Line profile of H Lyman α from dissociative excitation of H2 with application to Jupiter

Abstract

Observations of the H Lyman α (Ly α) emission from Jupiter have shown pronounced emissions, exceeding solar fluorescence, in the polar aurora and equatorial “bulge” regions. The H Ly α line profiles from these regions are broader than expected, indicating high‐energy processes producing fast atoms as determined from the observed Doppler broadening. Toward understanding that process a high‐resolution ultraviolet (UV) spectrometer was employed for the first measurement of the H Ly α emission Doppler profile from dissociative excitation of H2 by electron impact. Analysis of the deconvolved line profile reveals the existence of a narrow central peak of 40±4 mÅ full width at half maximum and a broad pedestal base about 240 mÅ wide. Two distinct dissociation mechanisms account for this Doppler structure. Slow H(2p) atoms characterized by a distribution function with peak energy near 80 meV produce the peak profile, which is nearly independent of the electron impact energy. Slow H(2p) atoms arise from direct dissociation and predissociation of singly excited states which have a dissociation limit of 14.68 eV. The wings of H Ly α arise from dissociative excitation of a series of doubly excited states which cross the Franck‐Condon region between 23 and 40 eV. The profile of the wings is dependent on the electron impact energy, and the distribution function of fast H(2p) atoms is therefore dependent on the electron impact energy. The fast atom kinetic energy distribution at 100 eV electron impact energy spans the energy range from 1 to 10 eV with a peak near 4 eV. For impact energies above 23 eV the fast atoms contribute to a slightly asymmetric structure of the line profile. The absolute cross sections of the H Ly α line peak and wings were measured over the range from 0 to 200 eV. Analytic model coefficients are given for the measured cross sections which can be applied to planetary atmosphere auroral and dayglow calculations. The dissociative excitation process, while one contributing process, appears insufficient by itself to explain the line broadening observed at Jupiter.