Galileo‐measured depletion of near‐Io hot ring current plasmas since the Voyager epoch

Abstract

The first mass‐discriminated, hot ion distribution moments (pressure, energy intensity) are determined for hot >50‐keV ions in Jupiter's inner magnetosphere at the outer edge of Io's plasma torus by using the Galileo energetic particle detector (EPD) data. These hot plasmas were significantly depleted during the Galileo encounter in 1995 as compared with the Voyager epoch of 1979. The depletion of the hot ions is apparently caused by enhanced charge exchange losses of hot ions, perhaps associated with enhanced emissions of neutral gases from the volcanoes of Io. Such neutral gas enhancements could simultaneously explain increases, reported elsewhere, in the densities of the cooler Io torus plasmas. The hot plasma changes may explain why radial transport interchange turbulence has been observed by Galileo in the Io torus regions, whereas such turbulence was not apparent during the Voyager encounters in 1979. The hot ion depletion could also play a role in explaining the apparent differences between the Jovian auroral configuration observed in recent years by the Hubble space telescope and ground observers and the configuration observed by Voyager. This possibility is much less certain, however.