Thermospheric Density and Temperature Derived from X-ray Occultations Observed by the Unconventional Stellar Aspect (USA) Experiment Aboard ARGOS

The X-ray Astronomy Branch of the Naval Research Laboratory launched the Unconventional Stellar Aspect (USA) experiment aboard the Advanced Research and Global Observation Satellite (ARGOS) in 1999. USA is a new X-ray timing experiment with a large collecting area and microsecond time resolution capable of conducting a broad program of studies of galactic X-ray binaries. USA consists of a collimated proportional counter X-ray telescope and two detectors with ~~1000 cm² effective area each, sensitive to photons in the 1-15~keV energy range. The X-ray Astronomy Branch is studying the composition and structure of the upper atmosphere using a novel method. The data analyzed are energy-resolved photon extinction curves of X-ray celestial sources occulted by the Earth's atmosphere. As each x-ray source is occulted by the mesosphere and lower thermosphere (80-160~km), the density profile is derived from the extinction curve and temperature is derived from the scale height; limited composition information may be derived from the energy-dependence of extinction. These data are compared to standard atmospheric models as well as data from the High Resolution Airglow and Aurora Spectrograph (HIRAAS), mounted alongside USA on the ARGOS satellite. 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 insensitive to nighttime neutral density.