JOURNAL OF GEOPHYSICAL RESEARCH, VOL. tee, NO. Ate, PAGES 19,473-19,486, OCTOBER t, 1995 Hot ions in Jupiter's magnetodisc: A model for Voyager 2 Low-Energy Charged Particle measurements M. Kane 1 Department of Physics and Astronomy, TheJohns Hopkins University, Baltimore, Maryland B. H. Mauk, E. P. Keath, and S. M. Krimigis Applied Physics Laboratory, TheJohns Hopkins University, Laurel, Maryland Abstract. The Low-EnergyCharged Particle(LECP) instrument on the Voyager 2 spacecraft acquired a comprehensive setof directional andenergy-dependent information on the natureof hot ions in the Jovianmagnetodisc. The LECP measurements in the energy range30 keV to 5 MeV, wherethe ion pressure dominates the total plasma pressure, have been successfully fit to a two-species convected K distribution function modelfor hot ions in the Jovian magnetodisc in the vicinity of neutral sheet crossings. The regions where themodel could be used ranged from60 to 30 Rj on thedayside (inbound) and 75 to 125 Rj onthe nightside (outbound). Withthis model, the full angular andspectral information from the lowest-energy LECP detectors has been deconvolved using a nonlinear least squares technique to reveal the heavy ion pressure, density, and temperature distinct from the corresponding hot proton parameters. The pressure is dominated by heavy ions in theouter magnetosphere. The temperature of protons remains nearly constant at 20 keV (dayside) and 10 keV (nightside), whereas the heavy ion temperature shows a distinct increase with radial distance paralleling the corotation or pickup energy of heavy ions. A neutral wind of heavy atoms, originating in the near-Io regions andionized during theirflightthrough the outer magnetosphere by solar radia- tion, may be the seed population for the heavy ionsmeasured by the LECP. The convec- tion velocity of the plasma is subcorotational, reduced from the rigid value by a factor of -2, butincreases withincreasing distance from 30 to 60 R• in thedayside region and from 75to 85R, in the nightside region. The trend stops beyond 85R•in the nightside region, but there J isstill a substantial corotational flow that extends from 85 R•toatleast 130 R•.In all the regions studied, the particle anisotropies in the LECP scan plane below -2 MeV are believed to result primarily from the Compton-Getting effectandnot from gradient anisotropies or particles executing nonadiabatic orbits as theyencounter the neutral sheet. Gradient anisotropies are not important evenin the distant nightside neutral sheet region (>85 R j) below -2 MeV. The large flow velocities and increasing heavy ion temperatures areconsistent with a strong corotational electric field andimply thatthe mass loading dueto lower-energy heavy ion plasma via outward transport from Io is insufficient to disrupt corotation within -60 R• during theVoyager 2 encounter. 1. Introduction The Voyager 1 and 2 encounters provideda wealth of informationon the nature of the hot plasmathat fills the Jovian magnetodisc. Yet many questions have remained unanswered. For example, to what extent does the magnetodisc corotate with Jupiter?What mechanisms es- tablish corotation, if present [e.g., Sands and McNutt, 1988; Carbary et al., 1981; Hill, 1979]? How are the hot ions energized [e.g., Barbosa, 1986; Cheng, 1990; Paranicas et al., 1990]? How are the heavy ions from •Also at Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland. Copyright 1995 by the American Geophysical Union. Paper number 95JA00793. 0148-0227/95/95JA-00793505.00 Io transportedto the outer (taken to be beyond 30 Rj) magnetosphere? With regard to corotation and flows in general,the an- gular response of the Low-Energy Charged Particle (LECP) detectors on Voyagers 1 and 2 to the convecting plasma of Jupiter's magnetodisc has been analyzed [Carbary et al., 1981] using the angular anisotropy intro- duced by the Compton-Getting effect [Comptonand Get- ting, 1935]. Their analysis was limited to -1 MeV protons, where a power law spectrum was assumed. Their calcu- lated convection velocities generally increase with increas- ing distance, with the peak values nearly rigidly corotational (at the rotationrate of Jupiter).Independent measurements have also been made using the plasmasci- ence (PLS) detector [Belcher et al., 1980; McNutt et al., 1979, 1981; Sands and McNutt, 1988]. Their calculations indicate a decreasing speed beyond -20 Rj but are in agreementwith the Carbary et al. [1981] determinations. These locations, however,are not at the peak locations of 19,473