Composition of 1–128 keV Magnetospheric ENAs

The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission has observed the inner magnetosphere since 2008. TWINS flies energetic neutral atom (ENA) cameras aboard two spacecraft in separate Molniya orbits. TWINS images the ENA emissions from the inner magnetosphere across a broad range of energies (1 to 128 keV for H, 16 to 128 keV for O, and higher energies for total ENAs). This allows TWINS to observe the evolution, on minute timescales, of the trapped and precipitating particles that dominate storm time dynamics. Presented here are ENA observations over this broad energy range of the large storm of 17 March 2015 (Dst < −223)—the St. Patrick's Day storm. The ENA observations are presented with a 15-min cadence for improved counting statistics. During the St. Patrick's Day storm the flux of ENAs and the concentration of O⁺/H⁺ increased significantly during the main phase. The concentration increased to a value of 1.0 for the 16 keV ions, and the temperature in the inner magnetosphere dropped during the time between the prestorm phase and the main phase. Comparing the results from this storm to a moderate storm on 22 July 2009, and a collection of nine other large storms, we find that the most energetic O⁺ ions in the ring current occur before the peak of the storm.