Sodium in the Jovian magnetosphere

Observations of sodium D-line emission from Io and the magnetosphere of Jupiter are reported. A disk-shaped cloud of sodium is found to exist in the Jovian magnetosphere with an inner edge at about 4 R_ch{cyrillic} and an outer edge at about 10 R_ch{cyrillic}. The gravitational scale height above the equatorial plane is a few Jovian radii. The data are interpreted in terms of a sputtering model, in which the sodium required to maintain the cloud is sputtered off the surface of Io by trapped energetic radiation-belt protons. Conditions on the atmospheric density are obtained. The Keplerian orbits attainable by such escaping sputtered atoms can provide the observed spatial distribution. The required 500-keV proton flux required to provide the 1-10 keV protons which will sputter the sodium at the surface of Io is consistent with the limiting trapped flux determined by ion-cyclotron turbulence.