TWINS observations of local time dependence of precipitating ions

Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) is the first stereoscopic magnetospheric imaging mission. TWINS is a NASA Explorer Mission-of-Opportunity performing simultaneous Energetic Neutral Atom (ENA) imaging from two widely-separated Molniya orbits on two separate spacecraft. Often the brightest signal in an ENA image is the low-altitude emission (LAE), the ENA signature of ions precipitating at 200 to 800 km altitudes. Unlike the optically thin emissions from the trapped ring current which involve only a single charge-exchange interaction along the imaging line of sight, LAEs result from multiple charge exchange and stripping interactions between energetic ions and the dense, neutral oxygen exosphere. The TWINS mission has developed the thick-target approximation (TTA), a mathematical treatment of ion-neutral interactions in the optically-thick region that enables quantitative extraction of ion flux spectra from LAEs. We use TTA analysis of TWINS LAEs to extract ion energy spectra and temperatures of precipitating ring current ions for pixels spanning several hours of magnetic local time (MLT). Two-spacecraft imaging provides nearly continuous coverage, and stereo imaging (when available) yields a broader range of MLT coverage than would be obtained from a single imager. We examine the MLT dependence of these precipitating ion spectra (and temperatures) for an ensemble of geomagnetic storm events spanning June 2008 through February 2012, to reveal the solar cycle and activity-level dependence.