0273-l 177(95)00318-S

Adv. Space Res. Vol. 16, No. 9, pp. (9)85-(9)94, 1995 Copyright 0 1995 COSPAR

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ULYSSES SOLAR WIND PLASMA OBSERVATIONS DURING THE DECLINING PHASE OF SOLAR CYCLE 22

J. L. Phillips,* S. J. Bame,* W. C. Feldman,* J. T. Gosling,* C. M. Hammond,* D. J. McComas,* B. E. Goldstein** and M. Neugebauer**

* Los Alumos National Laboratory, Los Alamos, NM 87545, U.S.A. ** Jet Propulsion Laboratory, Pasadena, CA 91109, U.S.A.

ABSTRACT

Since launch in October 1990, the Ulysses mission has included an in-ecliptic cruise enroute to Jupiter encounter in February 1992 and a post-Jupiter transit through a wide range of southerly latitudes and heliocentric distances. Here we present results from the solar wind plasma experiment through June 14, 1994, at which time Ulysses was at -68.2’ heliographic latitude. During the ecliptic phase of the mission, occurring just after solar maximum, the spacecraft encountered an irregular pattern of solar wind speed and sporadic coronal mass ejections, with mass ejections most prevalent during March 1991. Irregular, small-amplitude solar wind streams prevailed until mid-1992, after which Ulysses encountered a recurrent very high-speed stream from an equatorward extension of the South polar coronal hole. Encounters with the high-density, low-speed plasma from the coronal streamer belt ceased as Ulysses moved to increasing southerly latitudes in 1993. Many forward and reverse shocks associated with corotating interaction regions have been encountered; these shocks all had observable electron foreshocks. The shocks became less prevalent with increasing latitude, with the forward shocks disappearing first because of the tilted streamer belt and the resulting meridional shock propagation. After Ulysses passed -35’ in July 1993 the spacecraft encountered only high- speed wind, with a speed range of 700-800 km s-l and a density, scaled to 1 AU, averaging 3 cmW3. Latitudinal gradients in solar wind fluid parameters generally support previous findings, with the gradient in wind speed offset by a gradient in density such that mass and momentum flux vary relatively little.

INTRODUCTION

The Ulysses spacecraft was launched in October 1990 and proceeded in the ecliptic plane to 5.4 AU, where it was deflected southward by the gravity of Jupiter into a trajectory which would carry it to a peak southerly solar latitude of 80.2” at 2.3 AU. The Los Alamos solar wind plasma experiment on Ulysses comprises separate electrostatic ion and electron spectrometers which sample the solar wind plasma over energy ranges of 0.25 to 35.2 keV/q (positive ions) and 0.8 to 862 eV (electrons) /I/. The plasma instruments were turned on in November 1990, producing their first usable data on November 18 at I .I5 AU. The experiment has performed nearly flawlessly since turn-on, and overall data continuity has been exceptionally good. Science goals of the solar wind plasma experiment include investigation of systematic variations in the solar wind bulk properties with latitude, latitudinal effects on stream evolution, meridional gradients in the ion and electron distribution functions, and characterization of solar transient events at high latitudes. This study includes observations through the first encounter with the Jovian bow shock at 5.39 AU on February 2, 1992, and post-Jupiter data from February 16, 1992 through June 14, 1994, when Ulysses was at 2.9 AU and -68.2” heliographic latitude.

