CEDAR: Physics of the Hydrogen Geocorona

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Long-term measurements of the geocoronal Balmer-alpha emission are continued at the Pine Bluff Observatory near Madison, WI, and at Kitt Peak, NM using Fabry-Perot interferometers. Unprecedented instrument sensitivity due to low-noise image plane detection allows these geocoronal measurements to be extended to include measurements of the sub-Rayleigh Balmer-beta emission. Due to the dissimilar optical transport properties of these two emissions from the same source, it is possible to extract accurate values of atomic hydrogen (H) density profiles from the upper thermosphere to several thousand kilometers. That atomic H is the neutral source of the topside ionosphere, and can be used by virtue of charge exchange equilibrium to determine the density of atomic oxygen (O) in the upper thermosphere. Once determined, the atomic H altitude profiles are used to determine the vertical flux of H. The long term data base is also used to determine the solar cycle variability of exospheric H, and to explore the possibility of secular variability that relates to changes in whole-atmosphere hydrogenous species abundance that may be changing in concert with methane deposition in the lower atmosphere. High spectral resolution measurements at Pine Bluff are also used to quantify effective temperature near and above the exobase, and to quantify atomic cascade contributions to the Balmer line profiles.