TWINS Observations of Oxygen ENA Flux

As part of the science objectives of the TWINS (Two Wide-angle Imaging Neutral-atom Spectrometers) mission, we aim to enhance the knowledge of the workings of our magnetosphere by contributing compositional information of the observed energetic neutral atom (ENA) flux from the two TWINS instruments. We will present images of oxygen (O) ENA flux varying over time and space. Results from a few cases of TWINS flight data of geomagnetic significance will be presented with this analysis. One such example is a simultaneous and continuous observation of a minor geomagnetic storm (-60 nT) on 11 Oct 2008 as seen by both TWINS instruments. Initial TWINS images of this event show evidences of low-altitude emissions (LAEs) near the limb of the Earth and ring current signatures as well. Our compositional analysis is based on the secondary electron pulse height (PH) distributions measured by TWINS. Secondary electrons are generated as ENAs pass through an ultra-thin carbon foil in the TWINS instrument apertures. The secondary electron (PH) distribution varys as a function of the ENA’s energy and mass. The observed PH functions are then compared to the secondary electron PH distributions determined during instrument calibration. The flight data are simply treated as a linear combination of the two anticipated and most abundant species of magnetospheric ENAs, i.e., hydrogen (H) and oxygen (O).