Constraining Balmer Alpha Fine Structure Excitation Measured in Geocoronal Hydrogen Observations

Cascade contributions to geocoronal Balmer α airglow line profiles are directly proportional to the Balmer β ∕α line ratio and can therefore be determined with near simultaneous Balmer β observations. Due to scattering differences for solar Lyman β and Lyman γ (responsible for the terrestrial Balmer α and Balmer β fluorescence, respectively), there is an expected trend for the cascade emission to become a smaller fraction of the Balmer α intensity at larger shadow altitudes. Near-coincident Balmer α and Balmer β data sets, obtained from the Wisconsin H alpha Mapper Fabry-Perot, are used to determine the cascade contribution to the Balmer α line profile and to show, for the first time, the Balmer β∕α line ratio, as a function of shadow altitude. We show that this result is in agreement with direct cascade determinations from Balmer α 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 β ∕α line ratio poses a tight constraint on retrieved aeronomical parameters (such as hydrogen’s evaporative escape rate and exobase density).