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Ground-based observations of geocoronal Balmer alpha line profiles are presented. Observations were made with a large-aperture PEPSIOS spectrometer coupled to a flat mirror siderostat to eliminate contamination from nearby airglow lines and galactic emissions. Based on a single scattering Lyman beta excitation model, the observed line profiles are found to be narrower than predicted on the basis of Chamberlain's theory [1963, 1976] and also seem inconsistent with the Monte Carlo calculations of Hodges, et al. [1981]. Comparison with the OGO 5 Lyman alpha line width measurements [Bertaux, 1978] and Lyman alpha intensity measurements suggests that the narrow portion of the profiles may be due to emissions from satellite atoms with small eccentricities. The possibility that the narrow emissions are produced by cool hydrogen in the lower thermosphere is also discussed. The observed profiles are asymmetric, having an extended red wing greater than predicted by Chamberlain's theory [1963, 1976] but consistent with an enhanced escape flux due to charge exchange. The position of the red wing is also consistent with the presence of additional fine structure components possibly caused by a collisional excitation mechanism. Although some of the conclusions must be regarded as tentative until refined modeling including multiple Lyman beta scattering is completed, it seems clear that measurements of the type described can lead to improved understanding of geocoronal hydrogen.

Strong ultraviolet emissions from the upper atmosphere of Uranus suggest that both auroral and electroglow phenomena are of significant aeronomical consequences in the structure of the upper atmosphere. Combined modeling and data analysis were performed to determine the effect of electroglow and auroral phenomena on the global heat and atomic hydrogen budgets in the Uranus upper atmosphere. The results indicate that the auroral and electroglow heat sources are not adequate to explain the high exospheric temperature observed at Uranus, but that the atomic hydrogen supplied by these processes is more than sufficient to explain the observations. The various superthermal electron distributions modeled have significantly different efficiencies for the various processes such as UV emission, heating, ionization, and atomic hydrogen production, and produce quite different H2 band spectra. However, additional information on the UV spectra and global parameters is needed before modeling can be used to distinguish between the possible mechanisms for electroglow.