Temperatures and Altitudes of Jupiter's Ultraviolet Aurora Inferred from GHRS Observations with the Hubble Space Telescope

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We observed the jovian UV auroral regions with the Goddard high resolution spectrograph (GHRS) on board the Hubble Space Telescope (HST) on Apr. 29, May 2, and June 10, 1995. Observations of target areas were made in pairs in the two wavelength ranges 1257–1293 Å and 1587–1621 Å. Spectra in the long wavelength range are dominated by emissions of the H2 Lyman band system and show well separated rotational features, which we have used to determine the temperatures of the auroral emission regions. Spectra in the short wavelength range are mostly due to emission in the H2 Lyman and Werner band systems, but their intensities are reduced by hydrocarbon absorption. The brightest spectral pair was observed toward an area with longitude 155° and jovicentric latitude 58° when the central meridian longitudes (CMLs) were 191° and 203°. This area was found to be bright in our previous HST observations in 1993 and in HST faint object camera images. Assuming that electron impact excitation is the major source of the jovian aurora, we estimate total emission rates in the Lyman band system of about 270 and 46 kR for the long and short wavelength spectra of the pair, respectively. The attenuation of emission rate in the short wavelength spectrum implies a methane column density of about 3 × 1016cm−2, and a temperature of about 450 K is inferred from the long wavelength spectrum of the brightest pair. For all six pairs of observed spectra, we estimate methane column densities in the range (1–7) × 1016cm−2, which, when compared to a standard mid-latitude model, corresponds to a pressure range from a few μbar to a few tens of μbar. The temperatures derived are in the range 400–850 K with a possible tendency toward lower temperatures for higher methane column densities. This tendency and the uncertainty in the temperatures derived may indicate that the temperatures increases rapidly with altitude around the methane homopause in the auroral regions.