dynamical consequences of two modes of centrifugal ......jupiter’s rapid rotation rate may be...

Dynamical consequences of two modes of centrifugal instability in

Jupiter’s outer magnetosphere

M. G. KivelsonInstitute of Geophysics and Planetary Physics and Department of Earth and Space Sciences, University of California,Los Angeles, Los Angeles, California, USA

D. J. SouthwoodEuropean Space Agency, Paris, France

Received 7 April 2005; revised 16 August 2005; accepted 13 September 2005; published 13 December 2005.

[1] Important global-scale properties of the middle and outer portions of the Jovianmagnetosphere can be interpreted in terms of plasma experiencing either marginal orexplosive centrifugal instability as a response to rotational stresses. The process accountsfor characteristics of the plasmasheet, particularly a strong dawn-dusk asymmetry, inferredfrom magnetic field data. Confining forces externally imposed at the magnetopause arecritical to this analysis. Because of the vast length of magnetospheric flux tubes, mostplasma-disk ions do not bounce between mirror points as a flux tube rotates through localtime sectors. Rotational stresses can lead to irreversible heating and increase plasmaanisotropy, creating the conditions for firehose instability. The outer edge of the plasmadisk appears marginally stable prenoon, but there is weak loss of plasma from its outeredge. The plasma disk thickens further postnoon as it assimilates the empty flux tubes ofthe outer magnetosphere where small perpendicular scale instability enables Bohmdiffusion of plasma to refill depleted flux tubes. On the nightside, the plasmasheetbecomes strongly unstable, material flows out down tail, the plasma disk thins, and itsouter portions can break off leaving depleted closed flux tubes behind. This outflow mustdominate solar wind-controlled reconnection in the magnetotail. The depleted flux tubes atlarge radial distance on the morningside form a distinct low-density plasma/magneticregime that overlays a thin plasma disk. The dawn-dusk asymmetry in the UV aurora islikely associated with the larger tapping of the ionospheric flywheel in the afternoon sectorto drive the heating and instability that the model proposes.

Citation: Kivelson, M. G., and D. J. Southwood (2005), Dynamical consequences of two modes of centrifugal instability in Jupiter’s

outer magnetosphere, J. Geophys. Res., 110, A12209, doi:10.1029/2005JA011176.

1. Introduction

[2] This paper presents a physics-based conceptualframework that enables us to interpret the global structureof the Jovian magnetosphere as revealed by the extensivespacecraft observations now available. The approachrequires us to consider processes on both magnetohydrody-namic scales and kinetic scales as we seek to understand thelarge scale structure of the system. We focus on the role ofcentrifugal forces in creating the distinctive local timevariation of plasmasheet thickness and in controlling theplasma loss process. Centrifugal forces contribute to break-ing the dawn-dusk symmetry of a magnetosphere becausethey are not conservative; their effects on the plasma neednot be reversible as the plasma expands and contracts.[3] We first review some properties of Jupiter’s distant

magnetosphere that require interpretation, including notmerely the variation of plasmasheet thickness with local

time [Kivelson and Khurana, 2002] but also the presence offlow bursts principally in the postmidnight sector of themagnetotail [Woch et al., 2002], the clear distinctionbetween the plasmasheet and the outer magnetosphere inthe morning sector [Smith et al., 1974] and the bubbles ofplasma observed as magnetic ‘‘nulls’’ in the dayside outermagnetosphere [Southwood et al., 1993; Haynes et al.,1994; Leamon et al., 1995]. We propose that over themorningside magnetosphere the plasmasheet is marginallyunstable to centrifugally driven ballooning. Its unstableouter portion is interpreted as the source of the plasma inthe magnetic nulls in the outer magnetosphere. On theafternoonside, the closed flux tubes lengthen while rotatingfrom noon to dusk. In this sector, the ability of the plasma totap effectively into the centrifugal force provided by theplanet yields much more dramatic instability, although theoverall plasmasheet is contained radially by the pressure atthe magnetopause. Instability on short spatial scales willlead to mixing with previously emptied tubes hithertooverlaying the sheet. On the nightside, the absence of themagnetopause allows the unstable sheet plasma to develop

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, A12209, doi:10.1029/2005JA011176, 2005

Copyright 2005 by the American Geophysical Union.0148-0227/05/2005JA011176$09.00

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an outflow, strongest near dusk but sustained over much ofthe nightside. Because of the sustained outflow, the thickplasmasheet at dusk evolves into an increasingly thin sheetas the plasma rotates through the nightside into the post-midnight sector. As plasma flows down the tail, the innerportions of the flux tubes, depleted of plasma, can reconnectand form the plasmoids that are released in this sector. Theemptying of closed flux tubes by the loss of materialdowntail leads to a rapid inward and cross-tail flows inthe early morning sector [Krupp et al., 2001] of flux tubesthat will form the outer magnetosphere near dawn.[4] The picture we provide of the centrifugally driven

circulation does not invoke as a basic process any solarwind driven circuit (referred to as the Dungey cycle byCowley et al. [2003]). Such a cycle is undoubtedly alsopresent, but the low voltages that can be imposed by thesolar wind (of order 1% of the voltages associated withrotation) suggest that its inclusion is not fundamental in asystem dominated by rotation [Brice and Ioannidis, 1971;Vasyliunas, 1975]. In contrast to Cowley et al.’s [2003]speculation, we suggest that there is unlikely to be a distinctlimited region of local time in which the Dungey processdominates.

2. Key Observed Properties

[5] In this section we introduce important observationsrelated to the structure of the plasma disk, as well asproperties of the outer magnetosphere and the magnetotailthat bear on our subsequent interpretation of the process ofplasma loss. Undoubtedly, the most important observation isthat the plasma inside of 20 RJ rotates at a rate of roughly80% of corotation but the azimuthal flow speed does notincrease as fast as corotation outside of 20–30 RJ [Khuranaet al., 2004]. Beyond this distance the magnetosphericplasma continues to rotate at roughly half the speed ofcorotation out to the boundary on the dayside of themagnetosphere and through much of the nightside magne-tosphere (see Figure 25-1 in the work of Krupp et al. [2004]and the discussion of the figure). The significance ofJupiter’s rapid rotation rate may be better appreciated bynoting that Earth rotates 2=3� in the 160 s required for thesolar wind to travel from the nose to the terminator of itsmagnetosphere but Jupiter rotates 160� in the 4.5 hours ittakes the solar wind to travel from the nose to the terminatorof its vast magnetosphere. Furthermore, if a nominal 10% ofthe electric field of the solar wind were imposed on bothmagnetospheres through reconnection, just inside the day-side magnetopause the plasma flow speed would be 1.7times larger than the velocity of full corotation at Earth and�0.01 times the corotation velocity at Jupiter. The role ofthe solar wind is clearly of secondary importance.[6] Let us review the plasma disk in greater detail. It was

