Geocoronal H alpha intensity measurements using the Wisconsin H alpha Mapper Fabry-Perot facility

The Wisconsin Her Mapper (WHAM), a remotely operable, semi-automated Fabry- Perot located at Kitt Peak Observatory, has been making an all-sky survey o f interstellar hydrogen Balmer alpha (H alpha) emissions since 1997. Using the annular summing spectroscopy technique, WHAM has acquired similar to 37 ,000 spectra to date, spanning almost 100 nights of observations. Since all of the galactic emission spectral data contain the terrestrial H alpha (65 62.7 Angstrom) emission line, these measurements constitute a rich source o f geocoronal data for investigating natural variability in the upper atmosp here. The WHAM observations also serve as a benchmark for comparison with f uture data. Analysis of the first year of WHAM data shows only small day-to -day variations after shadow altitude variations are taken into account. Fo r example, at shadow altitudes of 2000 and 3000 km, the RMS scatter is with in approximately +/- 20%; this variability is expected to be reduced with a ccurate accounting of the smaller-scale effects of observational slant path , zenith angle, and azimuth on the H alpha intensity. This result is consis tent with past midlatitude Wisconsin data sets but different from observati ons made by other observers and instruments at the low-latitude Arecibo sit e. The multiple viewing geometries of the observations provide stringent mo deling constraints, useful in testing current modeling capabilities. Modeli ng of the WHAM data with a global nonisothermal resonance radiation transpo rt code (lyao_rt) indicates that the signal-to-noise of the data is suffici ent to determine relative variations in upper atmospheric atomic hydrogen c olumn densities to better than 5%. This paper describes the WHAM aeronomy p rogram and its observational scheme, analysis procedures, and results from data taken in 1997. Case study comparisons are made with past data sets and with predictions from the lyao_rt resonant radiation transport modeling co de of Bishop [1999].