Using the Unconventional Stellar Aspect (USA) experiment on ARGOS to determine atmospheric parameters by X-ray occultation

摘要

The Unconventional Stellar Aspect (USA) experiment is a multi-purpose experiment built around an X-ray sensor viewing celestial sources. The objectives include both basic research in X-ray astronomy and exploration of applied uses of X-ray sensors in space. The applied uses depend in large part upon exploiting understanding of celestial X-ray point sources. The experiment was launched on February 23, 1999 from Vandenberg AFB, CA aboard the Advanced Research and Global Observation Satellite (ARGOS). USA operated from April 1999 to November 2000. It consists of two proportional counters mounted in a two-axis gimbal for offset pointing from the nadir-pointed ARGOS. We present an overview of the experiment and then describe how it is used to provide a new atmospheric diagnostic that takes the form of redundant sets of atmospheric column density determinations. 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. This research is the first to study the neutral atmosphere using X-ray source occultations, and complements UV airglow remote sensing techniques used aboard ARGOS that are insensitive to nighttime neutral density.