The extreme ultraviolet dayglow of Jupiter

Abstract

Processes involving both resonant scattering and fluorescence of the incident solar flux in the wavelength region 300 Å–1300 Å have been considered for the upper atmosphere of Jupiter, and calculations have been performed for the intensities of the spectral features that would be observed from outside the atmosphere. The principal features were found to be the resonance lines of atomic hydrogen and helium at 1216 Å and 584 Å respectively. It is noted that a significant amount of radiation can be carried in the wings of the line when there are large amounts of scattering atoms present. The radiative transfer problem for this case is formulated and solved for the hydrogen and helium radiations. The emerging intensity of the resonantly scattered helium line depends upon the relative total amounts of H 2 and He in the atmosphere, the dependence arising through photoabsorption of the helium line by molecular hydrogen. The emerging intensity also depends upon the temperature and diffusion processes. The hydrogen Lyman alpha line will appear due to resonance scattering with a minor contribution due to the decay of excited hydrogen atoms formed through photodissociative excitation. The intensity of the HI Lyman alpha glow is a function of the amount of atomic hydrogen present, which in turn depends upon diffusion processes in the atmosphere and is less dependent upon the relative amounts of hydrogen and helium. Simultaneous measurements of these lines may be used to determine the relative total amounts of molecular hydrogen and helium in the atmosphere. Minor features in the Jovian ultraviolet spectrum will be the following. The HeII λ303 Å line is formed primarily through photoionization excitation and is found to be less than 5 R in intensity. The H 2 Lyman bands, produced through resonance fluorescence, will be of approximately 25 R in intensity. The Rayleigh and Raman scattered HI Lyman-α lines will be virtually unobservable, being of order 0.05 R or less.