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IN-ECLIPTIC OVERVIEW

J. L Phillips et al.

Figure 1 shows six-hour-avera 9

ed solar wind speed (upper trace, scale on left axis) and density (lower trace, right axis), scaled by R to 1 AU, for the preJupiter mission phase. Solar wind speed was generally slow and irregular through February 1991. In March 1991, a series of coronal mass ejections (CMEs) overtook the spacecraft, driving strong shocks and producing instantaneous wind speeds as high as 1000 km s-l 121. Additional fast CMEs were encountered in April, June, and August 1991 and produced many of the speed peaks visible in Figure 1 for that interval. From September 1991 until Jupiter encounter in February 1992, a weak solar-rotation periodicity emerged in wind speed, though the peaks were irregular in magnitude. A closer look at the data shows that the density peaks are compressive in nature (highest densities at upturns in speed), indicating the first clear sequence of corotating interaction regions (CIRs) observed at Ulysses. The pattern observed at Ulysses included two streams per rotation in July-August 1991; this periodicity nearly disappeared in September 1991, then gave way in October 1991 - January 1992 to a single stream per rotation. The hollow diamonds plotted at 700 km s-l in the second half of the plot mark 0” spacecraft Carrington longitude in order to allow better visualization of the solar-rotation periodicity in the observations. The prevailing interplanetary magnetic field (IMF) polarity for the fastest wind observed in the latter interval was inward, indicating a source south of the heliomagnetic streamer belt /cf. 3/.

5 800

$ v)

ero Carrington Longitude

vvvvvvv

18Nov90 14Feb9 1 13May91 2Feb92 2

Fig. 1. Six-hour averaged solar wind proton speed (top trace, labels on left axis) and proton density, scaled to 1 AU (bottom trace, labels at right), observed by the Ulysses solar wind plasma experiment from 18 November 1990 through I February 1992. Solid triangles at the top mark heliocentric distance of spacecraft; hollow triangles mark 0” spacecraft Carrington longitude for second half of plotted interval.

Ulysses Solar Wind Plasma Obsawtio~

OUT-OF-ECLIPTIC OVERVIEW

6987

Ulysses began its out-of-ecliptic mission phase by turning southward as it passed by Jupiter on February 8, 1992 at a heliocentric distance of 5.4 AU and heliographic latitude of -6’. Figure 2 shows six-hour averaged solar wind speed and density for this phase of the mission in the same format used in Figure 1. During February - June 1992. the solar wind at Ulysses was slow and irregular, with little discernible stream pattern. In early July, however, a well-defined high-speed stream was observed, with a location in solar longitude and an IMF polarity indicating a coronal source at an equatorward extension of the south polar coronal hole /3,4/. This streain was observed clearly fifteen times, recurring every -26 days through early-July 1993. During this interval, the length of the high-speed wind intervals gradually increased, and the low-speed excursions shortened, as Ulysses spent more time in the fast coronal hole wind and less time in the slow wind from the heliomagnetic streamer belt. A CME in November 1992 resulted in instantaneous wind speeds of -1000 km s-l, matching the previous Ulysses wind speed record from the in-ecliptic mission phase. In April 1993 the minimum wind speed increased from -400 to -550 km s-l, roughly at the same time as the last encounter with the heliospheric current sheet /5/.

In August 1993 the minimum wind speed increased to roughly 675 km s-l; since that time, Ulysses has been immersed continuously in the high-speed wind. Analysis of the IMF polarity confirmed that this wind originated from the south polar coronal hole. The coronal hole configuration was roughly constant during mid- to late-1993, indicating that the disappearance of low-speed wind was caused by the increasing southerly latitude of the spacecraft /6/. The envelope of modulation in the wind speed narrowed further with increasing latitude, with roughly a 700 to 800 km s-l range for the latest data shown in this study.

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1

I I I. J. ,I. _.I,’ ,_, I. , 3 10s ZOS 30s 40s 50s 60s

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16Feb92 4Aug92 21Jan93 lOJul93 27Dec93 15Jun94

Fig. 2. Six-hour averaged solar wind proton speed (top trace, labels on left axis) and proton density, scaled to 1 AU (bottom trace, labels at right), observed by the Ulysses solar wind plasma experiment from 16 February 1992 through 14 June 1994. Labels at top (bottom) indicate heliocentric distance (heliographic latitude) of spacecraft.

JASR 16:9-6

Q)88 J. L Phillips cf al.

