa et al. 2014). Ground-based telescopes seem to be 1738 ever expanding in wavelength and aperture. Io’s volcanism is monitored in the IR but perhaps 1739 the extension into the millimeter range of telescope systems such as Atacama Large Millimeter 1740 Array are allowing detection of the molecular species (SO2, SO, O2, H2). In the meantime, we urge 1741 the community to support semi-continuous monitoring the more observable species (Na, S+, O, 1742 O+…?) with modest-sized telescopes. 1743

Looking farther to the future, the development of ESA’s JUICE and NASA’s Europa Clipper 1744 missions promise close exploration of the Europa system with perhaps some forays closer to Io. 1745

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But to properly address the key scientific questions of Io’s peculiar role in the jovian system we 1746 need a mission that makes multiple close flybys of Io. 1747

1748

Acknowledgements 1749

We are grateful to our colleagues who have assisted us in pulling this review together and gave 1750 us feedback on earlier drafts: Tim Cassidy, Nick Thomas, Carl Schmidt, Joachim Saur, Kazuo 1751 Yoshioka, Peter Kollman, Barry Mauk. We thank Crusher Bartlett for support with graphics. 1752

1

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Figure 1: The Io-Europa Space Environment. The system comprises at least 10 components that span from the moons in to Jupiter and out into the interplanetary medium. Note that Io and Europa orbit Jupiter at 5.9 and 9.4 RJ where 1 RJ=radius of Jupiter = 71,494 km.(Credit: top – John Spencer, SwRI; bottom – Steve Bartlett)

Figure 2: Io and Europa Bulk Properties.

Io & Europa: Bulk & surface properties

Io Europa

Orbit Period 42.5 hours 85.2 hours

Semi-major axis 5.90 ± 0.024 RJ 9.38 ± 0.085 RJ

Radius 1822 km 1561 km

Mass 8.93 x 1022 kg 4.80 x 1022 kg

Density 3528 kg m-3 3013 kg m-3

Escape speed 2.6 km s-1 2.0 km s-1

Core : Rock : Water (% volume)

6 : 94 : 0 3 : 74 : 23

Albedo 0.62 0.64

Surface Temperature 110 K 110 K

Silicate lava. SO2 frost, concentrated at equator & longitudes with plumes.

Water Ice. Sulphates, hydrates, preferentially trailing/upstream side.

>425 volcanic basins, ~250 active volcanoes, 50-100 erupting

Magnetic perturbations show liquid ocean under the ice

Global resurfacing ~1 cm / year. Craters removed in ~1 million yearsTurns inside out 40 times in 4.5 Ga

Geophysical evidence ice 5-30 km. Geological features suggests thinner brittle layer over ductile ice.

Where/how are tides dissipated to drive volcanism?

How much contact between ocean and surface?

Figure 3: Io and Europa Surface Properties

Europa

Figure 4: Io and Europa Atmospheric Properties. a: Eruption of Tvashtar Patera imaged by

New Horizons' LORRI imager 2007; b: HST image Feaga et al. (2009); c,d: daytime column

density of SO2 Feaga et al. (2009); e: Galileo visible image of aurora and volcanic emissions

Geissler et al. (2001); f: scale of atmosphere; g, h, i, j: HST images Roth et al. (2016); k:

Cartoon of plume (NASA/ESA/SwRI); l: scale of atmosphere.

a b

c d

e f

g

i

k

h

j

l

Io EuropaOxygen Hydrogen

Atmospheric Property

Io Europa

Source (1) • SO2 sublimation in daylight hemisphere.

• Volcanoes at night?• Surface sputtering

negligible

• Sputtering by thermal, high energy ions (2)

• Pick-up ions? (3)• Plumes? (4)• Sub-solar source? (5)

Asymmetry(1, 6)

• Day/night• Upstream/downstream• Pole/equator<35 deg• Jovian/anti-jovian

• Upstream/downstream• Jovian/anti-jovian?• Day/night?

