Using USA and RXTE X-ray Source Atmospheric Occultations to Determine Atmospheric Densities and Temperature

The U.S. Naval Research Laboratory launched the Unconventional Stellar Aspect (USA) experiment aboard the Advanced Research and Global Observation Satellite (ARGOS) in 1999. USA is an X-ray timing experiment with a large collecting area and microsecond time resolution. USA consists of a collimated proportional counter X-ray telescope and two detectors with ~1000 cm² effective area each and sensitive to 1-15 keV photons. NASA's Rossi X-ray Timing Explorer (RXTE) has been operating since December 30, 1995. RXTE features unprecedented time resolution in combination with moderate spectral resolution. Time scales from microseconds to months are covered in an instantaneous spectral range from 2 to 250 keV. We have refined our new technique to measure the composition and structure of the upper atmosphere using atmospheric occultation of celestial x-ray sources. Both USA and RXTE provide energy-resolved photon extinction curves, and the combined energy ranges of USA and RXTE permit direct probing of the mesosphere and lower thermosphere (80-160 km). Roughly speaking, differential absorption among energy bands can provide composition information, and the shape of the light curve's total density versus altitude. The refined technique models the shape of the spectra for three different X-ray sources: the Crab Nebula, Cygnus X-2, and GX5-1. The source model is attenuated using basis functions derived from NRLMSISE-00 (Picone et al. 2000) and convolved with the USA or RXTE detector response. Finally, using a Discrete Inverse Theory maximum likelihood retrieval process, the attenuated and convolved models are fitted to the data to retrieve density and temperature information. In addition, these results are compared to results obtained from the High Resolution Airglow/Aurora Spectroscopy Experiment (HIRAAS) experiment. This research is the first to study the neutral atmosphere in this energy range, and complements UV airglow remote sensing techniques used aboard ARGOS that are less sensitive to nighttime neutral density. Enhanced Empirical Models of the Thermosphere, J. M. Picone, A. E. Hedin, D. P. Drob, R. R. Meier, J. Lean, A. C. Nicholas, and S. E. Thonnard, Physics and Chemistry of the Earth, Part C: Solar-Terrestrial and Planetary Science, 25(5-6), 537-42 (2000).