Large magnetic storms as viewed by TWINS: A study of the differences in the medium energy ENA composition

During large geomagnetic storms (Dst ≤-100 nT), oxygen can become a significant component of the energetic particles of the inner magnetosphere. Until recently, there were no available global observations of the medium energy (<50 keV) oxygen populations. Using observations from the Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) Energetic Neutral Atom (ENA) imagers we present a study of nine large storms of solar cycle 24 as a function of storm phase. For these storms we observe that the H and O ENA fluxes and their temperatures increase in tandem during the storm's initial phase. However, there is no increase in the O⁺/H⁺ ratio in the inner magnetosphere until the storm main phase. Also seen during the main phase is an energy dispersion with higher-energy (32 keV) H ENAs seen before the arrival of O ENAs of the same energy. The O ENAs take longer to return to prestorm levels during the recovery phases. This longer recovery time is likely because of the large difference between the storm time and prestorm O populations compared to H (i.e., there is always some prestorm H in the inner magnetosphere, but effectively no O prestorm). These results imply that medium-energy O ENAs evolve over long time scales (hours to days) as opposed to the shorter substorm time scales of the higher-energy (>52 keV) O ENAs. Key Points The first global observations of the medium energy H and O ENAs during storms The energy distribution and concentrations evolved over the storm phases For these storms a pattern has emerged in the medium energy ENA observations