A Systematic Program for Ground-Based Fabry-Perot Observations of the Neutral Hydrogen Exosphere

Large gains in the sensitivity of Fabry-Perots for geocoronal research have been achieved at the University of Wisconsin employing the technique of CCD annular summing spectroscopy. Earlier 'demonstration observations' of this technique lead to a significant new understanding of geocoronal hydrogen excitation. This paper will outline a new ground-based observing program which is building on these earlier observations in order to obtain definitive data regarding the physical processes which govern the abundance and transport of atomic hydrogen in the earth's atmosphere. Two double-etalon Fabry-Perot spectrometers have been installed at the University of Wisconsin's Pine Bluff Observatory (WI) for the purpose of making a systematic series of high spectral resolution (R approximately equals 100,000) line profile, and intensity observations of geocoronal hydrogen nightglow. For the first time it will be possible to obtain coincident observations of geocoronal hydrogen Balmer-alpha and Balmer- beta with sufficient signal-to-noise for detailed line profile studies. Because the geocoronal Balmer-beta emission is about one tenth the intensity of Balmer-alpha, the fitness of this line has frustrated past attempts to determine its profile; however, gains in sensitivity afforded by the annular-summing technique make these new observations possible. It is anticipated that these simultaneous observations will provide a means by which to isolate previously observed perturbations to the Balmer-alpha line, the components of which may arise from both contributions due to quantum mechanical fine structure and the non-Maxwellian dynamics of the hydrogen exosphere. Each of these instruments employs the annular summing technique in which the Fabry-Perot's annular fringe pattern is imaged onto a low noise CCD chip. Using the property that equal area annuli correspond to equal spectral intervals, software is used to divide the CCD image into equal area annular bins, whereby the Fabry-Perot interference pattern is converted into a useful spectral profile. This paper will describe the instrumentation, and how it relates to the planned observational program.