AURORAL X-RAY-IMAGING FROM HIGH-EARTH AND LOW-EARTH ORBIT

Observations of bremsstrahlung x rays emitted by energetic electrons i mpacting the Earth's atmosphere can be used for remotely sensing the m orphology, intensity, and energy spectra of electron precipitation fro m the magnetosphere. The utility of the technique is derived from the broad energy range of observable x rays (2 to > 100 keV), the simple e mission process, the large x-ray mean free path in the atmosphere, and negligible background. Two auroral x-ray imagers, developed for futur e spaceflights, are discussed. The Polar Ionospheric X-Ray Imaging Exp eriment is scheduled for launch on the NASA international Solar-Terres trial Physics/Global Geospace Science program POLAR satellite in 1994. The POLAR orbit, with an apogee and perigee of 9 and 1.8 R(e) (Earth radii), respectively, affords the opportunity to image the aurora from a high altitude above the north pole continuously for several hours. The Magnetospheric Atmospheric X-Ray Imaging Experiment (MAXIE) was la unched aboard the NOAA-I satellite on August 8, 1993. The 800-km polar orbit passes over both the northern and southern auroral zones every 101 min. MAXIE will be capable of obtaining multiple images of the sam e auroral region during a single satellite orbit. The experimental app roaches used to exploit these very different orbits for remote sensing of the Earth's auroral zones are emphasized.