ENA imaging from the Swedish micro satellite Astrid during the magnetic storm of 8 February, 1995

The generation of energetic neutral atoms (ENA) through charge exchange processes between hot (≥ 1 keV) plasma and cold (< 0.1 eV) exospheric neutrals makes possible imaging of hot magnetospheric plasma. Here we report on the ENA observations from a polar 1000 km orbit carried out by PIPPI (Prelude In Planetary Particle Imaging), the ENA imager onboard the Swedish micro-satellite Astrid. We analyse ENA images in the energy range 26–52 keV obtained from the 8 February 1995 during a moderate geomagnetic storm (Dst≈−80 nT). The equatorial ion distribution is modeled by a 13 parameter model using a three component (H, He and O) Chamberlain exosphere model with parameters from the MSISE90 extended thermospheric model. The equatorial ion distribution, deduced from the ENA-images through forward modeling is located at MLT (magnetic local time) 1900 hours and occupies L-shell 4–6. Very low ENA fluxes were detected at energies >37 keV. It is suggested that the ENAs are produced by precipitating/mirroring ions on auroral/subauroral field lines coming from the near Earth current sheet charge exchanging with exospheric neutrals at near-exobase altitudes (300–400 km).