first seen in the morning sector by Smith et al. [1974] onPioneer 10 inbound. They found that beyond �20 RJ,Jupiter’s morningside magnetosphere is dominated by athin current disk (half thickness of order 1 to 2 RJ) acrosswhich the field reverses from predominantly radially out-ward to predominantly radially inward. Smith et al. referredto this region, bounded within by a quasi-dipolar fieldregion, as the middle magnetosphere. Between the edge ofthe disk and the magnetopause in this morning sector lies a

region that Smith et al designated the outer magnetosphere,a region with strongly fluctuating magnetic field whoseorientation is predominantly southward. Such a region doesnot appear to be present at all local times and rather than usethe term ‘‘outer magnetosphere’’ we shall refer to it as thecushion region, a terminology first proposed by V. M.Vasyliunas (private communication, 1992).[7] The thickness of the plasmasheet can be inferred from

the form of the periodic variation of the radial component ofthe magnetic field. If the plasmasheet thickness is smallcompared with D, the maximum north-south excursion ofthe magnetic equator relative to the spacecraft, the radialcomponent reverses sign and changes magnitude in a smallfraction of the rotation period, after which it remains at aroughly constant value for a few hours, much as when aspacecraft is in the lobe of Earth’s magnetotail. If theplasmasheet thickness is �D, the 10 hour signature of theradial component is roughly sinusoidal and does not leveloff. The reversal signatures have been used to quantifythe plasmasheet thickness by K. K. Khurana (personalcommunication, 2005).[8] The form of the radial variations shows that system-

atically the plasmasheet is much thicker in the noon-earlyafternoon sector than in the prenoon sector. Postnoon, it canno longer be thought of as a disk. Indeed, Pioneer 11 on itsoutbound pass near noon encountered the plasmasheet at10 hour intervals at distances >20 RJ above the magneticequator. In this sector there also remains a cushion region atlarge radial distance. A thick plasmasheet was also encoun-tered periodically on the outbound Ulysses pass in the duskmeridian at distances >40 RJ from Jupiter and more than 26RJ below the equator [Lanzerotti et al., 1992], but near duskno cushion region was encountered.[9] There is a further feature to be remarked about the

cushion region seen from dawn round to past noon. In thisregion where the field is primarily southward, abruptdecreases of the magnetic field magnitude to levels muchbelow that of the ambient field have been commonlydetected. The decreases were first identified in data fromUlysses [Southwood et al., 1993; Haynes et al., 1994] andlater in reanalysis of Pioneer and Voyager data [Leamon etal., 1995]. Because the field magnitude approached zero inmany of the decreases, they were referred to as magneticnulls.[10] If pressure balance is assumed, the field decreases

occur where flux tubes filled with higher pressure plasmaare surrounded by flux tubes with lower plasma pressure.Southwood et al. [1993] show that the field drops in thenulls are well correlated with peaks in electron numberdensity. They interpret magnetic nulls as field decreasesproduced by the intrusion into the outer magnetosphere ofbubbles of warm plasma torn off of the dayside plasma disk.Occasionally, the field turns northward in the center of afield decrease, something that would occur if the perturba-tions were produced by magnetic field lines draped aroundbubbles of warm plasma. Their scales transverse to thesurrounding field appear to be at most some tens of Larmorradii in that field.[11] We argue here that the nulls are a basic feature of the

Jovian circulation. Their distribution can be inferred fromthe full local time survey that was completed by the Galileospacecraft. The magnetometer data from Galileo’s numerous

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passes through the outer magnetosphere in the afternoonsector reveal that plasma bubbles are not limited to themorning sector. Evidence of this is presented in Figure 1a inwhich the magnetic field (B, with components Br, Bq, Bj inSystem III coordinates) in the 1300 LT to 1400 LT sector isplotted across the transition region between the middlemagnetosphere (strong 10 hour periodicity of field orienta-tion with the dominant field radial except near the center ofthe current sheet) and the outer magnetosphere (dominantlysouthward field, less evidence of the 10 hour periodicity).Abrupt fluctuations of the field magnitude reveal numerousplasma bubbles, with the field occasionally reversing sign inthe center of the bubble. The fluctuations occur in abackground field magnitude of �15 nT, which is almostthree times larger than the background field (�5 nT) in theUlysses case studied by Southwood et al. [1993]. Assumingpressure balanced structures, the fluctuations imply greaterplasma pressure in this example from the postnoon sectorthan that needed to produce the field decreases found in theUlysses data near 1000 LT. The increased pressure isconsistent with the overall heating of the plasma as it movestoward and beyond noon. The nulls do not appear to extendthroughout the dayside. Figure 1b shows magnetic field datafor an outbound pass in the local time sector near 1600. Thenulls are no longer apparent but compressional fluctuations,consistent with a significant warm plasma population, arepresent through the entire outer portion of the pass right upto the magnetopause.[12] The magnetic field in the magnetotail has been

analyzed by Khurana [1997]. Well above and below thecenter of the current sheet, the field is almost purelyradially aligned relative to Jupiter in the premidnightquadrant. The orientation twists away from radial towardthe Jupiter-Sun direction in the postmidnight sector(referred to by Khurana as in the sense of corotationlag). Analysis of energetic particle anisotropies near thecenter of the plasmasheet by Krupp et al. [2001] and Wochet al. [2002] indicates that flow bursts in the antisolardirection occur frequently in the magnetotail beyond�100 RJ in the �2200 LT to �0500 LT sector. (Observa-tions are not reported outside of 100 RJ in the night sectorbetween �2000 LT and 2200 LT.) As regards the sheetthickness, data from Galileo orbits in the magnetotail[Kivelson and Khurana, 2002] demonstrate that theplasmasheet remains thick through the premidnight sector(Bz/jBj is not small in the center of the plasmasheet)but thins again near midnight and remains thin in thepostmidnight sector as one moves through to dawn.