Composition (7, 6) SO2, O, S, SONaCl, KCl O2, O, H, H2

Surface pressure (8, 6) ~ 2 nbar SO2 ~ 200 pbar O2

Column density (9, 10) [15-150] x 1015 cm-3 [0.2-2] x 1015 cm-2

Ionosphere peak density (11, 6) 28 x 104 cm-3 at 80 km 1.3 x 104 cm-3 at 50 km

Time variability • With distance from Sun (12)• With volcanic activity? (13)

• Plume activity? (4)• Dawn-Dusk (14)

Table 1: Io and Europa atmospheric properties. (1) Lellouch et al. 2007; (2) Cassidy

et al. 2013; (3) Ip 1996; Saur et al. 1998; (4) Roth et al. 2014a,b; (5) Oza et al.

2018; Plainaki et al. 2013; (6) McGrath et al. 2009; (7) Moullet et al. 2010, 2013; (8)

McGrath et al. 2004;(9) Spencer et al. 2005; (10) Hall et al. 1998; Hansen et al.

2005; Roth et al. 2016; (11) Hinson et al. 1998; (12)Tsang et al. 2012, 2013b; (13) de Kleer et al. 2019a,b; (14) Oza et al. 2018, 2019.

Figure 5: Plasma-Io Electrodynamic Interaction. a, b: schematic of interaction between the surrounding plasma and Io (from Schneider & Bagenal 2007). c: Hubble Space Telescope image of Jupiter's UV aurora (Clarke et al. 2004) showing auroral foot prints of Io, Europa and Ganymede. d: Pattern of Alfven waves generated by Io, bounding between hemispheres of Jupiter where bursts of radio emission are generated above the ionosphere (Gurnett & Goertz 1981).

a b

c d

Io EuropaGanymede

Property Upstream of Satellite

Io Europa

Range from magnetic equatorRange from centrifugal equator

±1 RJ±0.65 RJ

±1.5 RJ±1 RJ

Local jovian magnetic field 1720-2080 nT 420-480 nTPlasma-moon relative velocity 53-57 km/s 65-110 km/sElectron density 1200-3800 cm-3 65-290 cm-3

Alfven Mach number plasma sheet center 0.3 0.4

Thermal electron temperature 5 eV 10-30 eVHot electrons 0.2% at 40 eV 2-10% at 250 eV

Major ions S++ (20%) O+ (25%) S++ (20%) O+ (20%)

Average Ion temperature 20-90 eV 50-500 eVAverage ion gyroradius ~3 km ~10 km

Table 2: Plasma conditions upstream of Io and Europa (Kivelson et al. 2004; Bagenal et al. 2015)

Figure 6: Sketch of the concept of the multi-species chemistry approach of the local interactionat Io. A parcel of plasma of prescribed composition and energy is carried by the prescribedplasma flow into the atmosphere of Io. The ion-electron-neutral processes change thecomposition and energy of the plasma in the parcel, which are then collected along a specificGLL flyby of Io (here the J0 flyby in red) and compared to the observations. Such simulations aimat constraining the atmospheric distribution and composition of Io’s atmosphere. Dols et al.(2008, 2012, 2016).

Figure 7 Plasma-atmosphere interaction at Io. Plasma properties at Io from a multi-species chemistry approach of the atmosphere-plasma interaction sketched in Figure 6. The plasma flow is prescribed as an incompressible flow around a conducting ionosphere, which is assumed to extend to 1.26 RIo (see text). From upper left in clockwise direction: the prescribed SO2 neutral density of the corona; the plasma flow slowed upstream and downstream and accelerated on the flanks; the resulting average ion temperature, which increases where SO2

+ ions are picked up by a fast flow; the electron density resulting from the torus thermal electron ionization; the electron temperature, which decreases in the molecular atmosphere because of the efficient molecular cooling processes; and the resulting SO2

+ density. These simulations do not include the ionization caused by the parallel electron beams detected in the wake of Io and nor do they include a day-night asymmetry of the SO2 atmosphere.

