Constraining Balmer Alpha Fine Structure Excitation Measured in Geocoronal Hydrogen Observations

Cascade contributions to geocoronal Balmer alpha airglow line profiles are directly proportional to the Balmer beta/alpha line ratio and can therefore be determined with near simultaneous Balmer beta observations. Due to scattering differences for solar Lyman beta and Lyman gamma (responsible for the terrestrial Balmer alpha and Balmer beta fluorescence, respectively), there is an expected trend for the cascade emission to become a smaller fraction of the Balmer alpha intensity at larger shadow altitudes. Near-coincident Balmer alpha and Balmer alpha data sets, obtained from theWisconsin H alpha Mapper Fabry-Perot, are used to determine the cascade contribution to the Balmer.. line profile and to show, for the first time, the Balmer beta/alpha line ratio, as a function of shadow altitude. We show that this result is in agreement with direct cascade determinations from Balmer alpha line profile fits obtained independently by high-resolution Fabry-Perot at Pine Bluff, WI. We also demonstrate with radiative transport forward modeling that a solar cycle influence on cascade is expected, and that the Balmer beta/alpha line ratio poses a tight constraint on retrieved aeronomical parameters (such as hydrogen's evaporative escape rate and exobase density).