3. Plasmasheet Equilibrium in a RotatingMagnetosphere

[13] Let us now consider the physics controlling theplasmasheet in a nominal steady state. The fluid forcedensity (F?) experienced by the plasma in the directionperpendicular to the magnetic field in a frame rotating atangular velocity W, is given by

F? ¼ �r? p? þB2

2m0

� �þ pk � p? �

B2

m0

� �Rc

R2c

þ rw2r� 2rw v� �

?; ð1Þ

where r is density, pk is parallel pressure, p? isperpendicular pressure, v is velocity, r is the distance fromthe rotation axis, and Rc is the field radius of curvaturevector. The last term includes the pseudo-force arising fromcentrifugal acceleration and the Coriolis force. In steadystate, forces must balance so that the net force on the plasmamust be zero.[14] In the disk or extended sheet configuration that

concerns us here, there is an extended region in which thefield is relatively straight and radial. In this region thesecond term is small and the only contribution to the thirdterms comes from the Coriolis force and is small. Forcebalance perpendicular to the field is maintained largelyby pressure balance in the ẑ direction (where ẑ is parallelto W). This implies that there must be roughly total(magnetic plus perpendicular plasma) pressure balanceacross the sheet as a whole. If the field in the center ofthe sheet is much smaller than the external field (as is trueout to �30 RJ on the pass shown in Figure 1a), one deducesthat roughly

p?eq B2ext=2m0; ð2Þ

where the subscripts eq and ext denote equatorial andexterior values. Additional forces may act in the azimuthaldirection but forces arising both from azimuthal pressuregradients and from the azimuthal component of Rc thatdevelops where outflowing plasma lags corotation andproduces a bendback of the field are likely to be smallcompared with the radial component. (The radius ofcurvature arising from corotation lag is of order many 10sof RJ as shown in an equatorial plane projection by Khurana[1997], whereas the radius of curvature of field projectedinto the meridian plane is of order a few RJ.)[15] Of particular interest is the region near the equator

where the field rotates southward to close across the currentsheet. In the outer portions of the plasmasheet Rc isextremely small at the equator (Rc � r) and the secondterm in equation (1) becomes of primary importance. Let usconsider the radial stress balance where this conditionapplies. The contribution to F? from the pressure gradientis small because of the slow variation of pressurewith distance. One can argue that the scale of the firstterm in equation (1) is of order O(p?eq/r). In contrast, thesecond term’s order of magnitude is much larger, at leastO(p?eq/Rc). The order of magnitude of the third term isO(rw2r). The ratio of this term to B2/2moRc is (vcr

2 /vA2)(Rc/r)

� 1. For force balance to be possible, the dominantterm itself must be very small near the current sheet crossing(s � 0, where sis the distance measured along the flux tubewith s = 0 at the field minimum). This implies

pk � p? �B2

m0

0 s � 0 ð3Þ

i.e., the ‘‘firehose’’ condition must apply. From theperspective of equilibrium, it is the parallel pressure that isdestabilizing (causing the flux tube to expand radiallyoutward). The centrifugal term acts in the same radial sensebut, provided there is sufficient pressure anisotropy, thecurvature term will be more important.


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Figure 1


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[16] In order to understand how the anisotropy is gener-ated by centrifugal effects and leads to the centrifugalinstability of the plasmasheet, we need to examine theforces parallel to the field off the equator. The fluid forcesparallel to the field (Fk) are

Fk ¼ �rkpk þ pk � p?� �rkB

Bþ rw2r� 2rw v� �

k: ð4Þ

For individual particles of mass m in guiding center motion,the parallel momentum equation is

mvk@vk@s

¼ mw2r� 2mw v� �

k� m@B

@s: ð5Þ

[17] Rotation enters through the centrifugal and the Cori-olis terms in two significant ways. The stresses imposed bythe rotating plasma lead to ballooning of the magnetic field(discussed in the next section), thereby producing the disk-like plasma distribution. In addition if the thermal motion isnegligible and the velocities associated with rotation dom-inate, the motion of particles parallel to the distorted field isdominated by the centrifugal term. In a closed field config-uration, particles move both parallel and antiparallel to thefield (while conserving m). One can write the parallelpressure in terms of Uk, the mean of the velocity ofparticles moving up and down the field as

pk rU 2k : ð6Þ

As for the value of Uk itself, the ions trapped on thedistended field are traveling along the field in bothdirections under the magnetic mirror force, but beyondabout 25 RJ they do not, in general, complete full bouncesbetween mirror points as they rotate through the daysidemagnetosphere. For example, a 1 keV ion of mass 20 mp(mp is the proton mass) with thermal velocity purely alongthe field moves only �5 RJ in an hour. From equation (5), ifan ion moves from a mirror point at rm (where vk = 0) to r >rm (using distances measured perpendicular to the spinaxis), its parallel velocity will exceed its thermal speed byroughly

Uk ¼ O w r2 � r2m� �1=2h i

: ð7Þ

If the ion of mass 20 is corotating, it gains �20 keV as itmoves from 45 to 50 RJ. Equations (6) and (7) demonstratethat the parallel pressure near the portion of a flux tubecrossing the equator at req is pk rw2(req2 � rm2 ) where rmrepresents the distance from Jupiter at which the bulk of the

plasma mirrors. The substantial growth in rUk2 that this

implies means that as the plasma rotates into the afternoon,rUk

2 > Bext2 /mo and rUk

2 > p? and it is no longer possible tosatisfy equation (3). The flux tubes at the outer edge of thedisk can no longer remain at the stable/unstable margin.Rather, throughout the sheet the equilibrium condition (3) isviolated and the sheet becomes explosively unstable.

4. Interchange and Ballooning

[18] The discussion of the previous section focused on theprocesses relevant to rotating plasma in a disk. The presenceof the disk geometry, which imposes a distinctive structureon the global magnetosphere, is well described by a modelin which partially corotating plasma injected in the innermagnetosphere by Io at the rate of 1 ton/s [Hill, 1980] isconfined near the magnetic equator by centrifugal force[Hill, 1980; Bagenal, 1994]. The material injected by Iomust move outward and in the dipolar regions of themagnetosphere it seems probable that this proceeds byinterchange motion [Southwood and Kivelson, 1987,1989] whose weak magnetic effects and plasma signatureshave been detected [Kivelson et al., 1997; Bolton et al.,1997; Thorne et al., 1997]. The interchange motion isspontaneous because the system is naturally unstable; theinterior source ensures that flux tubes fill until the gradientreaches the marginal condition for instability.[19] Interchange motions, by definition, make little or no

change to the field configuration; emptier flux tubes fartherfrom the planet move in, while fuller ones move out withoutmajor reconfiguration of the field. However, once the forcesassociated with the plasma itself become comparable to thefield forces, a different process must take over from inter-change motion and outward transport must be associatedwith field reconfiguration. One expects the transport ofmaterial inward and outward to be associated with ‘‘bal-looning’’ motions of the plasma. In ballooning, the fieldbulges out (see Figure 2a followed by Figure 2b). Ulti-mately, the bubble may ‘‘burst’’ with much of the originalcontent of the flux tube B breaking off in the bubble Dleaving a depleted tube C.[20] What goes up must come down, and one would

expect the depleted tube to collapse back toward the planetand eventually, in equilibrium, to find a mechanism to refillitself. How this occurs depends on the spatial scale of theinstability and the geometry of the system and these featuresare important elements of our speculations in this paper.[21] Even in a nonrotating magnetosphere, a steady

plasma source deep within the equatorial magnetospherewill create density gradients exceeding the adiabat and driveinterchange loss in the inner regions and ballooning loss at

