Geocoronal Fine-Structure Cascade Excitation Constraints for Ground-Based Observations and Modeling

Night-time Geocoronal hydrogen Balmer-alpha emission line-shapes, obtained by Fabry-Perot at Pine Bluff, WI, indicate a decrease in cascade contribution to the total Balmer-alpha observed intensity with viewing geometry (shadow altitude). Accurately accounting for cascade’s redwing line-shape contribution is critical to interpreting individual line-shape observations for residual exospheric dynamic signatures. Poor cascade (or Galactic background) model fits can mask sought after dynamics, leading to misinterpretation of the Balmer-alpha line profile, and erroneously high effective exospheric temperatures retrieved from the data-model fits.

Roesler et al. (2014) showed relative cascade contributions to Balmer-alpha profiles could be determined with near simultaneous Balmer-beta observations (i.e., by Balmer-beta/Balmer-alpha line ratio). Roesler et al. (2014) also noted that, due to multiple scattering differences in geocoronal hydrogen for Lyman-beta and Lyman-gamma (responsible for Balmer-alpha and Balmer-beta respectively), there is a trend for the cascade to become a smaller fraction of the Balmer-alpha intensity at larger shadow altitudes.

We have used near coincident Balmer-alpha and Balmer-beta data, obtained from the Wisconsin H-alpha Mapper (WHAM) Fabry-Perot, to parameterize the cascade contribution to the Balmer-alpha line profile as a function of shadow altitude. This result is in good agreement with direct cascade determinations from time-averaged Balmer-alpha line profile data, obtained by high resolution Fabry-Perot at Pine Bluff, WI. We will discuss the sensitivity of this line ratio to solar Lyman flux, and how it could be used to constrain the underlying Geocoronal hydrogen distribution.