A Fabry-Perot annular summing spectroscopy technique has been sued at the University of Wisconsin's Pine Bluff Observatory to acquire geocoronal Balmer-(alpha) line profile data with significantly improved precision and height resolution. The double-etalon Fabry-Perot interference pattern is imaged onto a photometrics PM512 CCD chip, thus enabling light to be gathered in multiple spectral bins simultaneously. In comparison with scanning systems we used earlier, the high quantum efficiency of the CCD and the multi-channel detection associated with the Fabry-Perot annular summing technique have enabled us to save a factor of about 10 in the integration time required for studies of the line profile. As a result, we are now able to both more precisely observe the line shape of the very faint Balmer- (alpha) emission and obtain data using shorter integration times. Our data illustrate the scientific potential for using this technique for the study of very faint extended emission line sources. The increase in the signal-to-noise of our data has enabled us to examine Balmer-(alpha) profile asymmetries which we have found to be compatible with predictions that on the order of 10 percent of the geocoronal Balmer-(alpha) excitation arises from cascades due to higher-member solar Lyman series excitation. This fine structure was overlooked in previous Balmer-(alpha) studies aimed at determining non-Maxwellian dynamical properties of exospheric hydrogen; we find that cascade excitation largely masks the expected very small dynamical perturbations to the line profile at low shadow heights, and must be more thoroughly studied before drawing conclusions about exospheric dynamics. Accounting for cascade laos leads to more realistic determinations of exospheric hydrogen temperatures near the exobase.
[1]
J. D. Garcia,et al.
Energy Level and Line Tables for One-Electron Atomic Spectra*
,
1965
.
[2]
S. Atreya,et al.
Doppler profile measurements of the geocoronal hydrogen Balmer alpha line
,
1975
.
[3]
R. J. Reynolds.
A measurement of the hydrogen recombination rate in the diffuse interstellar medium
,
1984
.
[4]
Ronald J. Reynolds,et al.
Fabry-Perot/CCD multichannel spectrometer for the study of warm, ionized interstellar gas and extragalactic clouds
,
1990,
Astronomical Telescopes and Instrumentation.
[5]
Robert R. Meier,et al.
Hydrogen Balmer alpha intensity distributions and line profiles from multiple scattering theory using realistic geocoronal models
,
1987
.
[6]
J. Meriwether,et al.
Measurements of the spectral profile of Balmer alpha emission from the hydrogen geocorona
,
1980
.
[7]
F. Roesler,et al.
Geocoronal Balmer Alpha Line Profiles and implications for the exosphere
,
1985
.
[8]
F. Roesler,et al.
Intensity variations of geocoronal Balmer alpha emission: 1. Observational results
,
1985
.
[9]
Raymond G. Roble,et al.
How will changes in carbon dioxide and methane modify the mean structure of the mesosphere and thermosphere
,
1989
.
[10]
A. Hedin.
Extension of the MSIS Thermosphere Model into the middle and lower atmosphere
,
1991
.