Figure 1. Magnetic field measured by the Galileo magnetometer [Kivelson et al., 1992] versus UT (a) on the I32 outboundpass near 1300 LT (b) on the G29 pass near 1600 LT. The upper panel shows the azimuthal component in red and the radialcomponent in black. The lower panel shows the q-component (positive for a southward field) in red and the field magnitudein black. The nulls are identified as the sharp field decreases between 0400 UTon 18 October and the end of 19 October; in anull Bq beomes small and may even reverse sign. Labels across the bottom give the radial distance from Jupiter in RJ and thelocal time in hours and minutes. Grid marks are shown at 10 hour spacing. A vertical marker identifies the transitionbetween the region in which the ten hour periodicity is clear and the q-component dominates only near the current sheet andthe outer magnetosphere in which the periodicity is absent and the q-component remains comparable with the fieldmagnitude. Magnetosheath intervals are indicated by gray markers. See color version of this figure at back of this issue.


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large distances. Indeed, Southwood et al. [2001] showevidence of apparent interchange in Earth’s plasmasphere.At Earth the process does not play a major role in plasmatransport but at Jupiter the story is different. If the solarwind at large distance were absent, ballooning at Jupitercould occur at any local time and would have to fitseamlessly to the interchange motions feeding in plasmaat low radial distances. Both interchange and the ballooningmotions are fluid instabilities analogous to the Rayleigh-Taylor instability, the basic instability of a heavy fluidsuspended over a light fluid under gravity, but in a rotatingsystem with a tendency to move denser material outwardagainst gravity,. Under some circumstances the relevantfluid instabilities in the small amplitude regime growpreferentially for the shorter wavelengths. Effects likesurface tension can inhibit them. In magnetohydrodynam-ics, the field can play the role of surface tension if it has tobe strongly bent in the unstable motion. If the basic unstablemotion takes place transverse to the field, one expects theshortest (and therefore most unstable) wave numbers to beof order the inverse Larmor radius, rL, and the mostunstable wavelengths to be of order 2prL.[22] The above discussion gives some ‘‘rules of thumb’’

to guide our thinking about the centrifugal instability of theJovian plasmasheet. We do not attempt more than qualita-tive descriptions of instability in this paper nor shall weallow for temporal variation of sources or boundary con-

ditions. However, even in the steady state, the preciseinstability conditions will depend strongly on the overallboundary conditions whose influence need next to beconsidered.

5. Effect of the Outer Boundary Conditions

[23] Jupiter’s magnetosphere is immersed in the solarwind and the external conditions that lead to confinementof plasma at the outer edge of the disk-like plasmasheet varywith local time. Only on the nightside is the pressureexerted by the external region negligible. Elsewhere theforce imposed at the outer boundary restrains outwardexpansion of plasmasheet plasma within the magnetosphere.The effect of the magnetopause boundary is particularlyclear in the dusk sector where the plasmasheet is very thick(20–30 RJ). In part the thickening results from the extremeheating that can take place in the afternoon sector. The thicksheet extends far out, close to the magnetopause, consistentwith the expectation that in the afternoon sector plasma lossis inhibited by the magnetopause pressure.[24] Why the plasmasheet should thicken in the afternoon

sector is illustrated in the sketch of Figure 3. The thick sheetis associated with a more substantial north-south fieldthroughout much of the plasmasheet and, in particular, atits outer edge. The solar wind, containing its own magneticfield, overlays the boundary and exerts a normal pressure aswell as potentially exerting an effective surface tension ifthe magnetosheath field has a strong component in thedirection of flow. Although surface waves may occur onthe boundary, it is clear that both the pressure and themagnetic tension of the magnetosheath acting at the mag-netopause will render it difficult for the amplitude of a waveon the outer boundary of the plasmasheet to grow largeenough to allow the field to break. Accordingly, loss fromthe plasmasheet is probably inhibited on the afternoonside.[25] The evidence that the outward moving internal

plasma is actually confined within the magnetopause pro-vides a constraint on the plasma density of the internalplasma. From observations of magnetopause crossings it isknown that the magnetopause flares by 50% between thenose of the magnetopause and the terminator [Joy et al.,

Figure 2. Ballooning and loss of plasma bubble. (a) Initialflux tube configuration. (b) Change of magnetic configura-tion arising from radially outward plasma pressure in therotating system. (c) Bubble broken off the outer portion ofthe flux tube.

Figure 3. Meridional cut through the plasmasheet on theduskside showing the magnetopause as a boundary thatprevents outflow and increases radius of curvature byexerting a relatively uniform inward pressure.


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2002]. For the most probable nose distances of 60 and 90RJ, the plasma just inside the boundary rotating from noonto dusk in roughly 5 hours expands radially outwardthrough 30 or 45 RJ, respectively. The expansion requiresan average speed, vms, of 120–180 km/s. If the magneto-spheric plasma density at the magnetopause is rms and thesolar wind plasma density is rsw, pressure balance requiresrmsvms

2 � (n̂ � x̂)rswvsw2 or rms � (n̂ � x̂)rsw(vsw/vms)2 atlocations on the boundary where the outward normal is n̂.Taking the geometric factor as �0.5, a nominal solar windvelocity (400 km/s) and density rsw/mp �0.3 cm�3, thisinequality requires rms/mp < 1.7 cm

�3, a condition that isreadily satisfied in the outer magnetosphere. For example,the electron density in the outer magnetosphere both in-bound and outbound on the Ulysses flyby of Jupiterremained less than roughly a tenth of a particle cm�3

[Phillips et al., 1993].

