COMPARISON OF IUE AND HST DIAGNOSTICS OF THE JOVIAN AURORAE

Analysis of international ultraviolet explorer (IUE) and Voyager ultra violet spectrometer (UVS) spectra, of the Jovian auroral emission indi cates that the Jovian auroral brightness is modulated in longitude (br ighter near 180 degrees in the north and 20 degrees in the south) and that there is a color ratio asymmetry associated with this brightening . The purpose of this study is to investigate the origin of this appar ent asymmetry. To that end, we use. a series of six typical images of the north auroral region taken in the. H-2 Lyman bands with the. faint object camera (FOG) aboard the Hubble. space telescope (HST) and whic h cover a full Jovian rotation. Although the images do not display any strong brightening near 180 degrees, once we have: simulated the sign al IUE would,see through its aperture, we find the characteristic long itudinal modulation. We attribute most of this modulation to a combina tion of viewing geometry effects near the. east and west ansae of the auroral oval (already taken into account in previous studies) and of t he spatial degradation of the source by the. IUE instrument function ( never considered so far), and we. suggest qualitatively that these eff ects may also affect the color ratio asymmetry. Nevertheless, part of the asymmetry seems to be due. to an intrinsic modulation associated w ith a bright feature crossing the polar cap along the 160 degrees meri dian (transpolar emission) and present in roost of the images. We then use a series of FOC images taken during an atypically strong auroral event, and we. show that the same effects can again account for the an omalous brightness variations observed simultaneously with IUE.