Pergamon Adv. zyxwvutsrqponmlkjihgfedcbaZYXWVUT Space Res. Vol. 20, No. 46, pp. 1055-1060, 1997 @I997 COSPAR. Published by Elsevier Science Ltd. All rights reserved printed in Great Britain 0273-1177/97 $17.00+ 0.00 PII: SO273-1177(97)00560-7 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM ENERGETIC NEUTRAL ATOM IMAGING BY THE ASTRID MICROSATELLITE S. Barabash*, P. C:son Brandt*, 0. Norberg*, R. Lundin*, E. C. Roelof**, C. J. Chase**, B. H. Mauk** and H. Koskinen*** * Swedish institute of Space Physics, Box 812, 98128 Kiruna, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON Sw eden **The Johns Hopkins University/Applied Physics Laboratory, Johns Hopkins Road, Laurel MD 207234099, U.S.A. ***Finnish Meteorological Institute, Box 503. SF-OOIOIHelsinki, Finland ABSTRACT The microsatellite Astrid carried the first instrument (PIPPI, Prelude in Planetary Particle Imaging) specifically designed to perform energetic neutral atom (ENA) imaging. It made measurements from a low altitude ( 1000 km) polar orbit in the energy range -13 - 140 keV. The ENA images, obtained from near-pole vantage points, adequately reflect general morphological features of the ring current such as a global dawn - dusk asymmetry. The detected ENA peak fluxes (500 - 2000 cm-*s-lsrlkeV-1 for 26 - 37 keV ) and structure of the ENA images correlate well with magnetospheric activity throughout the entire data set. The Astrid results demonstrate a considerable potential for ENA imaging from low altitude polar orbits. High ENA fluxes, large angular size of the generation region and simultaneous sampling over all local times are major advantages of such imaging. 01997 COSPAR. Publishedby Elsevier Science Ltd. INTRODUCTION Any energetic plasma immersed in a neutral gas will emit energetic neutral atoms (ENAs) generated by the charge - exchange process. ENAs are not affected by electromagnetic fields and propagate essentially rectilinearly, like photons. A direction-responsive neutral particle detector can thus image the emitting region. The main region of the Earth’s magnetosphere which is occupied by a hot plasma with a significant neutral background is the ring current. High energy ( > 10 keV) neutral atoms (HENAs) emitted from the ring current have, indeed, been detected by charged particle detectors on several occasions (Hovestadt and Scholer, 1976; Roelof et al., 1985; Voss et al., 1993). but not, as yet, by a specifically designed ENA imager. The most advanced of these early analyses produced the first-ENA image of a storm-time ring current from ISEE 1 data (Roelof, 1987) and can be considered as a proof-of-concept of ENA imaging. So far, general attention has concentrated on the ENAs generated within the high altitude ring current, because they are supposed to be used in global magnetospheric imaging from high altitude ( > 20000 km) spacecraft. However, in the aurora1 region where the ring current/radiation belt particles plunge into the dense exosphere/upper atmosphere, the charge-exchange process should be much more effective and the ENA emissions much more intense. Despite that McEntire and Mitchell (1989) mentioned high ENA flux and two dimensional character of ENA emissions in auroral/sub-aurora1 zone (due to the very quick rise of exospheric densities), up to now ENA imaging from low altitudes and polar latitudes has not been considered in details and advantages of such vantage points have not been realized. Only after the flight of Swedish microsatellite Astrid, has Roelof (1996) developed a basic approach to the analysis of such images. This report discusses the basic features of ENA imaging from a low altitude (-1000 km) polar orbit and presents the first images obtained by the ENA camera PIPPI (Prelude In Planetary Particle Imaging) onboard the low altitude orbit Swedish microsatellite Astrid.