The polar trajectory of Ulysses allows calculation of latitudinal variations in fundamental solar wind parameters, subject of course to the inherent limitations of single-spacecraft measurements, a non- circular orbit, and a changing Sun. Figure 3 shows a solar-rotation-averaged synopsis of these variations for solar wind speed, density, mass flux, and momentum flux, in heliographic coordinates. Note that the relatively smooth variation in wind speed from 10’ to 40* results from varying sampling of fast and slow wind, even though the actual separation between these two wind types is quite distinct. As shown in previous studies of meridional gradient8 in the solar wind /e.g., 7/ mass flux and momentum flux are much more constant than either speed or density as a result of the well- documented negative correlation between the latter two quantities. Latitudinal variations in the internal state of the solar wind plasma observed by Ulysses have been presented elsewhere for ions /8/ and electrons /9,10/. Results of those studies included a finding that the solar wind proton distributions are much hotter at high latitudes than at low latitudes, and that the proton entropy in the high-latitude wind has a nearly adiabatic radial profile 181.

IV,“.‘. 1 .‘.I 5 15 25 35 45 55 65

I. I. I. I. I.

5 15 25 35 45 55 65

5 15 25 35 45 55 65

II’ ’ ” . ’ . ’ ” 1

15 25 35 45 55 65 South Heliographic Latitude (deg)

Fig. 3. Median solar wind fluid parameters as functions of heliographic latitude for post-Jupiter observations. Plotted values are medians of solar-rotation bins, for mass- weighted proton and helium measurements. Parameters shown are wind speed (top panel), mass density (panel 21, mass flux (panel 3), and momentum flux (bottom). All parameters except speed are scaled to 1 AU by multiplying by R* (AU).

Ulysses Solar Wind Plasma Obserwtioas

COROTATING INTERACTION REGIONS AND SHOCKS

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Ulysses results have significantly added to our knowledge of corotating interaction regions (CIRs), shocks, and global heliospheric structure. Although distinct CIRs were observed during the in- ecliptic mission, it was after Jupiter encounter that they became most striking. Compressions driven by fast wind overtaking slow wind can steepen into forward- and reverse-propagating shock waves, most often at heliocentric distances beyond -2 AU. Figure 4 shows solar wind speed (top) and density (bottom) for an 80-day interval including CIRs and numerous shocks at 1Y to 19” south heliographic latitude. Forward (reverse) shocks are marked by solid (dashed) vertical traces. The first CIR shown is relatively simple, with a forward shock and reverse shock bounding a compressed high-density region (days 226, August 15, through 234, August 23) which is expanding into the lower-density wind on both sides. The third fast stream also produced a clear-cut CIR (days 279, October 7, through 286, October 14), while the second stream involves a much more complicated pattern with multiple forward and reverse shocks.

900

800

700

600

500

400

300

200

-Forward Shock - - - - - Reverse Shock -Foreshock

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225 235 24.5 2.55 265 275 285 295 305

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225 235 245 255 265 275 285 295 305

5.30 AU Day of Year 1992 5.18 AU

15” s 19” s

Fig. 4. Solar wind speed (top) and proton density (bottom) for days 225 (August 14) through 305 (November 2) of 1992. Forward (reverse) shocks are marked by solid (dashed’) vertical traces, and electron foreshocks are marked by heavy bars.

(9)90 J. L. Phillips et al.

