Ulysses plasma electron observations in the Jovian magnetosphere

J. L. Phillips, S. J. Bame, B. L. Barraclough, D. J. McComas, R. J. Forsyth, P. Canu, P. J. Kellogg

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Abstract

The electron analyzer of the solar wind plasma experiment aboard the Ulysses spacecraft measured the plasma properties of the Jovian magnetosphere and surrounding regions during the February 1992 encounter with Jupiter. In addition to sampling a new region of the Jovian magnetosphere, the Ulysses electron measurements were unique in that they included fine directional sampling over nearly 4π sr in look direction, for electron energies up to 862 eV. In this paper we present an overview of electron bulk parameters and a sampling of distribution shapes for the magnetosphere and adjacent plasma regions. The magnetopause was crossed 10 times, with each crossing characterized by a boundary layer with mixed magnetospheric and magnetosheath-like electron distributions. The spacecraft transited the dayside, prenoon, equatorial magnetosphere inbound, and the near-terminator, dusk magnetosphere at mid-latitudes outbound. The middle and inner magnetosphere contained a distinct plasma sheet, well-ordered in magnetic latitude, while the outer magnetosphere was less cleanly structured. Field-aligned electron anisotropies were observed throughout the magnetosphere. Unique electron spectra were seen in probable open-field line regions at the highest sampled magnetic latitudes in the inner magnetosphere. Azimuthal flow velocities indicated significant departures from rigid corotation, perhaps representing a combination of corotation and tailward flow. Radial plasma flows were slightly outward in the prenoon plasma sheet and strongly outward throughout the dusk terminator region, but were inward at higher latitudes on the dayside. Strong southward flows were observed in the outer magnetosphere on the dusk side.

Original languageEnglish (US)
Pages (from-to)877-892
Number of pages16
JournalPlanetary and Space Science
Volume41
Issue number11-12
DOIs
StatePublished - 1993

Bibliographical note

Funding Information:
and M. F. Thomscn (LANL). B. E. Goldstein. M. Neugcbauer During February 1992, U/~~.Y.YtLrãnSs ited the Jovian and R. Sakurai (JPL). and J. C. Chavez (Sandin National Lab- plasma environment. The SWOOPS solar wind plasma oratories). WC thank A. Balogh and R. G. Stone. respectively, experiment returned ;t unique set of ion and electron Ibr providing ohscrvations from the Magnetomctcr and IJnified measurementsi n the magnetosphere,m agnetosheatha nd Radio and Plastna Wave expcrimcnts. We especially apprecintc magnetopause boundary layer. Subject to the inherent COSPIN cxpcrimcnt in advance of’ publication, court&y 01 access to plasma velocity determinations from the I ‘/~~xsc~s limitations of solar wind instruments in a planetary K. Staincs.W ork at Los Alamos WIS carriedo ut under the auspices environment. the instruments performed flawlessly. In this of’ the U.S. Dcpartmcnt of’ Energy with financial support from paper WC have presented the complete set of c’l~~.s.s~~.s NASA. plasma electron observations from the Jovian magneto- spherea nd boundary layer. These observations are unique in that they included the transit of the duskside, near- terminator magnetosphere. and in that the instrument returned spectra with a wealth of directional detail. Noteworthy observational results include : (I ) each of the IO magnetopause crossings showed evidence for a boundary layer, with coexisting sheath-like and hottci magnetospheric electron populations: (2) no obvious magnetic-latitude ordering exists for the magnetopause location, though any such ordering would likely be masked by compressive effects; (3) while the inner magnetosphere transits showed clear, periodic plasma sheet crossings near the lowest sampled magnetic latitudes. the structure was much less distinct beyond SO 60 R,: (3) the plasma sheets were characterized by an approximately tenfold increase in density and by slight decreases in total electron temperature; (5) the mag- nctosphcric plasma in general exhibited anisotropics in the senseo f T, > T, , with most pronounced anisotropics in the plasma sheet; (6) at highest magnetic latitudes in the inner magnetospheret he spacecraft transited regions of apparently open magnetic field. Within these regions, the hot magnetospheric plasma was depleted, 21m id- energy component resembling the solar wind or map- netosheath was observed, and an unexplained modi- fication of the spectra at lowest observable energies occurred : (7) azimuthal plasma flows are sub-corotating or anti-corotating in the prenoon sector and super- corotatinp on the dusk side. suggestinga combination of co-

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