[26] Our analysis of the external force that accounts forthe plasmasheet thickness in the afternoon sector alsosuggests that when the plasma rotates into the region whereit is no longer confined by the magnetopause, the outwardpressure that has been hitherto countered by the magneto-pause will cause the plasma to stream outward fairlyuniformly in the postdusk sector. The conditions at theoutermost boundary will rapidly become not merely unsta-ble but, with unconstrained outflow, explosively unstable.[27] One must question whether flux tubes continue to

corotate in the postdusk sector. Parallel currents that imposerotational stresses are carried between the equatorial plasmaand Jupiter’s ionosphere by Alfvén waves. Pressure anisot-ropy reduces the Alfvén wave speed to

vA 1� bk þ b? 1=2

¼ B2=mo � pk þ p?� �

=r� �1=2

; ð8Þ

where vA = B/(mor)1/2, bk = pk/B

2/2mo and b? = p?/B2/2mo.

Where the firehose instability condition applies, the factormultiplying vA vanishes, Alfvén waves do not propagate andthe equatorial plasma is effectively decoupled from theionosphere.[28] A description of the plasma behavior in the magneto-

tail beyond �200 RJ, a region not probed by Galileo,becomes highly speculative. Two possibilities can be con-sidered. If the outer portion of the equatorial plasma is closeto firehose unstable as it rotates into and beyond �1600 LT,the plasma flowing tangent to the magnetopause willexperience only locally imposed forces that act principallynormal to the magnetopause. The plasma will continue toflow past dusk and down the evening flank of the magneto-tail. This is illustrated in the much-reproduced Vasyliunas[1983] cartoon (Figure 4a) of magnetotail flow at Jupiter.However, because the outer field lines of the thickenedplasmasheet on the afternoonside have large Rc, it ispossible that the firehose condition of equation (3) is notsatisfied at the outer edge of the thickened plasmasheet. Inthis case, corotation-enforcement stress may be imposed asthe plasma moves through the dusk sector. The UV emis-sions observed throughout the region of putative closedfield lines in the polar aurora [Clarke et al., 1998; Prangé etal., 1998; Pallier and Prange, 2001, 2004] in the dusk andnight sectors seem to require that field-aligned current flowas far out as the magnetopause in the dusk sector andbeyond. It is possible that the plasma decouples from theionosphere only after the flux tubes have rotated into thenight sector and have begun to blow down tail. In this case,corotation-enforcement currents may be able to imposeazimuthal stress on the plasma through and somewhatbeyond the dusk meridian. The purported outflow of plasmawould significantly reduce Rc and the connection with theionosphere would terminate when the firehose conditionagain dominated the dynamics. Thereafter the plasma wouldbe decoupled from the ionosphere. In this picture, the heavyion plasma of the plasmasheet would not flow down theflanks of the magnetotail but instead be confined to a morelimited region of the nightside.[29] If the latter description is pertinent, magnetotail

plasma could be distributed as illustrated schematically inFigure 4b. The confinement to the near tail in the figureacknowledges that the rate of plasma expansion down tail is

Figure 4. Schematics of the local time variation of theJovian plasma. (a) Illustration from Vasyliunas [1983]showing the equatorial plane on the left and meridionalcuts representing the formation and detachment of plas-moids. Flow of the outermost internal plasma follows theeveningside magnetopause while flow from regionsincreasingly closer to the planet are directed partiallydawnward. (b) Illustration of an alternate interpretation ofmagnetotail flow. Here plasma of the plasmasheet is shownonly within a limited radial range. Bubbles of plasma thathave broken off through an explosive ballooning instabilityare shown continuing to move across the tail at close to theirvelocity of release. The outer edge of the plasmasheetretracts after a release and then starts again to expand.Reconnected lobe flux tubes are shown flowing slowlysunward.


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bounded by the sound speed. For a heavy ion plasma, hereheated to a characteristic energy of order 20 keV, the soundspeed is of the order of 300 km/s = 15 RJ/hr. Assuming thatthe dusk boundary lies near 150 RJ, the closed flux tubeswould penetrate no further than �350 RJ down tail duringthe �10 hours required for them to rotate through the nightsector. Beyond the boundary of closed flux tubes, themagnetotail might resemble a terrestrial magnetotail withnorth and south lobes separated by plasma of solar windorigin flowing sunward interspersed with regions of plasmaof planetary origin torn off of the unstable outer plasma-sheet and flowing down the tail. We next consider theimplications of these models.

6. Reconnection in the Magnetotail

[30] Reconnection in the magnetotail of Earth conservesthe magnetic flux of the planet by converting flux tubesfrom the north and south lobes, opened by dayside recon-nection, into closed and disconnected flux tubes at a neutralline in the distant magnetotail. Following reconnection, theclosed flux tubes convect sunward. At Jupiter as well asEarth, a distant neutral line is likely to exist in the tail. Atdowntail distances where flaring of the magnetopause hasceased, the magnetic fields of the lobe and of the solar windmust be close inmagnitude. For 10% reconnection efficiency,the north-south convection speed will then be �10% ofthe solar wind speed, meaning that the distant neutral line

will form at roughly ten times the half width of themagnetotail (roughly 200 RE down tail at Earth and2000–3000 RJ or 1–1.5 AU at Jupiter). Sunward of thatdistance plasma should return sunward at an average speedof order or less than 0.1 vsw Bsw/Bt (�40 km/s), where vswand Bsw are the solar wind speed and magnetic field,respectively and Bt is the magnetic field at the neutral sheetof the magnetotail. This average speed is considerablysmaller than the sound speed that we estimated as the rateof plasma outflow.[31] In the Cowley et al. [2003] picture, the inward

flowing plasma will be diverted across the tail to the dawnsector before returning to the dayside in a narrow channel.However, it seems possible to us that the outflowing plasmawill develop structure on the scale �2prL as previouslymentioned and that fingers of oppositely directed flow maydevelop in the dusk to midnight sector as illustrated inFigure 4b. Whether or not such structure develops, in theregions of outward flow the plasmasheet will thin. Thethinning of the plasmasheet as the flux tubes move fromdusk toward midnight is consistent with the decrease of Bzbetween 1800 LT and 0000 LT reported by Kivelson andKhurana [2002]. As the flux tubes expand down the tail,portions closer to Jupiter than roughly the distance to themagnetopause near dusk will be partially depleted ofplasma, a situation that facilitates reconnection and thesubsequent loss of a plasma-filled bubble. In regions ofreduced plasma density, the flux tubes should thin andultimately break as illustrated in Figure 5. When plasmabubbles break off the sheet, some lobe field may possiblyreconnect, the newly closed portion overlaying partiallydepleted plasmasheet flux tubes from which the plasmoidseparated. As these flux tubes continue to rotate around theplanet, they can return flux to the dayside from any startingpoint on the nightside, eliminating the need to return fluxthrough a narrow channel on the morningside as proposed byCowley et al. [2003]. The plasmoid itself should continue tomove outward at close to the sound speed estimated above.Meanwhile, the depleted inner portion of the flux tube that isstill linked to Jupiter is likely to speed its azimuthal motionbecause the corotation enforcement currents can more read-ily accelerate the plasma on flux tubes with reduced fluxtube content. The breaking off of plasmoids from theplasmasheet is also described in the Vasyliunas [1983] andCowley et al. [2003] scenarios, which predict that the feet ofthe flux tubes will rotate around to the midtail before areconnection X-line forms.[32] Recalling that the plasmasheet is continually fed

