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D. E. Shemansky, J. Tew Hallett, X. Liu, P. Gangopadhyay (Univ. Southern California), Cassini UVIS Team
A hydrogen physical chemistry model operating at the rotational structure level has been developed for application to the outer planet atmospheres. The model has been applied to a preliminary analysis of the Cassini UVIS observations of the Jupiter dayglow, and to comparison with the H2 quadrupole emissions obtained by Trafton in observations of Uranus. The Jupiter EUV/FUV H2 band emissions show dominance by very cold electron excitation mixed with solar photon flux fluorescence. The major source of emission is exospheric. The spectra show strong Rydberg series lines excited primarily also by cold electrons in the exosphere. The model predicts dominance of H3+ throughout the ionosphere with the exception of the extreme upper exosphere. The H2 predicted ground state vibrational and rotational population is damped by the ambient ionosphere electrons, but the activated gas shows populations as much as 10 to 15 orders of magnitude above LTE. H2 vibrational populations are forced by a positive feedback system in one branch of the recombination of H3+, by hot atomic hydrogen collisions with H2, and 3-body recombination of atomic hydrogen. Uranus, with lower ionosphere densities, is predicted to be more efficient in building H2 substantial ground state vibration populations. The present model, however, falls far short of predicting the observed H2 quadrupole emission power at Uranus, we believe primarily because the H + H2 vibrational excitation reaction is not fully incorporated into the model architecture. The determination of the extent of the role of direct solar input in the development of the dayglow will be determined only by a top-down ionospheric model calculation in order to define photoionization rates in vibrationally excited H2. On the basis of these calculations, heating of the upper thermospheres is primarily determined by electron impact dissociation of vibrationally excited H2. Funding for this work is supported by Cassini Program support of the UVIS experiment, and NASA Grant NAG5-8939 to the University of Southern California.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.