Spatially Resolved Far Ultraviolet Spectroscopy of the Jovian Aurora

Abstract

Spatially resolved spectra in four 50-Å FUV spectral windows were obtained across the jovian aurora with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope. Nearly simultaneous ultraviolet imaging makes it possible to correlate the intensity variations along the STIS slit with those observed in the images and to characterize the global auroral context prevailing at the time of the observations. Spectra at ∼1-Å resolution taken in pairs included an unabsorbed window and a spectral region affected by hydrocarbon absorption. Both sets of spectra correspond to an aurora with a main oval brightness of about 130 kilorayleighs of H 2 emission. The far ultraviolet color ratios I(1550–1620 Å)/ I(1230–1300 Å) are 2.3 and 5.9 for the noon and morning sectors of the main oval, respectively. We use an interactive model coupling the energy degradation of incoming energetic electrons, auroral temperature and composition, and synthetic H 2 spectra to fit the intensity distribution of the H 2 lines. It is found that the model best fitting globally the spectra has a soft energy component in addition to a 10 erg cm −2 s −1 flux of 80 keV electrons. It provides an effective H 2 temperature of 540 K. The relative intensity of temperature-sensitive H 2 lines indicates differences between the auroral main oval and polar cap emissions. The amount of methane absorption across the polar region is shown to vary in a way consistent with temperature. For the second spectral pair, the polar cap shows a higher attenuation by CH 4, indicating a harder precipitation along high-latitude magnetic field lines.