HYDROGEN FROM JUPITER ATMOSPHERE IN THE JOVIAN MAGNETOSPHERE

The passage of Ulysses through Jupiter's magnetosphere presents a new opportunity to investigate the contribution to the Jovian magnetospher e of ions of atmospheric origin. A determination of the magnetospheric H+/He2+ flux ratio allows an estimate of the relative abundance of io nospheric material in the Jovian magnetosphere. We find that the H+/He 2+ flux ratio, measured in the energy/charge range between 0.65 and 60 keV/e, steadily increases from a solar wind level of 25 at the magnet opause to a value of 700 at the point of closest approach, and then st eadily decreases whilst approaching the magnetopause on the outbound p ath. We conclude from this that: (1) there is a significant solar wind component throughout the outer and middle magnetosphere; and (2) a si gnificant fraction of the protons in the middle magnetosphere are of n onsolar origin.