SO2 V SO2+

ne Te Ti

Io

Figure 8: Net Production at Io. Estimate of the contribution of each plasma-neutral process above an exobase (at 150km altitude) using the multi-species modeling approach described in Figure 6. These calculations use a prescribed MHD flow different from the flow shown in Figure 7. Although these rates depend on the flow and the atmosphere prescribed, they provide a reasonable estimate of the relative contribution of each process. The most important loss processes are the SO2 electron-impact dissociation, which provide slow atomic neutrals and a cascade of SO2 resonant charge-exchanges, which provide faster neutrals depending on the location of these charge exchanges.

Io

Figure 9 Plasma-atmosphere interaction at Europa. Plasma properties at Europa from a multi-species chemistry approach of the atmosphere-plasma interaction sketched in Figure 6. The plasma properties can be compared to Io’s in Figure 7. The flow is similar while O2 is the main atmospheric component and O2+ the main ion. As Europa’s atmosphere is more tenuous than Io’s, the interaction is weaker and the plasma properties do not vary as strongly as at Io.

O2 V O2+

ne Te Ti

Europa

Figure 10: Net Reactions at Europa. Estimate of the contribution of each plasma-neutral O2 process

above an exobase at 70 km altitude. The plasma-neutral processes on H2 are not a significant loss of H2

and are not shown here. Although the rates vary with the prescribed flow and neutral distribution, they

are much lower than the rates at Io shown in Figure 8. Europa does not provide much plasma to the

magnetosphere of Jupiter.

Europa

Figure 11: Densities of the plasma downstream of the interactions with (above) Io and (below) Europa (Dols et al. 2008, 2012, 2016).

Plasma-Atmosphere

Io Europa

Galileo flybys J0, I24, I27, I31, I32 E4, E6, E11, E12, E14, E15, E16, E17, E19, E26

Flow diversion 95 % (1) 60 - 80 % (23, 2) Magnetic perturbation 700 nT (3) 80 nT (4, 23)Main ion-neutral process Collision and chex (1, 5) Collisions and chex (2, 6)

Ionization • Thermal electron impact (3, 5)• Photoionization ~ 15% (5)• Electron beams (3, 7)

• Thermal and hot electron impact (2, 6)

• Photoionization ~ 10 %? (2)Main ion produced SO2

+ ~200 kg/s (1, 5, 8) O2+ ~20 kg/s (2, 6)

WakeNe~30,000 cm-3

slow flow (~ 1 km/s) cold ions (~ 1 eV) (8, 9)

No clear plasma wake at Galileo altitude? (10)

Auroral emissions Mainly O (11) also S (12), Na, K (13)

Mainly O also H Lyman-alpha (14)

Aurora location

• Equatorial flanks (15)• polar emission stronger on side of

plasmasheet (11)• Corona (11)

• Polar emissions stronger on side of plasma sheet (14)

• No flank emissions• Corona (16)

Electron beams location In wake J0, above poles (I31, I32) (17) No beams detected by GalileoElectron beams energy • >150 eV Powerlaw distribution (18)

Footprint emissions Multi-spot structure + long tail (19)

Double-spot structure + short tail (20)

Induced mag field in subsurface Yes (21) No (22, 23) Yes (24)

Table 3 – Plasma-atmosphere interactions. (1) Saur et al. 1999; (2) Saur et al. 1998 calculated for a small upstream density ~ 40 cm-3; (3) Kivelson et al. 1996a; (4) Kivelson et al. 1997; (5) Dols et al. 2008; (6) Dols et al. 2016; (7) Saur et al. 2002; (8) Bagenal 1997; (9) Frank et al. 1996; (10) Kurth et al. 2001; (11) Retherford et al. 2003, 2007; (12) Wolven et al. 2001; Ballester et al. 1987; (13) Bouchez et al. 2000; Geissler et al. 2004; (14) Roth et al. 2016; (15) Roth et al. 2011; (16) Hansen et al. 2005; (17) Williams et al. 1996, 2003; Frank et al. 1999; (18) Mauk et al. 2001; (19) Gerard et al. 2006; Bonfond et al. 2008; (20) Bonfond et al. 2017; Grodent et al. 2006; (21) Khurana et al. 2011;(22) Roth et al. 2017b; (23) Blocker et al. 2018; (24) Zimmer et al. 2000; Schilling et al. 2004.

