Published online by Cambridge University Press: 27 October 2009
Voyager 1 and 2 performed the first unambiguous low-energy (E ≥ 30 keV) ion measurements in and around the Jovian magnetosphere in 1979. The magnetosphere contains a hot (kT ~ 30 keV), multicomponent (H, He, O, S) ion population dominated by convective flows in the corotation direction out to the dayside magnetopause and on the nightside to ~ 130–150 Rj beyond which the ion flow direction changes to predominantly antisolar, but with a strong component radially outward from Jupiter. This tailward flow of hot plasma, the magnetospheric wind, accounts for the loss of ~ 2 × 1027 ions/s and ~ 2 × 1013 W from the magnetosphere. Comparison of energetic (≥ 30 keV) ion to magnetic field pressure reveals that particle and magnetic pressures are comparable from the magnetopause inward to at least ~ 10 Rj, that is, magnetosphere dynamics is determined by pressure variations in a high-β plasma. This particle pressure is responsible for inflation of the magnetosphere and it (rather than the planetary magnetic field) determines the standoff distance with the solar wind. The ion spectrum can be described by a convected Maxwellian component at E ≤ 200 keV, and a nonthermal tail at higher energies described by a power law of the form E−γ. New theoretical techniques were developed in order to interpret the low-energy solid-state detector measurements of temperature, number densities, pressures, and flow velocities in this novel hot-plasma environment.