DETECTION OF SELF-REVERSED LY-ALPHA LINES FROM THE JOVIAN AURORAE WITH THE HUBBLE-SPACE-TELESCOPE

We present Ly alpha profiles taken at high spectral resolution (simila r to 70 m Angstrom) in the Jovian aurorae, using the Goddard High-Reso lution Spectrograph aboard the Hubble Space Telescope, They exhibit a strong central reversal, reminiscent of solar and stellar spectra, Thi s effect was predicted by models as a consequence of radiative transfe r effects on photons excited deep in the atmosphere by energetic charg ed particles from the magnetosphere. However, it had not been detected until these observations, the first detection of a reversed planetary Ly alpha line. The peaks on both sides of the reversal are separated by about 0.1 to 0.15 Angstrom depending on the region, indicating fain t and variable atomic hydrogen vertical column densities of about (1-5 ) x 10(16) cm(-2) above the auroral source. We find that the line prof iles are asymmetric, with differences in intensity between the blue an d red peaks. The reversals and the nearby H-2 emission lines seem to b e tied to the planet's rest frame, whereas the auroral photons may be shifted by as much as a few kilometers per second in either direction. Finally, the auroral profiles display almost symmetric weak wings ext ending over more than 1.5 Angstrom, possibly because of emission from a superthermal hydrogen population.