An unexpected finding is that the CIR-related forward and reverse shocks have extended electron foreshocks. Enhanced field-aligned fluxes of suprathermal (-50 to -500 eV) electrons are commonly observed streaming away from the shocks upstream (prior to forward shock passage and after reverse shock passage) of these shocks /l I/. Foreshocks are marked by heavy bars in the top panel of Figure 4; note that the foreshocks end at forward shocks and/or start at reverse shocks. The suprathermal electrons indicate magnetic connection to the shocks; shock-heated electrons leak into the upstream region and stream generally sunward along the IMF, thus opposing the normal solar wind electron heat flux /ll/. The signatures of these foreshocks are superficially similar to the counterstreaming electron beams sometimes used for identification of CMEs in the solar wind /e.g., 12/, thus complicating the CME identification process beyond -2 AU. Of the two forward shocks with no observable foreshocks, the one on day 273 (October 1) was clearly driven by a fast CME, based on analysis of plasma properties following shock passage. The other shock with no foreshock, on day 298 (October 26), was probably also CME-driven, though results are less conclusive for this event, CME-related forward shocks usually have geometries which cause suprathermal electrons upstream from the shock to stream antisunward, in the same sense as the prevailing electron heat flux, thus hindering foreshock identification. Figure 5 shows schematically the magnetic geometries expected for CME-driven forward shocks and for CIR-driven forward and reverse shocks.

CME - DRIVEN SHOCK

COROTATING SHOCKS

Slow Wmd

Fig. 5. Schematic showing IMF-shock geometry appropriate for interpreting counterstreaming electron events upstream from shocks in interplanetary space. Solid arrows indicate foreshock electron flux directions, while hollow arrows indicate normal solar wind electron heat flux and counterstreaming heat flux directions in CMEs.

Ulysses Solar Wind Plasma Observations (9)91

During 1993, a pattern emerged in corotating shock occurrence and characteristics. Simply put, the forward shocks vanished while the reverse shocks persisted /13/. The last forward and reverse corotating shocks observed at Ulysses through June 14, 1994 are marked on Figure 2 and occurred on June 29, 1993 at -33.6” and April 3, 1994 at -58.2”, respectively. Figure 6 illustrates the pattern of corotating shock occurrence. Shocks that are clearly CME-related are not shown here, but a few unrecognized CME-driven shocks may be included. Shock strength, as measured by the downstream- to-upstream density ratio minus unity, is plotted (above the zero axis for forward shocks, below for reverse shocks) versus heliographic latitude from -10” to -60”. Note that, with a single exception, no

forward shocks were observed after the spacecraft passed a latitude corresponding to the heliomagnetic current sheet tilt of 29’, while reverse shocks were encountered far southward of that latitude, Flow deflections downstream of the forward (reverse) shocks were usually equatorward

(poleward), indicating equatorward (poleward) propagation of the forward (reverse) shocks /13/.

Forward

Reverse

L’I k 18% s 311 k (‘I’# 0 * ~‘~“~I

-12 -20 -30 -40 -50 -60

Heliographic Latitude (Deg)

Fig. 6. Shock strength versus latitude for corotating shocks observed by Ulysses from -10” to -60” heliographic latitude. Forward shocks are plotted above the horizontal axis, and reverse shocks are plotted below. The shock marked by a question mark occurred during a data gap and has uncertain strength. Adapted from 1151 by deletion of obviously CME-related shocks.

These phenomena were interpreted /13,14,15/ to result from the tilt of the heliomagnetic streamer belt relative to the heliographic equator. The subsequent tilt of the region of compressed plasma in interplanetary space caused equatorward propagation of the leading forward shock and poleward propagation of the trailing reverse shock. Figure 7 contains a schematic illustrating these processes. A band of slow wind from the heliomagnetic streamer belt encircles the Sun with a modest tilt corresponding to the tilt of the solar magnetic equator relative to the heliographic equator. This band is surrounded by regions of fast solar wind from the polar coronal holes. Fast wind overtakes slow wind in interplanetary space along interfaces that are inclined relative to the equator in the same sense as the streamer belt. With increasing heliocentric distance the interaction regions expand as the forward and reverse waves bounding the CIRs propagate into the slow wind ahead and the fast wind behind, respectively. The forward waves propagate westward (to the right in Figure 7) and equatorward in both hemispheres, while the reverse waves propagate eastward (to the left) and poleward. Eventually the waves steepen into shocks, as in the sequence of CIRs shown in Figure 4. Downstream flow deflections are westward and equatorward for the forward shocks and eastward and poleward for the reverse shocks. The reverse shocks propagate to higher latitudes than the band of slow solar wind. Thus as the spacecraft moved southward, it exceeded the high-latitude limit for forward shocks long before the corresponding limit for reverse shocks /13,14,15/.

