Detection of self-reversed Lyα lines from the Jovian Aurorae with the Hubble Space Telescope

We present Lyα profiles taken at high spectral resolution (∼70 mÅ) in the Jovian aurorae, using the Goddard High-Resolution Spectrograph aboard the Hubble Space Telescope. They exhibit a strong central reversal, reminiscent of solar and stellar spectra. This effect was predicted by models as a consequence of radiative transfer effects on photons excited deep in the atmosphere by energetic charged particles from the magnetosphere. However, it had not been detected until these observations, the first detection of a reversed planetary Lyα line. The peaks on both sides of the reversal are separated by about 0.1 to 0.15 Å depending on the region, indicating faint and variable atomic hydrogen vertical column densities of about (1-5) × 10¹⁶ cm^-2 above the auroral source. We find that the line profiles are asymmetric, with differences in intensity between the blue and red peaks. The reversals and the nearby H₂ emission lines seem to be 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 extending over more than 1.5 Å, possibly because of emission from a superthermal hydrogen population.