The Ultraviolet Spectrum of the Jovian Dayglow
Session 33 -- Solar System
Display presentation, Wednesday, June 14, 1995, 9:20am - 6:30pm

## [33.03] The Ultraviolet Spectrum of the Jovian Dayglow

W.Liu, A.Dalgarno (CfA)

The ultraviolet spectra of the molecules H$_{2}$ and HD by solar fluorescence and photoelectron excitation are calculated and compared with the Jovian equatorial dayglow spectrum obtained by the Hopkins Ultraviolet Telescope at $3$~\AA\ resolution near the maximum of solar cycle 22. The Jovian dayglow is accounted for in both the brightness and the spectral shape by solar fluorescence and photoelectron excitation, and no additional energy source is required. It is characterized by an H$_{2}$ rotational-vibrational temperature of $540$~K and an H$_{2}$ column density of $7 \times 10^{19}$~cm$^{-2}$. The dayglow spectrum is in agreement with the presence of cascade contribution to the Lyman band emission from the high-lying $E,F^{1} \Sigma_{g}^{+}$ states. The incorrectly perceived absence of appreciable cascade effects from comparison of the dayglow spectrum with the room-temperature laboratory spectrum of H$_{2}$ is caused by the fact that the spectrum of H$_{2}$ is sensitive to the temperature and the Jovian atmosphere is hotter. The observed weakness of the dayglow emission at short wavelengths is mainly caused by self-absorption by H$_{2}$. The dayglow spectrum suggests minimal absorption by hydrocarbon on Jupiter.

Wavelength coincidences of solar emission lines and absorption lines of H$_{2}$ and HD occur, resulting in strong fluorescence of H$_{2}$ and HD. The strong coincidence of the solar Lyman-$\beta$ line at $1025.72$~\AA\ and the $P(1)$ line of the $(6,0)$ Lyman band of H$_{2}$ at $1025.93$~\AA\ together with other line coincidences produce unique line spectra which are identified in the dayglow spectrum. The fluorescence due to absorption of the solar O~VI line at $1031.91$~\AA\ by the vibrationally excited H$_{2}$ via the $Q(3)$ line of the $(1,1)$ Werner band at $1031.86$~\AA\ is a sensitive measure of the atmospheric temperature. Despite strong coincidence of the solar O~VI line at $1031.91$~\AA\ and the $R(0)$ line of the $(6,0)$ Lyman band of HD at $1031.91$~\AA, the molecule HD remains to be undetective due to the weakness of the HD signature and the dominance of the H$_{2}$ emission.