Solar cycle variations of geocoronal Balmer α emission

Abstract

Observations of the geocoronal Balmer α nightglow have been made from Wisconsin for more than a solar cycle with an internally consistent intensity reference to standard astronomical nebulae. These measurements were made with a double‐etalon, pressure‐scanned, 15‐cm aperture Fabry‐Perot interferometer. The resulting long time line data provides an opportunity to examine solar cycle influence on the mid‐latitude exosphere and to address accompanying questions concerning the degree to which the exosphere is locally static or changing. Our exospheric Balmer α absolute intensity measurements show no statistically significant variations throughout the solar cycle when the variation with viewing geometry is removed by normalizing the data to reference exospheric model predictions by Anderson et al. However, the relative intensity dependence on solar depression angle does show a solar cycle variation. This variation suggests a possible related variation in the exospheric hydrogen density profile, although other interpretations are also possible. The results suggest that additional well‐calibrated data taken over a longer time span could probe low‐amplitude variations over the solar cycle and test predictions of a slow monotonic increase in exospheric hydrogen arising from greenhouse gases.