Galileo ultraviolet spectrometer observations of Jupiter's auroral spectrum from 1600–3200 Å

Abstract

In 1996 and 1997 the Galileo Ultraviolet Spectrometer (UVS) obtained the first measurements of Jupiter's nightside midultraviolet (MUV) polar auroral spectrum from 1620 to 3231 Å at 13 Å 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 H 2 continuum in the a ‐ b dissociative emission transition. This is the first direct identification of the H 2 a ‐ b transition in astronomy. The a ‐ b emission is excited by electron impact exchange reactions with H 2 that peak in cross section near 15 eV. The emission threshold is at 1216 Å, and the continuum peaks in intensity in the 2000–2500 Å range. Jupiter's observed wavelength‐integrated MUV H 2 a ‐ b emissions (1620–3231 Å) have a photon flux ∼8 times smaller than simultaneously observed wavelength‐integrated far‐ultraviolet (FUV) H 2 band emissions (1230–1650 Å). Because the FUV H 2 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.