Spatial and temporal variations of the Jovian auroral electrons deduced from HST-STIS ultraviolet spectroscopy

Jovian auroral spectra have been observed with the Space Telescope Imaging Spectrometer (STIS) in both hemispheres since 1997, including during the recent Cassini Jupiter millenium campaign. The time-tagged G140L STIS spectra used for this study cover the 1100-1700 Ì· range which includes the Lyman (B-X) bands and continuum and the Werner (C-X) bands. Time-dependent spectroscopy with spatial resolution along the 25 arcsec slit makes it possible to follow spectral and temporal variations in different regions of the aurora. FUV images observed on the same HST orbit were obtained to put the spectral data into the global morphology context. Synchronous observations of spectral domains absorbed or not by CH4 and other hydrocarbons are used to determine the FUV color ratio and the depth of the auroral emission relative to the methane homopause. Large variations of the color ratio across the auroral oval are observed. The time evolution of the mean electron energy and its possible correlation with the precipitated energy flux are investigated. It is found that the mean electron energy generally positively correlates with the auroral brightness. In contrast, flares occasionally observed in the polar cap can correspond to a softening or a hardening of the precipitation. The mean electron energy associated with the Io trail is approximately constant and shows less hydrocarbon absorption than the higher latitude aurora.