from the deep magnetosphere by interchange motions, ourargument suggests that the release of bubbles occursrepeatedly as the flux tubes rotate through the tail. Thismeans that even a tube that has blown out and lostmaterial through reconnection in the premidnight sectorcan refill and again begin to move outward. It is likelythat the central tail does not contain a steady X-line butrather that material bubbles off in localized plumes fromthe explosively unstable outer edge of the plasmasheet;regions of rapid outflow may be separated by regions ofslow inflow. It is likely that any tube containing Iogenicplasma has a high probability of moving in a generallyantisolar direction across most of the tail.

Figure 5. Release of plasmoid on the nightside. From topto bottom are shown the initial strong outflow that thins theplasmasheet, depletion of the inner part of the flux tube, andreconnection that produces a plasma bubble. Dotted linesrepresent lobe flux tubes (open to the solar wind) initiallydraped over plasmasheet flux tubes that can participate inthe process.


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[33] Unfortunately, the region in which measurementscould distinguish the two scenarios described lies beyondthe portion of the tail explored by Voyager and Galileo. Thepicture of spatially localized lobe reconnection on anazimuthal scale small compared with the dusk to dawnextent of the tail is, however, consistent with the failureof the Galileo orbiter instruments to observe major tailreconfiguration analogous to that observed in terrestrialsubstorms.

7. Cushion Region in the Morning Sector

[34] The flux tubes that rotate into the morning sectorinclude those threading the inner portion of the plasmasheetthat do not lose plasma through plasmoid formation on thenightside and those that participate in the bubbling/breakingprocess and thereby lose much of their plasma content onthe nightside. As flux tubes rotate into the morning sector,they again begin to respond to the presence of the magne-topause at large radial distance. In this sector it is commonto observe an outer magnetospheric region beyond theplasmasheet where the field is not distended, no longermanifests a 10 hour periodicity and is generally southwardover tens of degrees of latitude near the equator. Thisregime, the cushion region, forms a new outer boundaryto the plasmasheet. We take the existence of a region ofrelatively empty flux tubes adjacent to the magnetopause inthe morning as perhaps the best evidence that closed tubesthat have lost plasma through the nightside remain outsideof the flux tubes that have not lost plasma and that they donot immediately start mixing with the plasmasheet.

[35] On the dayside, the cushion serves as the externalenvironment for the plasmasheet. If the external boundary iscompletely quiescent, there is no reason for mixing ofmaterial from the sheet with the cushion region becauseboth move inward as they rotate toward noon. However,significant outward motion of the magnetopause associatedwith solar wind dynamic pressure variation can reduce theconfining pressure and allow plasma bubbles to escape fromthe outer edge of the plasmasheet, although it is likely thatlittle plasma is lost in the morning sector. Once a bubble isdetached, the flux within it must be a closed loop. Thereforethere is no loss of magnetic flux and the bubble itself, nolonger attached to the planet, moves tangential to itsdirection of motion at the time of detachment.[36] When material bubbles off the outer edge of the

plasmasheet in this sector, the detached bubble finds itselfsurrounded by a southward field containing only low-pressure plasma. This has been suggested as the explanationof the plasma bubbles that have been seen in the morningsector [Southwood et al., 1993; Haynes et al., 1994;Leamon et al., 1995]. As the bubble moves through thenorth-south field, the surface forces exerted on the weaklymagnetized bubble will prevent the plasma from movingacross the external field (thereby slowing its motion towardthe magnetopause) but allow it to expand along the field inresponse to pressure gradient forces. Figure 6 shows thisschematically and also shows that if the transverse scale ofthe flux tube decreases to the scale of an ion gyroradius,ambient fluctuations can cause the plasma to diffuse off theflux tube.[37] Although the detachment of plasma in the dayside

sector is associated with loss of plasmasheet plasma, theplasma loss rate is probably much smaller than the Io sourcerate and there may be little plasma lost to the magnetopause.

8. Evolution of the Plasmasheet Through theDayside Magnetosphere

[38] We next apply the ideas discussed in the previoussections to describe the changes of structure as the plasma-sheet rotates through the dayside magnetosphere. Figure 7summarizes how the plasmasheet changes its size and shapeat different local times as inferred from data. The back-ground of the figure represents the equatorial magneto-sphere with the magnetopause and bow shock locationsshown as broad curves to reflect typical ranges of theirpositions [Joy et al., 2002]. Arrows at different localtimes link to schematics of meridional cuts through themagnetosphere.

8.1. Plasmasheet From the Morningside to Noon

[39] As the plasmasheet rotates through the morningsector it experiences an increasing pressure at large distancethat produces three effects. First, the compression shouldaccelerate and/or heat the particles in the sheet. Second, theincreasing external pressure at its outermost edge isexpected gradually to thicken the outer portion of theplasmasheet. Finally, as the outer portion of the plasmasheetmoves inward, its rotation should speed up and it shouldbecome easier for the ionospheric torque to maintain rota-tion on a shortened flux tube [Southwood and Kivelson,2001; Cowley and Bunce, 2001]. This latter effect reduces

Figure 6. Flux tubes of the dayside cushion regionassimilating a plasma bubble separated from the outerplasmasheet. (a) Bubble embedded in flux tubes of thecushion region. (b) At a later time, plasma has moved alongflux tube in response to pressure gradient forces and the fluxtube has narrowed in response to magnetic forces. (c) If theflux tube narrows to the scale of gyroradii, plasma candiffuse onto nearby flux tubes.


