Energetic neutral atom imaging at low (<= 10 keV) energies from Astrid: observations and simulations

Intense (10(6) cm(-2) sr(-1) s(-1)) fluxes of upflowing ENAs from the polar cap have been observed in the energy range 0.1-13 keV (hydrogen assumed) f rom the Astrid satellite at 1000 km altitude. If a source altitude of 400 k m is assumed, the ENA emissions come from an are-like region at magnetic la titudes 70-85 degrees extending from dusk over to the nightside. Simulated images show that the observed emissions may be the ENA-albedo effect of the auroral ion precipitation. It is also possible that the observed emissions may originate from upward accelerated ions with cone-like pitch-angle dist ributions charge exchanging with the upper atmosphere. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.