Determination of Total Atmospheric Density in the 80-150 km Altitude Range Using X-ray Occultation Sounding

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. The Proportional Counter Array (PCA) experiment on NASA's Rossi X-ray Timing Explorer (RXTE) has been operating since December 30, 1995. The PCA 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 demonstrate that X-ray Occultation Sounding (XOS) is a viable method to determine the structure of the upper atmosphere in terms of the total number density of atmospheric constituents by using atmospheric occultation of celestial x-ray sources. We now have an ensemble of approximately 30 occultations of two different X-ray sources using both the USA and PCA experiments. In general, our measurements demonstrate general agreement with the nominal NRLMSISE-00 model (Picone et al 2000), but with some differences of up to 30% at some altitudes for some occultations. Also, we demonstrate that XOS is very sensitive to satellite position determination and the time resolution of the occultation data. In addition, some of 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.