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the need for field-aligned current to maintain corotation asplasma diffuses across L shells and probably explains theregion of low emission poleward of the main oval in themorning sector [Clarke et al., 1998; Prangé et al., 1998].[40] Some of these ideas seem to be borne out by the

Galileo magnetic field data from near midday illustrated inFigure 1. In Figure 1a, one sees that the current sheetsignatures with their 10-hour periodicity are present in thedata out to a radial distance of 31 RJ beyond which there is a‘‘cushion’’ region where the field is primarily southwardextending out to the magnetopause, encountered at a radialdistance of around 50 RJ. The current sheet itself is clearlythicker than at earlier local time as evidenced by the factthat the field does not flip sharply across the current sheet asit does, for example, in measurements taken near 0600 LTby the Pioneer 10 spacecraft. In addition, the presenceof compressional oscillations in the field at all parts of the10-hour cycle suggests that dense plasma is present and thatthe spacecraft does not ever fully exit the sheet. In thecushion region beyond 30 RJ innumerable dips in the fieldstrength can be seen. In these magnetic ‘‘nulls’’ [Haynes et

al., 1994; Southwood et al., 1993; Leamon et al., 1995] thefield dips from �15 nT outside down to a few nT and attimes the actually reverses and becomes northward. Thesenulls are consistent with a source plasma that has broken offfrom the sheet.

8.2. Plasmasheet in the Afternoon Sector

[41] The magnetopause distance from Jupiter increases byabout 50% from the nose standoff (probable distances oforder 60 and 90 RJ) to the dusk meridian (probabledistances of �90 and 135 RJ). Were the magnetospherenot rotating, one would expect that the plasmasheet wouldthin as it rotated from noon to dusk, expanding in responseto the decreasing pressure at large radial distance. Theopposite happens. In Figure 1b which shows a pass in the1500–1600 LT sector, the transition to a thick current sheet(with Bq/jBj � 1) occurs at about 40 RJ and compressionalfluctuations are present everywhere outside of 25 RJ. Thesignatures of magnetic nulls are not observed in this localtime sector. Because the pass though the region dominatedby Bq lasted more than a week, it is reasonable to assume

Figure 7. The background shows an equatorial cut of the magnetosphere with a range of probablelocations of the magnetopause in cyan and of the bow shock in gray from the model of Joy et al. [2002].Surrounding the equatorial cut are meridional cuts taken at different local times indicated by arrows. Atdawn the plasmasheet is thin and elongated. Across the dayside at later local times, the plasmasheet iscompressed by forces exerted by the magnetopause. It becomes thicker, but its outer edge becomesmarginally unstable, so the outer magnetosphere is populated with bubbles of plasma that have broken offthe outer edge. At dusk, the clear transition from the plasmasheet to the outer magnetosphere is no longerpresent and the sheet is thick and irregular. As the sheet rotates into the nightside, outflow (at a speed or�10 RJ/s pushes the outer edge increasingly far down the tail and by midnight the plasmasheet is againvery thin and greatly elongated. As the flux tubes rotate into the postmidnight sector, the inner portionsevacuate, and two images grouped within a dashed contour represent the stretched sheet and the sheetfollowing detachment of a plasmoid at a neutral line.


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that the absence of nulls is a permanent characteristic of thislocal time sector. All through the afternoonside theplasmasheet thickens as the plasma moves toward dusk.[42] To understand this, we have proposed that a new

mode of plasma acceleration comes into play when the outerboundary recedes with increasing local time. We attributethe changing form of the plasmasheet to the increase of themean parallel thermal speed as plasma moves outward inthe rotating system. The dynamic stress rUk

2 exerted by theplasma as it passes through the equatorial bend of the fieldsubstantially increases as the plasma rotates from noontoward dusk. Our inference regarding the importance ofpressure anisotropy has not been tested in the afternoonsector, but it is consistent with evidence from Voyager 1and 2 that anisotropy force dominates other forces in theneutral sheet from 18 to 35 RJ on the nightside [Paranicaset al., 1991]. Also in the morning sector near 0900–1000 LT, Mauk and Krimigis [1987] report that somethingother than an isotropic pressure gradient is needed toproduce stress balance beyond 22 RJ. Near the outerboundary, external forces change the configuration of thestretched plasmasheet. The solar wind pressure at themagnetopause causes the plasmasheet to thicken, therebyrelaxing the sharp curvature and increasing the radius ofcurvature R near the equator.[43] It is important to understand that there is no coun-

terpart to this effect in the morning sector and that the scaleof heating achieved through the parallel centrifugal force islarge.

9. Refilling of Evacuated Flux Tubes: CushionRegion as Part of the Rotational Cycle

[44] In our discussion of the morning sector we tentativelyidentified the cushion region as consisting of relativelyempty tubes that are likely to have been emptied on theirpassage across the nightside. The region, characterized bypredominantly southward field and at most minor modula-tion at the rotation period of Jupiter, appears at radialdistances beyond plasmasheet. It is still present near noonbut is not evident in the dusk sector. In this local time sector,at large distance from Jupiter (�60 RJ on the G29 outboundpass at �1600 LT) the q component and the radial compo-nent are of comparable magnitude (unlike the cushion regionin which the q component dominates). The radial componentreverses sign rather irregularly but its spectrum shows clearpeaks near the rotation period of Jupiter. These character-istics of the data lead us to suggest that the plasmasheet,albeit sometimes distorted, extends to the magnetopauseeven though the 10-hour periodicity of the current sheetcrossings is obscured.[45] Only a small portion of the flux in the cushion region

is likely to have been removed by reconnection with thesolar wind on the dayside because the estimated voltagefor reconnection is 1 MV [Cowley et al., 2003], well belowthe tens of MV across the region in the morning sector. Thelikelihood is that the cushion has been assimilated into theplasmasheet through interchange occurring on the balloon-ing instability scale of the plasma Larmor radius. With theinstability taking place on such scales, the process ofassimilation might be as effective as Bohm diffusion (scat-tering of one Larmor radius in of order a Larmor period) in

transferring the plasmasheet plasma onto the previouslydistinct tubes. Such a process would produce a plasmasheetfilled to its outer boundary with Iogenic plasma and poisedto expand rapidly and lose much of its plasma content as itrotates through the night sector where it begins to flowoutward as we have discussed.

