A survey of the plasma electron environment of Jupiter: A view from Voyager

Abstract

A survey of the plasma environment within Jupiter's bow shock is presented in terms of the in situ, calibrated electron plasma measurementa made between 10 eV and 5. c .5 keV by the Voyager Plasma Science Experiment (PLS). These measurements have been analyzed and corrected for spacecraft potential variations; the :'.ata have been reduced to nearly model independent macroscopic parameters of the local electron density and temperature. The electron parameters are derived without reference to or internal calibration from the positive ion measurements made by the PL$ experiment. Extensive statistical and direct comparisons with other determinations of the local plasma charge density clearly indicate that the analysis procedures used have successfully and routinely discriminated between spacecraft sheath and ambient plasmas. These statistical cross correlations have been performed over the density range of 10-3 to 2 x 10 2 /cc. These data clearly define the bow shock, the magnetosheath (30-50 eV) the magnetosphere (10 -2 /cc, 2-3 keV) as well as the periodic appearances of the plasma sheet which are illustrated to be routinely cooler than the surroundings. The proximity of the plasma sheet define:. a regime in the magnetosphere where very cold electron plasma (as low as 50 eV) at 40 R can be seen in unexpected density enhancements. These plasma "spikes" in the density can often represent an order of magnitude enhancement above the ambient density and are correlated with diamagnetic depressions. These features have been seen at nearly all magnetic latitudes within the plasma sheet. The temperature within these spikes is lowered by similar factors indicating that the principal density enhancements are of cold plasma. The plasma sheet when traversed in the outer magnetosphere has a similar density and temperature morphology as that seen in these 11 3pikes". In all oases the plaama sheet crossing lasts for intervals commensurate with that defined by the diamagnetic depression in the simultaneously measured and uisplayed magnetic field. The electron temperatures in the plasma sheet in the outer and middle magnetosphore appear to have a positive radial gradient with jovicentric distance. The electron temperature is observed to be lower on the centrifugal, side of the minimum magnetic field strength seen in each sheet, while the suprathermal, electron density is enhanced symmetrically about the locally indicated magnetic equator. The electron distribution functions within the plasma sheet are markedly non-Maxwellian; during the density enhancemeW s of the plasma sheet tie thermal sub-population is generally enhanced more than the suprathermal population. The suprathermal fraction of the electron densf..y within the plasma sheet is an increasing function of jovicentric distance.Direct, in situ sampling of the electron plasma environment of lo's torus clearly illustrates that the system is demonstrably removed from local, thermodynamic equilibrium; these measurements illustrate that between 5.5 and 8.9 R, there are s...

IMF control and convection in the Jovian magnetosphere.

“…Namely, although there is a low density region beyond the outer boundary of the plasma disc it contains numerous spiky density enhancements where the electron temperature is low as compared to where the density is not enhanced. It has been noted that the enhanced density and reduced temperature within the spikes are similar to those observed in the outermost region of the plasma disc (SCUDDER et al, 1981). A very similar structure has been detected also in the Saturnian magnetosphere.…”

Excitation of the Ganymede Ultraviolet Aurora

“…Since the ambient plasma sheet electrons (Scudder et al 1981) do not suffice to generate even the relatively faint di †use aurora, we conclude that a local acceleration mechanism, analogous to that operating on Earth, is needed to explain both the 300 R bright spots and the di †use emission. We Ðnd that the tenuous nature of the upper atmosphere (Eviatar et al 2001) makes direct impact excitation by the energetic electrons observed by Galileo (Williams et al 1997a(Williams et al , 1997bParanicas et al 1999) most ine †ective.…”

“…Eviatar et al (2001) have developed a model of the polar Ganymedean ionosphere from which they estimate with nominal emission efficiency, obtained from Hall et al (1998), that a steady state air glow or continuous aurora on the order of 20 R can be maintained without a further input of energy. In this region of the Jovian magnetosphere, the electron population in the plasma sheet was found by V oyager to be made up of a core population of enhanced density cm~3 ) with a (n e B 5È20 temperature of about 20 eV, a suprathermal halo of about 1/10 the number density, and a temperature of 2 keV (Scudder et al 1981). This population can support at most an emission rate of about 10È40 R (or 3È12 R at 60¡ latitude).…”

“…We must therefore rule out the possibility that local thermal electrons are creating the background emission. If proper conditions hold, the plasma sheet electrons observed by V oyager (Scudder et al 1981) may contribute a signiÐcant fraction of the di †use aurora. In general, however, it appears more plausible to attribute it to a local acceleration mechanism.…”

Jupiter Radio Bursts and Particle Acceleration

“…The agreement between the computed dispersion and burst leading edge is obvious. For this calculation, a sheath density of 0.85 cm -3 and ray path length through the sheath of 35 R } were used, consistent with typical in situ observations of the magnetosheath from Voyager (Lepping et al 1981;Scudder, Sittler, & Bridge 1981). It is assumed that all frequencies are generated simultaneously in the source region.…”