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

A survey of the plasma environment within Jupiter's bow shock is presented in terms of the in situ, calibrated electron plasma measurements made between 10 eV and 5.95 keV by the Voyager Plasma Science Experiment (PLS). These measurements have been analyzed and corrected for spacecraft potential variations; the data 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 PLS 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 × 10²/cm³. These data clearly define the bow shock, the magnetosheath (30–50 eV) the magnetosphere (10 −2 /cm³,2–3 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 defines a regime in the magnetosphere where very cold electron plasma (as low as 50 eV) at 40 R J 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 ‘spikes.’ In all cases the plasma sheet crossing lasts for intervals commensurate with that defined by the diamagnetic depression in the simultaneously measured and displayed magnetic field. The electron temperatures in the plasma sheet in the outer and middle magnetosphere 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 enhancement of the plasma sheet the thermal sub‐population is generally enhanced more than the suprathermal population. The suprathermal fraction of the electron density within the plasma sheet is an increasing function of jovicentric distance. Direct, in situ sampling of the electron plasma environment of Io'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 J there are sizeable systematic variations of the macroscopic and microscopic parameters; there are at least three electron thermal regimes within the torus. These three regimes have mean electron energies in the outer, temperate, and inner torus of the order of 100, 10–40, and less than 5 eV, respectively. The distribution functions in these regimes are always non‐Maxwellian with the suprathermal population an increasing fraction of the density and partial pressure with increasing distance from Jupiter. The common non‐Maxwellian character of the electron torus plasma unequivocally implies that the electrons and ions cannot locally have the same temperature if binary Coulomb collisions are the only scattering present in the plasma torus. The direct in situ torus electron spectra are shown to be compatible with a number of indirect assessments of the electron state in the torus including observations of plasma hiss, whistler Landau damping, gyro‐harmonic emissions, possible asymmetric sink for collisional ionization of sodium, and capacity to ionize sulfur whose presence is implied by the optical and EUV measurements. It is also suggested that the Io plasma torus is the limiting form of the plasma sheet, possibly being its complete direct source, since a progression in the fractional number in the cold, or thermal number density is clear: this fraction is 0.999 in the inner torus, but only 0.5 in the plasma sheet at 40 R J . We have tentatively concluded that the radial temperature profile within the plasma sheet is caused by the intermixing of two different electron populations that probably have different temporal histories and spatial paths to their local observation. The cool plasma source of the plasma sheet and spikes is probably the Io plasma torus and arrives in the plasma sheet as a result of flux tube interchange motions or other generalized transport which can be accomplished without diverting the plasma from the centrifugal equator. The hot suprathermal populations in the plasma sheet have most recently come from the sparse, hot mid‐atitude ‘bath’ of electrons which are directly observed juxtaposed to the plasma sheet. As the cool plasma is diluted by filling an increasing volume as it undergoes radial expansion, the outer hot bath of electrons can increasingly dominate until at sufficient radial distance the sheet per se does not exist anymore.