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 greater than or equal to 15 and greater than or equal to 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 x 108K, density about 5 x 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 (greater than or equal to 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 approx. 900 to approx. 1300 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