Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001jgr...10628797k&link_type=abstract
Journal of Geophysical Research, Volume 106, Issue A12, p. 28797-28818
Physics
10
Atmospheric Composition And Structure: Airglow And Aurora, Atmospheric Composition And Structure: Thermosphere-Composition And Chemistry, Atmospheric Composition And Structure: Transmission And Scattering Of Radiation, Ionosphere: Topside Ionosphere
Scientific paper
Measurements of the geocoronal Balmer-alpha (Hα) brightness have been made at the Arecibo Observatory during 11 separate periods since 1983 using both a Fabry-Perot interferometer and a tilting filter photometer. The tilting filter photometer is calibrated for absolute sensitivity using a constant brightness source traceable to National Institute of Standards and Techniques (NIST) standards and is used to cross-calibrate the Fabry-Perot interferometer. Since the observational technique has not changed since 1983, and since the data analyses technique are uniform, these data provide a measure of the solar cycle variation of Hα brightness at Arecibo. Unlike earlier studies, we discern no systematic discrepancy between the Hα brightness and estimates of the solar Lyman-beta flux that pumps the resonant fluorescent Hα emission. Rather, we conclude that geocoronal hydrogen abundance is generally (but not always) larger than models suggest, although not systematically so. The largest Hα emission is measured during solar minimum conditions, when brightness is approximately 50% greater than during solar maximum conditions, for measurements at solar depression angles from 15° to 35°. Above about 40° solar depression (corresponding to an illuminated column with a base at approximately 2000 km), no solar cycle variation is evident, and the brightness there is persistently greater than models. Intricacies of the Lyman-beta radiative transport problem make detection of an early morning maximum of hydrogen density near the exobase difficult using brightness data alone, although the Hα brightness is generally asymmetric with respect to local midnight, with brighter emission in the postmidnight sector. Studies of the semidiurnal (nighttime) variation demonstrate that the Hα brightness can vary by a factor of 2 over a period of days.
Doe Richard A.
Friedman Jerome
Garcia Rafael
González Sixto A.
He Xuehao
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