Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid - Observations at low (not greater than 10 keV) energies

Intense (not greater than 10 exp 6/sq cm sr s) fluxes of upflowing energetic neutral atoms (ENAs) from the polar cap have been observed in the energy range 0.1-10 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 emissions maps to magnetic latitudes 70-85 deg extending from dusk over nightside. There were no measurements in the midnight sector below approximately 80 deg. It is shown that the emissions cannot originate from an isotropically emitting source, which rules out any optical emissions. Local charged particle detection is ruled out on the basis of no correlation 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 110-140-deg pitch angle interval (source altitude 400 km). It is suggested, with support from low altitude rocket observations, that the ENAs are produced by low altitude (400-600 km) ion conics of several hundreds eV charge exchanging with the upper atmosphere. (Author)