The University of Wisconsin’s Long-Term Geocoronal Hydrogen Data Set

The University of Wisconsin has observed geocoronal hydrogen Balmer-alpha emissions since the late 1970s from Wisconsin and during Solar Cycle 23 from the Kitt Peak, AZ, observatory. These observations were all obtained using ground-based double etalon Fabry-Perot Interferometers. The signal-to-noise of the observations has increased along with the sensitivity of the instrumentation. Earlier observations used scanning photomultiplier detection, and the Solar Cycle 23 observations employed CCD annular summing spectroscopy. We are using this long-term data set to investigate the impact of the solar cycle variation upon upper atmospheric hydrogen, a byproduct of radiatively important water vapor and methane, below. The Wisconsin Northern hemisphere mid-latitude data set shows consistent solar cycle dependence over two solar cycles and three solar minima with column emission intensities a factor of ~1.5 higher during solar maximum than solar minimum conditions. We review the Wisconsin long-term data record and present ongoing efforts to compare these observations to empirical and climate models of the upper atmosphere through the use of forward modeling. The observed trend of higher intensities during solar maximum periods is consistent with predictions. However, there are significant differences in magnitude between the predicted and observed intensities and solar cycle variation.