| Abstract |
Long term observations of geocoronal hydrogen offer potential to
contribute to our understanding of vertical coupling processes affecting
hydrogen-containing species. For example, models predict a 40-75%
increase in exospheric hydrogen associated with a doubling of
tropospheric concentrations of carbon dioxide and methane [Roble, AGU
meeting, Spring 2005; Roble and Dickinson, 1989; Ehhalt, 1986].
Understanding the influence of the solar cycle on the upper atmosphere
is required to isolate signatures of natural variability from those due
to human-caused change. Mid-latitude ground-based Fabry-Perot
Interferometer observations of geocoronal hydrogen fluorescence
emissions by Wisconsin observers now span more than two decades and
include observations taken near Madison, Wisconsin, as well as more
recent observations from the Kitt Peak Observatory, near Tucson Arizona.
Observations of thermospheric + exospheric Balmer-alpha column
emissions by the Wisconsin H- alpha Mapper (WHAM) Fabry-Perot (Kitt
Peak, Arizona) show a statistically significant solar cycle variation
over solar cycle 23 with intensities at midrange shadow altitudes
approximately 45% higher during solar maximum compared with solar
minimum conditions. The higher signal-to-noise WHAM observations
corroborate suggestions of a solar cycle trend in observations of
Balmer-alpha emissions from Wisconsin over solar cycle 22. Geocoronal
hydrogen column emissions are a function of both the vertical density
structure and the incoming solar UV radiation, both of which change over
the course of the solar cycle. Multiple scattering of solar Lyman line
radiation below the Earth's shadow complicates the analysis of the
observations requiring the use of detailed data/forward modeling
comparisons to retrieve geophysical information such as the hydrogen
column abundance from the observations [Bishop, 1999; Bishop et al.,
2004]. We will discuss our previous solar minimum observations and plans
for observations during the upcoming solar minimum period, including
the challenges and strategies associated with comparing data taken with
different, but similarly designed, instruments at different mid-latitude
observatories. We will also discuss work in progress on parameter
definitions in the forward modeling technique, tested in terms of
sensitivity under differing observing applications (e.g. lone station
measurements versus coincident measurements acquired by several
independent instruments). |