Monitoring the Outer Region of the Neutral Atmosphere

Hydrogen is a primary constituent of the geocorona and is a chemical byproduct of species below such as methane and water vapor, two greenhouse gases. The solar cycle is a dominant source of natural variability in this region and must be accounted for when isolating the effects of coupling processes from below, including that due to potential long-term change in the region. Observations by the Wisconsin H-alpha Mapper Fabry-Perot of geocoronal hydrogen Balmer-alpha emissions over solar cycle 23 have quantified a factor of 1.5 ± 0.15 higher intensities at solar maximum than at solar minimum. These observations are consistent with Fabry-Perot observations from Wisconsin during solar cycle 22. All observations have been consistently calibrated for intensity using the North American Nebula. We used the LYAO_RT radiative transfer code of Bishop to compare the observed Balmer-alpha intensities with intensities calculated using the hydrogen density distribution in the Mass-Spectrometer-Incoherent-Scatter (MSIS) model, a major empirical model used by the middle and upper atmospheric research communities. The MSIS distribution yields a solar maximum to minimum ratio similar to that observed; however, significant differences in magnitude between predicted and observed intensities highlight the need for improved upper atmospheric hydrogen density determinations.