10. Conclusions

[46] We have outlined the way in which the dynamicsof the Jovian plasmasheet varies as a function of localtime under steady state boundary conditions. The plasma-sheet is a central feature of the Jovian magnetosphere andthe transport of Iogenic plasma through it is the system’smost important transport process. Figure 8, with interpre-tations of the key processes added to a diagram fromVasyliunas [1983], summarizes the ideas that we have putforward here.[47] Much of what we say is consistent with the picture of

circulation drawn by Vasyliunas [1983], particularly on thenightside. However, what we newly emphasize and explainis a fundamental dawn-dusk asymmetry in the plasmasheet,it being thickest near dusk and thinnest in the morningsector near dawn, which is probably basic to rotationallydriven magnetospheres. We argue that as flux tubes moveacross the nightside, an outflow of material downtail pro-duces the thinning. The same outflow leads to flux tubesbreaking and leaving closed depleted flux tubes that moveonto the dayside at large radial distance on the morningside(forming what is referred to as the cushion region in thispaper). The residual sheet thickens as flux tubes rotate tonoon. During this rotation, the appearance of magnetic nullsin the cushion region provides some evidence of weak lossof plasma from the outer edge of the plasmasheet. Amassive change takes place in the afternoon sector wherethe plasma of the plasmasheet energizes and thickens as itmoves from noon to dusk and assimilates the empty fluxtubes of the cushion. We speculate that this assimilation isaccomplished through a short perpendicular scale centrifu-gally driven ballooning leading to Bohm diffusion ofplasma to refill the previously emptied tubes. The well-known dawn dusk asymmetry in the UV aurora [Clarke etal., 1998; Prangé et al., 1998] is very likely associated withthe vastly larger tapping of the ionospheric flywheel in theafternoon sector that our model proposes.[48] We have emphasized how the properties of the

plasma-field configuration are affected by the radial motionof the equatorial portion of the flux tubes, particularly in theregions where the flux tubes move radially outward in thepostnoon and night sectors. If the solar wind conditions areunsteady, inward and outward displacement of the magne-topause can occur at a large range of local times. As notedin section 7, outward motion of the magnetopause will leadto loss of stability of the outer edge of the plasmasheet inthe morning sector. Analogously, inward motion of themagnetopause would stabilize the outer portions of theplasmasheet on the afternoonside, where the heating thatwe proposed in section 8.2 may become unimportant whenthe magnetopause is moving inward. A full consideration ofthe effects of local time structure and time varying externalboundary conditions is outside the scope of this paper but isa topic that is worth pursuing.


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[49] Our description of particle acceleration and associatedheating of the plasmasheet implies that physical processesnot modeled in magnetohydrodynamics are central to under-standing Jupiter’s magnetosphere. The effects that we havedescribed require rapid rotation and large spatial scales. Therotation provides the particle acceleration, produces pressureanisotropy and leads to associated effects on the magneticstructure. The large spatial scale is required so that theparticle bounce periods can be long compared with thefractional rotation period. These conditions are not presentin Earth’s magnetosphere but are likely to apply in someportion of Saturn’s magnetosphere where we trust that ideasput forward here will find further application.

[50] Acknowledgments. We acknowledge frequent intense discus-sions of the subjects of this paper with Krishan Khurana, Ray Walker,and Fran Bagenal, and most useful criticisms from Vytenis Vasyliunas. Wethank Todd King, Steve Joy, and Joe Mafi for creative programming andother support and Rose Silva for general assistance. This work wassupported in part by NSF’s Division of Atmospheric Sciences under grantNSF ATM 02-05958.[51] Arthur Richmond thanks the reviewers for their assistance in

evaluating this paper.

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Cowley, S. W. H., E. J. Bunce, T. S. Stallard, and S. Miller (2003), Jupiter’spolar ionospheric flows: Theoretical interpretation, Geophys. Res. Lett.,30(5), 1220, doi:10.1029/2002GL016030.

Haynes, P. L., A. Balogh, M. K. Dougherty, D. J. Southwood, A. Fazakerley,and E. J. Smith (1994), Null fields in the outer Jovian magnetosphere:Ulysses observations, Geophys. Res. Lett., 21, 405.

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Joy, S. P., M. G. Kivelson, R. J. Walker, K. K. Khurana, C. T. Russell, andT. Ogino (2002), Probabilistic models of the Jovian magnetopause andbow shock locations, J. Geophys. Res., 107(A10), 1309, doi:10.1029/2001JA009146.

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Khurana, K. K., M. G. Kivelson, V. M. Vasyliunas, N. Krupp, J. Woch,A. Lagg, B. H. Mauk, and W. S. Kurth (2004), The configuration ofJupiter’s magnetosphere, in Jupiter: The Planet, Satellites and Magneto-sphere, edited by F. Bagenal, T. E. Dowling, and W. B. McKinnon,p. 593, chap. 24, Cambridge Univ. Press, New York.

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Krupp, N., et al. (2004), Dynamics of the Jovian magnetosphere, in Jupiter:The Planet, Satellites and Magnetosphere, edited by F. Bagenal, T. E.Dowling, and W. B. McKinnon, pp. 593–616, Cambridge Univ. Press,New York.

Figure 8. The equatorial plane of Jupiter’s magnetosphere as represented by Vasyliunas [1983]. Labelsindicate the interpretation of the dynamics in the schematic in terms of the processes discussed in thispaper.


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Thorne, R. M., D. J. Williams, R. W. McEntire, T. P. Armstrong, S. Stone,S. Bolton, D. A. Gurnett, and M. G. Kivelson (1997), Galileo evidencefor rapid interchange transport in the Io torus, Geophys. Res. Lett., 24,2131.

Vasyliunas, V. M. (1975), Concepts of magnetospheric convection, inThe Magnetospheres of the Earth and Jupiter, edited by V. Formisano,pp. 179, Springer, New York.

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Woch, J., N. Krupp, and A. Lagg (2002), Particle bursts in the jovianmagnetosphere: Evidence for a near-Jupiter neutral line, Geophys. Res.Lett., 29(7), 1138, doi:10.1029/2001GL014080.

��M. G. Kivelson, Institute of Geophysics and Planetary Physics and

Department of Earth and Space Sciences, University of California, LosAngeles, 6843 Slichter Hall, 405 Hilgard Avenue, Los Angeles, CA 90095-1567, USA. ([email protected])D. J. Southwood, European Space Agency, 8-10 rue Mario-Nikis,

F-75738 Cedex 15, Paris, France. ([email protected])


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Figure 1

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Figure 1. Magnetic field measured by the Galileo magnetometer [Kivelson et al., 1992] versus UT (a) on the I32outbound pass near 1300 LT (b) on the G29 pass near 1600 LT. The upper panel shows the azimuthal component in red andthe radial component in black. The lower panel shows the q-component (positive for a southward field) in red and the fieldmagnitude in black. The nulls are identified as the sharp field decreases between 0400 UT on 18 October and the end of19 October; in a null Bq beomes small and may even reverse sign. Labels across the bottom give the radial distance fromJupiter in RJ and the local time in hours and minutes. Grid marks are shown at 10 hour spacing. A vertical marker identifiesthe transition between the region in which the ten hour periodicity is clear and the q-component dominates only near thecurrent sheet and the outer magnetosphere in which the periodicity is absent and the q-component remains comparable withthe field magnitude. Magnetosheath intervals are indicated by gray markers.

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dynamical consequences of two modes of centrifugal ......jupiter’s rapid rotation rate may be...

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