GALILEO ULTRAVIOLET SPECTROMETER OBSERVATIONS OF JUPITERS AURORAL SPECTRUM FROM 1600-3200 ANGSTROM

In 1996 and 1997 the Galileo Ultraviolet Spectrometer (WS) obtained th e first measurements of Jupiter's nightside midultraviolet (MUV) polar auroral spectrum from 1620 to 3231 Angstrom at 13 Angstrom resolution . The reduced polar spectra, after removal of off-axis scattered radia tion from the sunlit dayside of Jupiter, contain a spectrum that match es laboratory spectra of the H-2 continuum in the a-b dissociative emi ssion transition. This is the first direct identification of the H-2 a -b transition in astronomy. The a-b emission is excited by electron im pact exchange reactions with H-2 that peak in cross section near 15 eV . The emission threshold is at 1216 Angstrom, and the continuum peaks in intensity in the 2000-2500 Angstrom range. Jupiter's observed wavel ength-integrated MUV H-2 a-b emissions (1620-3231 Angstrom) have a pho ton flux similar to 8 times smaller than simultaneously observed wavel ength-integrated far-ultraviolet (FUV) H-2 band emissions (1230-1650 A ngstrom). Because the FUV H-2 emissions have an emission cross section that peaks at higher energies near 50 eV, this FUV/MUV brightness rat io is diagnostic of the secondary electron energy distribution and is consistent with a ''warm'' distribution of electrons.