A study of the auroral upper atmosphere of Uranus with Voyager UVS data.

Voyager 2 Ultraviolet Spectrometer (UVS) data over 500-1700 A for the darkside auroral emission spectra of Uranus, obtained during Jan.25-30, 1986 have been used to produce thirteen averaged spectra. The analysis of the intensities of six emission peaks at 925, 972, 1027, 1045, 1110, 1258 A and the total EUV intensity over 874-1143 A have shown strong correlations among them: between total EUV intensity and $I\sb{925},\ I\sb{972},\ I\sb{1110}.$ Also, $I\sb{925}{-}I\sb{1258},\ I\sb{1027}{-}I\sb{972}$ and $I\sb{1045}{-}I\sb{1258}$ correlate well. For comparison, one spectrum each for the darkside north polar aurora of Jupiter and Saturn has also been reduced. Model emission spectra have been produced including H$\sb2$ band and H Lyman series transitions via downward auroral electron transport through plane-parallel atmospheric layers. Volume emission rates emerging from the top of the atmosphere have been calculated as a function of wavelength, taking into account absorption and re-emission. By comparing model emission peak intensities against the correlations of observation, the total energy input at the top of the local auroral atmosphere has been determined as 0.04-0.06 erg/cm$\sp2$/s with an excitation efficiency of $\approx$10%. The range of the average electron energy per particle $(\alpha)$ has been determined to be very narrow $\alpha = 3{-}5$ keV. The model analysis has revealed that there seem to be two distinctly different aurora in the darkside of Uranus. The ones observed near the terminator have the auroral altitude temperatures of 600-1000$\sp\circ$K, $\alpha = 3{-}4$ keV and very intense. The location, temperature and emission intensity are well matched with those of the north polar aurora observed by the Voyager. This leads to another conclusion that the other aurora with locations away from the terminator, a much higher temperature (1500$\sp\circ$K), a higher $\alpha$ of $\sim$5 keV and less intense emission could be the south polar aurora. Hence, this study has shown that the north and south polar aurora seem to be different from each other in those aspects and their parameter ranges have been established. When the case of emitted radiation undergoing only absorption is compared with the case of absorbed radiation re-emitted, spectra with re-emission seem to represent the auroral spectra of Uranus better than absorption-only spectra. The lack of emission features near 1600 A might require increase in the abundance of ethylene (C$\sb2$H$\sb4$) via possible elevation of the homopause due to increased temperature. The spectral shape in the extreme ultraviolet region seems mainly determined by the ratio $I\sb{972}$/$I\sb{1027}$ which depends strongly on H$\sb2$ Werner band. The Werner band is very sensitive to temperatures greater than 1000$\sp\circ$K. Careful consideration of the contributions of H Ly-$\beta$ and Ly-$\gamma$ in this spectral region is also suggested.PhDAstronomyPhysics, Atmospheric SciencePure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131014/2/9825269.pd