Observations of Solar Cyclical Variations in Geocoronal H Alpha Column Emission Intensities

Understanding the influence of the solar cycle variation on the Earth's upper atmosphere is important for determining the basic state of this region, as well as for distinguishing between natural variability and possible observational signatures of climate change [Roble, 1995]. Atomic hydrogen spans the thermosphere and exosphere, becoming increasingly dominant with altitude. Geocoronal hydrogen is the byproduct of middle and upper atmospheric chemical, photolysis, and charge exchange reactions involving its hydrogenous source species below such as methane, water vapor, and molecular hydrogen. Observations of thermospheric exospheric Hα column emissions by the Wisconsin Hα Mapper (WHAM) Fabry-Perot (Kitt Peak, Arizona) over the 1997-2001 rise in solar cycle 23 show a statistically significant solar cycle variation. The variation observed by WHAM in the thermospheric exospheric hydrogen column emission intensities over the solar cycle is small compared with variations in hydrogen exobase densities, and the solar excitation flux, with higher Hα emissions seen during solar maximum periods of the solar cycle. The magnitude of the increase in these Hα column emission intensities between solar minimum and solar maximum conditions depends upon viewing geometry; at the mid range shadow altitude of 3000 km, WHAM terrestrial Hα column emission intensities are about 45% higher during solar maximum conditions. The higher signal-to-noise WHAM observations corroborate suggestions of a solar cycle trend seen in Wisconsin Hα emission observations over the previous solar cycle (cycle 22). We present comparisons of WHAM 1997 and 2000-2001 winter solstice Hα observations toward regions of the sky with low galactic emissions. We will also show analysis currently in progress to compare these data with those taken during the previous solar cycle (cycle 22). We will discuss factors necessary to isolate possible geophysical trends from long term comparisons of thermospheric exospheric Hα emission observations. Requirements include a stable calibration source, cross-calibrated, well-understood instrumentation, reproducible observing conditions, separation of the terrestrial from the galactic emission line, and consistent data analysis, accounting for differences in viewing geometry.