HST spectra of the Jovian ultraviolet aurora: Search for heavy ion precipitation

Ultraviolet spectra using Hubble Space Telescope sampled between 1250 and 1 680 Angstrom, at spectral resolution less than or equal to 0.57 Angstrom ar e reported for characteristically bright regions of Jupiter's morning and a fternoon northern aurora. Several observed spectra exhibit sharply enhanced resolution. We interpret this as bright auroral emission foreshortened on the morning limb with a maximum intensity at least as high as 2000 kR. We h ave searched for evidence that the primary precipitating particles exciting the aurora include the heavy ions known to exist in Jupiter's plasma torus and magnetosphere. We have also searched for such ambient heavy ions and n eutrals at rest in the amoral ionosphere, the end products of previous prec ipitation, excited by the auroral cascade. We argue that primary emission w ould be characterized by a dramatically Doppler-broadened (similar to 10-15 Angstrom) and redshifted line profile resulting from the cascade process a nd the angle between the line of sight and the magnetic held lines in the a tmosphere. In contrast, ambient emission would be distinguished by narrow e mission lines. We have modeled the theoretical sulfur and oxygen line shape s for ion precipitation and conclude that electron precipitation is respons ible for most of the H-2 emissions. O ions contributed <13% of the precipit ating energy flux, and S ions contributed < 50%. This dominance suggests th at field-aligned magnetospheric currents are more important than energetiza tion of energetic ions and subsequent scattering by plasma waves as a mecha nism for generating the Jovian aurora. We set an upper limit over our spect ra of 35-43 R to the emission from ambient oxygen and sulfur ions and their neutrals, except that for the S II 1256 triplet, the upper limit for the n ominally brightest line, at 1260 Angstrom, is 74 R. Hence, we find no evide nce for the accumulation of sulfur in the auroral ionosphere. A single narr ow emission line from an unidentified ambient specie near 1254 Angstrom may be detected at the 4 sigma level, introducing the possibility of complex a uroral aeronomy. Differences were observed in the auroral spectral hydrocar bon absorption at different locations, which cannot be interpreted without ambiguity between auroral and atmospheric structural causes. We have found that the brighter emission in an auroral sector consistently shows more spe ctral hydrocarbon absorption than the dimmer emission. We suggest two alter native physical explanations for this phenomenon.