Figure 12: Neutral Clouds of Io (based on Wilson et al. 2002). Left - Io's sodium cloud on three spatial scales, as imaged by ground-based observations of sodium D-line emission (the bottom image is from Burger et al. 1999). The features observed on the left are explained by the three atmospheric escape processes shown schematically on the right.

Figure 13: Io (left) and Europa (right) neutral clouds as modeled by Smith et al. (2019). The color contours show local densities (cm−3) in the XY plane (−Y axis points toward the Sun). Bottom Left is a plotted the vertically-integrated column density along the Y=0 axis in the equatorial plane.

Sources & Losses

Io Europa

Local Loss processes(1, 2)

• Exospheric collisions, chex• Atmospheric sputtering• SO2 electron impact

dissociation• Direct volcanic injection

negligible• Surface sputtering negligible

• Jeans escape?• Exospheric collision• O2 electron impact

dissociation • Direct plume injection

negligible

Net Neutral loss (1, 2) 250 - 3000 kg/s 25 - 70 kg/sNeutral clouds (3, 4)DetectedExpected

S, O, Na, K, SO2 , SO ?

H, O, Na, K, H2 , O2 ?

Sodium sources (5) 10 - 30 kg/s ??Local plasma source (6, 7) ~200 kg/s ~20 kg/s

Extended plasma source (11) 700- 2000 kg/s ~60 kg/s

Table 4 – Neutral and plasma sources at Io and Europa. (1) section 3.1 Figures 7, 8; (2) section 3.2 Figures 9, 10; (3) Thomas et al. 2004; (4) Smyth & Marconi 2005; Smith et al. 2019; (5) Thomas et al. 2004; Wilson et al. 2002; (6) Bagenal 1997; Saur et al. 2003; Dols et al. 2008; (7) Saur et al. 1998; Dols et al. 2016; (11) Delamere et al. 2004; Nerney et al. 2019.

Warm Torus

RibbonS+ at 6731Å

S++ at 685Å

UV emission

Cold Disk Figure 14: Io Plasma Torus

a

b

c

d

Three regions of the Io plasma torus: Cold disk, ribbon, warm torus. (a) This computed image shows optical S+ emission (b) EUV S++ emission. Note that S+

dominates the cold torus and S++ dominates the warm torus. The ribbon is a tall, narrow ring which appears bright at the torus ansa because of projection effects. The ribbon is typically the most prominent of the three regions for S+, while in S++

emission the ribbon is a slight brightening at the inner edge of the warm torus (Schneider & Bagenal 2007). (c) net emission across the EUV spectrum as observed by Cassini UVIS (Steffl et al. 2004a). The structure of the torus can exhibit strong longitudinal variations, and the relative brightnesses of different regions can vary with time.

Cold Disk Ribbon Warm Torus Europa Orbit

Location (RJ) 4.7 - 5.7 Height 0.2

5.6 - 5.9 ?Width 0.2

6 - 8 9.4

Mass 35 kton1%

200 kton10%

2 Mton90%

~72 kton/RJ

Radial Transport (/RJ)

Inward:months

?? Outward: 20 - 60 days

~20 hrs

Ne (cm-3) ~1000 ~3000 ~2000 ~160

Te (eV) <2 ~5 ~5 ~10 - 30

Hot Electrons ~0 <0.1% 0.2% @ 40eV 2-10% @ 250eV

Ti (eV) <2 20 - 40 20 - 90 50 - 500

Ions S+, O+ O+, S+, S++ O+, S++, S+

S+++, O++O+, S++, O++

S+++ , S+

Table 5 – Plasma Torus Properties (see section 5).

Figure 15 – Models of the Io plasma torus. Top: two-dimensional model of the electron density based on Voyager in situ measurements and distribution along field lines under diffusive equilibrium. Bottom: Cassini UVIS observations and physical chemistry models constrain torus composition (Nerney et al. 2017).

