Geocoronal hydrogen Balmer‐α line profiles obtained using Fabry‐Perot annular summing spectroscopy: Effective temperature results

Abstract

A Fabry‐Perot annular summing spectroscopy technique has been used at the University of Wisconsin's Pine Bluff Observatory to acquire geocoronal Balmer‐α line profile data with significantly improved precision and height resolution. The double‐etalon Fabry‐Perot interference pattern is imaged onto a Photometries PM512 charge‐coupled device (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 multichannel 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 (1–10 R) Balmer‐α 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. We present exospheric effective temperatures obtained from line profile data acquired during the period of 1992–1993. When these data are compared with predictions from the Anderson et al. [1987] and MSIS90 models, there are points of agreement as well as some discrepancies. Included in this paper are discussions of both technical issues associated with applying annular summing spectroscopy for geocoronal Balmer‐α observations and results of data obtained using this technique.