Hot Plasma Environment at Jupiter: Voyager 2 Results

Abstract

Measurements of the hot (electron and ion energies ≥20 and ≥ 28 kiloelectron volts, respectively) plasma environment at Jupiter by the low-energy charged particle (LECP) instrument on Voyager 2 have revealed several new and unusual aspects of the Jovian magnetosphere. The magnetosphere is populated from its outer edge into a distance of at least ∼ 30 Jupiter radii (R_J) by a hot (3 x 10⁸ to 5 x 10⁸ K) multicomponent plasma consisting primarily of hydrogen, oxygen, and sulfur ions. Outside ∼ 30 R_J the hot plasma exhibits ion densities from ∼ 10^–1 to ∼ 10^–6 per cubic centimeter and energy densities from ∼ 10^–8 to 10^–13 erg per cubic centimeter, suggesting a high β plasma throughout the region. The plasma is flowing in the corotation direction to the edge of the magnetosphere on the dayside, where it is confined by solar wind pressure, and to a distance of ∼ 140 to 160 R_J on the nightside at ∼ 0300 local time. Beyond ∼ 150 R_J the hot plasma flow changes into a "magnetospheric wind" blowing away from Jupiter at an angle of ∼ 20° west of the sun-Jupiter line, characterized by a temperature of ∼ 3 x 10⁸ K (26 kiloelectron volts), velocities ranging from ∼ 300 to > 1000 kilometers per second, and composition similar to that observed in the inner magnetosphere. The radial profiles of the ratios of oxygen to helium and sulfur to helium (≤ 1 million electron volts per nucleon) monotonically increase toward periapsis, while the carbon to helium ratio stays relatively constant; a significant amount of sodium (Na/O ∼ 0.05) has also been identified. The hydrogen to helium ratio ranges from ∼ 20 just outside the magnetosphere to values up to ∼ 300 inside; the modulation of this ratio suggests a discontinuity in the particle population at ∼ 50 to 60 R_J. Large fluctuations in energetic particle intensities were observed on the inbound trajectory as the spacecraft approached Ganymede, some of which suggest the presence of a "wake." Five-and 10-hour periodicities were observed in the magnetosphere. Calculations of plasma flow velocities with the use of Compton-Getting formalism imply that plasma is mostly corotating to large radial distances from the planet. Thus the Jovian magnetosphere is confined by a plasma boundary (as was implied by the model of Brice and Ioannidis) rather than a conventional magnetopause. Inside the plasma boundary there exists a discontinuity at ∼ 50 to 60 R_J we have named the region inside this discontinuity the "inner plasmasphere."