Energy Spectral Evolution of Precipitating Ring Current Ions Using TWINS Low-Altitude Emissions (LAEs) and in-Situ NOAA Observations.

Low Altitude Emissions (LAEs) are the energetic neutral atom (ENA) signature of precipitating ring current ions at 200-800 km. The LAE signature is produced in the so-called thick target region, i.e., by multiple charge exchange and stripping interactions in the oxygen exosphere, with each such interaction resulting in a small net energy loss of about 30 eV per interaction. We present analysis of LAE observations from 1 July 2011 obtained by the dual-spacecraft Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) ENA imagers. For this event, TWINS provides nearly continuous magnetospheric observations from at least one of the TWINS spacecraft and stereo viewing during selected intervals. Using TWINS LAE measurements at several energy bins spanning 1-100 keV, we infer spectral properties of LAEs, and compare the LAE energy spectra with simultaneous NOAA in-situ observations. We also determine to first order whether the energy lost in the thick target region significantly shifts the thermal peak of TWINS-measured precipitating ENAs.