(9192 J. L. Phillips etal.

Fast

East Limb

West Llmb

interaction Region Far From Sun

Fig. 7. A sketch illustrating the origin of tilted interaction regions in interplanetary space in terms of a tilted-dipole geometry at the Sun. From /13/.

The validity of the meridional shock propagation model can be demonstrated by examining the last corotating shock observed by Ulysses, at -58.2”. Figure 8 shows 90 minutes of solar wind data surrounding this shock: wind speed (top) and polar angle (bottom), with positive polar angle corresponding to northward flow. Note that at shock time (1518 UT) the flow shifted northward, corresponding to a southward upstream-to-downstream flow deflection and to southward propagation of this reverse shock. The bottom panel shows the source surface heliomagnetic neutral line calculated from Wilcox Solar Observatory photospheric field observations, as well as the position of Ulysses on April 3, mapped back to the corona based on observed wind speed. The shock was observed at a mapped solar longitude just trailing the southerly excursion of the neutral sheet. This position is exactly what would be expected for poleward propagation of a reverse shock in a tilted streamer belt configuration (see Figure 7). Thus the reverse shocks associated with the main equatorial CIR can propagate well poleward of the actual interface between coronal hole and streamer belt wind, with important ramifications for cosmic ray propagation and energization of solar wind particles.

SUMMARY

The solar wind plasma experiment aboard Ulysses has returned, and continues to return, comprehensive solar wind measurements along the unique spacecraft trajectory. During the in- ecliptic mission phase, Ulysses encountered a variety of fast, slow, and CMF! solar wind. After Jupiter encounter, the irregular wind structure quickly gave way to a pattern of fast wind from the south polar coronal hole and slow wind from the equatorial streamer belt. Poleward of 35” south latitude, Ulysses has encountered only fast wind. Meridional gradients in solar wind fluid parameters are qualitatively in agreement with previous results from the ecliptic plane and from remote measurement techniques.

Ulysses Sok Wind Phma obpcrvotous

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Carrington Longitude (deg)

Fig. 8. Ninety-minute time series of solar wind speed (top panel) and flow polar angle (middle) surrounding a CIR-related reverse shock (vertical trace). The bottom panel shows the source surface neutral line position for the corresponding Carrington rotation, plus the position of Ulysses at shock time, mapped ballistically to the corona.

The rich Ulysses plasma data set has yielded new results on many topics including, for example, ion distribution functions /16,17/ and high-latitude CMEs /e.g., 181. We have focused here on CIRs and shocks because of their ramifications for global heliospheric structure. Ulysses results on this subject have shown that the influence of the main heliomagnetic streamer belt can extend to latitudes well poleward of the actual streamer belt plasma. The resulting shocks can accelerate solar wind ions or interplanetary pickup ions to high energies, can create backstreaming suprathermal electrons which may contribute significantly to the halo electron population of the solar wind in the inner heliosphere, and may affect the access to the inner solar system by galactic cosmic rays.

ACKNOWLEDGMENTS

Data reduction support by R.K. Sakurai (JPL) and S.J. Kedge (LANL) were crucial to this study. We appreciate the use of Wilcox Solar Observatory source surface field calculations by J.T. Hoeksema. Work at Los AIamos was carried out under the auspices of the U.S. Department of Energy. Work carried out at the Jet Propulsion Laboratory of the California Institute of Technology was performed under contract to the National Aeronautics and Space Administration.

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18. J.T. Gosling, Proceedings ofthe Third SOHO Workshop, Kluwer, in press (1994).