STEREO Observations of the Earth's Quiet-Time Ring Current using Energetic Neutrals

On November 6, 2006 the two STEREO spacecraft successively observed Energetic Neutral Atoms (ENA) from the night side magnetosphere with their Supra-Thermal Electrons (STE) Solid State Telescopes. STE was designed to study solar wind electrons in the 2-100keV with ~1keV FWHM energy resolution, but has recently demonstrated its sensitivity for imaging 4-20keV ENAs (Hydrogen) from the heliosheath [Wang et al 2008, Nature]. STEREO A and B made close passes separated by approximately 50 minutes at ~600km on the Earth's night side and near the equator. We present here the ENA spectra and spatial distribution of the observations. Further, we invert the ENA spectrum to give an L-integrated ion flux from 4- 100keV and compare this result to in situ observations of the quiet-time ring current. Preliminary analysis suggests that this detector can make the most sensitive ENA measurements of the ring current to date from low Earth orbit.