Low-Energy Charged Particle Environment at Jupiter: A First Look

The low-energy charged particle instrument on Voyager was designed to measure the hot plasma (electron and ion energies ≳ 15 and ≳ 30 kiloelectron volts, respectively) component of the Jovian magnetosphere. Protons, heavier ions, and electrons at these energies were detected nearly a third of an astronomical unit before encounter with the planet. The hot plasma near the magnetosphere boundary is predominantly composed of protons, oxygen, and sulfur in comparable proportions and a nonthermal power-law tail; its temperature is about 3 × 10⁸ K, density about 5 × 10^–3 per cubic centimeter, and energy density comparable to that of the magnetic field. The plasma appears to be corotating throughout the magnetosphere; no hot plasma outflow, as suggested by planetary wind theories, is observed. The main constituents of the energetic particle population (≳200 kiloelectron volts per nucleon) are protons, helium, oxygen, sulfur, and some sodium observed throughout the outer magnetosphere; it is probable that the sulfur, sodium, and possibly oxygen originate at Io. Fluxes in the outbound trajectory appear to be enhanced from ∼90° to ∼130° longitude (System III). Consistent low-energy particle flux periodicities were not observed on the inbound trajectory; both 5- and 10-hour periodicities were observed on the outbound trajectory. Partial absorption of > 10 million electron volts electrons is observed in the vicinity of the Io flux tube.