Geocoronal Hydrogen Studies Using Fabry-Perot Interferometers

Ground based Fabry-Perot observations of solar excited geocoronal hydrogen fluorescence emissions are one of the primary means of studying the neutral upper atmosphere [Atreya et al., 1975; Meriwether et al., 1980; Yelle and Roesler, 1985; Shih et al., 1985; Kerr et al., 2001a,b; He et al., 1993; Nossal et al., 1993, 1998, 2004; Bishop et al., 2001; Mierkiewicz, 2002; and references therein]. Excellent reviews of early ground-based geocoronal Balmer α observations are found in: Krassovsky et al. [1966], Krassovsky [1971], Donahue [1964, 1966], Tinsley [1974], Fahr and Shizgal [1983] and Kerr et al. [2001a]. Instruments onboard satellites and rockets also observe the geocorona, but these observations will not be the focus of this paper, except in terms of collaboration with ground based passive optical instruments (see e.g., Bishop et al. [2004]).

The tenuous uppermost reach of the earth’s neutral atmosphere is commonly referred to as the exosphere or geocorona. For an overviews of the geocorona with a historical perspective, see e.g., Chamberlain [1963], Tinsley [1974], Donahue [1977]. The exosphere is a unique region of the atmosphere characterized by low densities, long mean free paths, and non-Maxwellian orbital dynamics. In addition to its interesting physics, geocoronal hydrogen is important because of its involvement in many upper atmospheric chemical, photolysis, and charge exchange reactions. Geocoronal hydrogen is the by-product of lower and middle atmospheric hydrogenous species chemistry below involving radiatively important species such as methane and water vapor. As such, observations of thermospheric+exospheric hydrogen offer the potential as verification of the representation by atmospheric models of vertical coupling in hydrogenous species chemistry and as a possible upper atmospheric footprint of global change. Understanding of sources of natural variability such as the influence of the solar cycle is needed to characterize this region and to isolate signatures of natural variability from those due to human caused change.