Spectroscopic Evidence for High-Altitude Aurora at Jupiter from Galileo Extreme Ultraviolet Spectrometer and Hopkins Ultraviolet Telescope Observations

Abstract

The Galileo Extreme Ultraviolet Spectrometer (EUVS) and the Hopkins Ultraviolet Telescope (HUT) acquired UV spectra of Jupiter Aurora in the period from 1995 through 1997. The EUVS spectra spanned the wavelength range 540–1280 Å and the HUT spectra measured the extreme ultraviolet and far ultraviolet (EUV+FUV) wavelength range 830–1850 Å. Both sets of spectra present evidence of high-altitude, optically thin H 2 band emissions from the exobase region. The analysis of the UV spectra with a two-stream electron transport model and a jovian model auroral atmosphere indicates that the primary electron flux is composed of both soft and hard electrons with characteristic energies in the soft electron energy range of 20–200 eV and the hard electron range of 5–100 keV. The soft electron flux causes enhanced EUV emission intensities below 1100 Å. The soft electron flux may explain the high temperature of the upper atmosphere above the homopause as measured from H 3 + rovibrational temperatures in the IR. For the deep aurora, a high primary characteristic energy above 5 keV is known to be present. The Galileo Energetic Particle Detector (EPD) has measured the electron distribution functions for energies above 15 keV in the middle magnetosphere. The high-energy distribution functions can be modeled by a combination of Maxwellian and kappa distributions. However, the EUV (800–1200 Å) portion of the HUT spectrum cannot be modeled with a single distribution of hard electrons as was possible in the past for the FUV (1200–1650 Å) spectrum measured by itself. The combination of EUV and FUV spectral observations by HUT serves to identify the amount of soft electron flux relative to the hard primary flux required to produce the high-altitude aurora in the neighborhood of the exobase.