Figure 16: Flow of (a) particles and (b) energy through the Io Plasma Torus,

applying the physical chemistry model of Delamere & Bagenal 2003 for typical

torus conditions. High charge-exchange rates (McGrath & Johnson 1989) cause

about half of the material (primarily O) to be lost from the system as fast

neutrals, while the remainder is transported out as plasma. Most of the energy

is radiated away via UV emissions. Input of energy from hot electrons is key for

maintaining the torus.

S S+ S++ S+++

O O+ O++

4.30.36 4.8

18.60.979.2

5.7

9.119

3.4

17

6.12.2

101.48 2.7

0.00710.60

20420.58

8.81.454

18.80.128.1

1.390.00050.22

RecombinationIonization (thermal e-)Ionization (hot e-)

Charge ExchangeRadial Transport

5.1

100%=6.4 (10-4 cm-3 s-1)

Particle Flow for D&B 2004 Inputs for Cassini 01/14/2001

The electron density is determined at each iteration by the quasi-neutrality condition (assuming nH+/ne = 0.1): ne=( nS+ + 2nS++ + 3nS+++ + nO+ + 2nO++ )/0.9

4.9

1.15

1.15

Particle Budget

S S+ S++ S+++

O O+ O++

Hot e-

Core e-

0.450.038

0.51

10

0.54

5.1

3.2

0.61

1.33

8.20.97

2

1.06

0.30.121

0.630.093

1.97

0.148

0.00039

0.033

1.463.1

0.043

2.4

0.39

15

0.32

63

0.470.0750.65

0.0065

0.43

0.0740.00002670.0118

0.49

0.82

9.4

1.6813 4

0.332

0.19

0.63

1.3

0.038

1.78

Recombination

Ionization (thermal e-)

Ionization (hot e-)

Coulomb Collisions

Charge ExchangeRadial Transport

Radiation

Ionization Potential91.4

0.32

Energy Flow for D&B 2004 Inputs for Cassini 01/14/2001

100%=0.66 (eV cm-3 s-1)

0.31

0.061

Total radial transport = 3% Total CHEX = 6% Total radiated = 91%

Energy Budget

Figure 17 – (a) Ground-based observations of S+ emissions from the torus (Schneider & Trauger 1995). While S+ is a minor constituent of the warm outer torus it dominates the ribbon and the cold inner torus. (b) Voyager PLS measurement of ion species in the cold inner torus. (c) Herbert et al. (2008) re-analyzed Nick Schneider's 1991 observations of the cold torus to derive the 3-D distribution of S+ density shown in (d). The red and green lines in (c) are the centrifugal and magnetic equators derived from the JRM09+CAN magnetic field model of Connerney et al. (2018).

a

d

b

c

Figure 18 – Top – equatorial plasma density of electrons and main ion constituents (based Voyager data presented by Dougherty et al. 2017). Mass-discriminated density of energetic (~100 keV) ions (middle) and fluxes of oxygen ions for different energies (bottom) measured by the Galileo EPD instrument (based on Allen et al. 2019).

Io Europa

Fluxes at ~100 keV

Oxygen Fluxes at Various Energies

Plasma Densities

Europa

Observed Energetic Neutral Atoms ~100 keV H, O

Figure 19: Europa Neutral Cloud as observed by Cassini's MIMI instrument (Mauk et al. 2004) via very Energetic Neutral Atoms (H, O) at energies of ~100 keV.

Figure 20: Jovian Nebulae of (very) Energetic Neutral Atoms (based on Krimigis et al. 2002). Inserted bottom right is a histogram of counts versus mass/charge summed over days 338-362, 2000, before the Cassini bow shock encounter. Major peaks are identified on the plot but counts have not been normalized, so no relative elemental abundances are implied.

Mendillosphere

100s eV - FNAsThermal ions charge

exchange near Io

10s-100s keV - ENAsEnergetic ions charge exchange with Europa neutral cloud