Ultraviolet Spectroscopy of Jupiter's Auroras from Galileo

W. R. Pryor, C. W. Hord, C. A. Barth, A. I. F. Stewart, W. E. McClintock, K. E. Simmons (LASP/U. of Colorado), J. M. Ajello (JPL), W. K. Tobiska (Nyma/TELOS/JPL), R. A. West, G. James (JPL), D. E. Shemansky (U. of Southern California), B. R. Sandel (U. of Arizona), J. T. Clarke (U. of Michigan)

Jupiter auroral results from the EUV (54-128 nm) and UVS (113-432 nm) spectrometers on Galileo will be presented. EUV is on the orbiter spinning section, while the UVS is on the scan platform. Several orbits of data are examined, showing both spatial and temporal variations in the aurora. EUV (54-128 nm) spin-scan images of Jupiter show polar emissions from the (B, B', B'', C, D, D' - X) systems of H2 and Lyman alpha emission from H. EUV spectra show H2 self-absorption, due to an H2 column penetration depth of 10**20 cm**-2. Absorption by atomic H is seen below 91.2 nm. UVS FUV auroral spectra (113-192 nm) are similar to UVS lab spectra of 100 eV electron impact on H2. Differences are mostly due to CH4 absorption. Slant methane columns of 2*10**16 cm**-2 in the north and 5*10**16 cm**-2 in the south were seen on the G1 orbit. Simultaneous EUV and FUV spectra constrain the vertical distribution of CH4 in Jupiter's atmosphere. The FUV spectra are similar to almost simultaneous HST spectra. UVS MUV spectra (162-323 nm) of Jupiter's dark side contain H2 a-b continuum emission. This band system is diagnostic of secondary electron energy, because its excitation cross-section is strongly peaked at 15 eV. The UVS spectra obtained together with the G7 SSI auroral image were particularly bright.