Galileo ultraviolet spectrometer observations of Jupiter's auroral spectrum from 1600-3200 A

In 1996 and 1997 the Galileo Ultraviolet Spectrometer (UVS) obtained the first measurements of Jupiter's nightside mid-UV (MUV) polar auroral spectrum from 1620 to 3231 A at 13-A resolution. The reduced polar spectra, after removal of off-axis scattered radiation from the sunlit dayside of Jupiter, contain a spectrum that matches laboratory spectra of the H2 continuum in the a-b dissociative emission transition. This is the first direct identification of the H2 a-b transition in astronomy. The a-b emission is excited by electron impact exchange reactions with H2 that peak in cross section near 15 eV. The emission threshold is at 1216 A, and the continuum peaks in intensity in the 2000-2500 A range. Jupiter's observed wavelength-integrated MUV H2 a-b emissions (1620-3231 A) have a photon flux about eight times smaller than simultaneously observed wavelength-integrated FUV H2 band emissions (1230-1650 A). Because the FUV H2 emissions have an emission cross section that peaks at higher energies near 50 eV, this FUV/MUV brightness ratio is diagnostic of the secondary electron energy distribution and is consistent with a 'warm' distribution of electrons. (Author)