High resolution geocoronal Balmer alpha observations

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 airflow lines and galactic emissions. The observed line profiles are narrower than predicted on the basis of Chamberlain's theory (1963, 1976) and also seem inconsistent with the Monte Carlo calculations of Hodges, Rohrbaugh, and Tinsley (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 is caused by emissions from satellite atoms with small eccentricities. The possibility that the narrow line profiles are caused by emissions from the cool region of the thermosphere is also discussed. The observed profiles are also asymmetric, having an extended red wing greater than predicted by Chamberlain's theory (1963, 1976). A red wing in the line profile is the signature of escaping atoms, therefore the enhanced red wing may be evidence for an additional escape flux, probably due to charge exchange. The asymmetry is larger than predicted by the Monte Carlo calculations of Hodges, Rohrbaugh and Tinsley (1981), which include charge exchange for the night side of the globe, but is consistent with the Monte Carlo calculations on the day side