Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Observations at low (<= 10 keV) energies

Intense (less than or equal to 10(6) cm(-2) sr(-1) s(-1)) fluxes of upflowi ng energetic neutral atoms (ENAs) from the polar cap have been observed in the energy range similar to0.1-10 keV (hydrogen assumed) by the ENA imager PIPPI (Prelude In Planetary Particle Imaging) onboard the Astrid satellite at 1000 km altitude. If a source altitude of 400 km. is assumed, the ENA em issions maps to magnetic latitudes 70 degrees -85 degrees extending from du sk over nightside. There were no measurements in the midnight sector below approximately 80 degrees. It is shown that the emissions cannot originate f rom an isotropically emitting source, which rules out any optical emissions . Local charged particle detection is ruled out on the basis of no correlat ion with local electron measurements and local magnetic field configuration . The ENA emissions can be explained by a conically pitch angle distributed ion source emitting ENAs in the 110 degrees - 140 degrees pitch angle inte rval (source altitude 400 km). It is suggested, with support from low altit ude rocket observations that the ENAs are produced by low altitude (400-600 km) ion conics of several hundreds eV charge exchanging with the upper atm osphere.