Fabry-Perot Observations of the Hydrogen Geocorona

Within the framework of this dissertation, the Fabry-Perot annular summing optical system was completely redesigned and constructed in order to minimize vignetting and to facilitate improved intensity, wavelength, and linewidth calibration for the study of geocoronal Balmer α. A signal to noise ratio of approximately 50 was obtained for a typical geocoronal Balmer α intensity in a ten minute integration, covering a 75 km/s velocity interval with 3.75 km/s velocity resolution, from a 1.5° beam on the sky.

This newly designed Fabry-Perot annular summing spectrometer was operated for two years (2000–2001) at the University of Wisconsin's Pine Bluff Observatory (PBO). An extensive geocoronal Balmer α data set of approximately 1500 spectra over 71 nights was obtained; this represents the highest quality geocoronal Balmer α line profile data set to date.

This dissertation reviews past geocoronal observations, the atomic physics associated with geocoronal Balmer α emission, and Fabry-Perot annular summing spectroscopy. The design, calibration, and performance of the PBO Fabry-Perot are discussed at length. The full 2000–2001 PBO geocoronal Balmer α data set is presented including: Balmer α intensities, Doppler widths, and data regarding cascade contributions to the emission. A diurnal signal is clearly observed in the line intensity, but not in the line width. A significant decrease in Balmer α Doppler width with increasing shadow altitude was detected every night in which a wide range of shadow altitudes was observable. Cascade contributions to the Balmer α emission were found to be approximately 5%, consistent with recent estimates. Preliminary applications of the nonisothermal radiative transport code lyao_rt to a subset of the PBO data indicates good general agreement both with regard to trends in the emission line intensity and to line width, indicating that a new level of understanding of the geocorona is likely to emerge through forward-modeling analysis.