Storm-time accumulation of O+ ions throughout the nightside magnetosphere, as observed by MMS

During intense geomagnetic storm activity, the contribution of O+ ions to the ring current population becomes important, reaching levels comparable to that of H+ ions. The presence of O+ in the inner magnetosphere plays an important role in magnetospheric dynamics by, for instance, affecting the development of the ring current and formation of an oxygen torus during active times, reconnection rates and onset at magnetotail reconnection sites, and the cross-polar cap potential. There are several ways for O+ ions of various energies to be accumulated in different regions of the magnetosphere following a Storm Sudden Commencement (SSC). For example, O+ ions could be transported from the ionosphere: i) directly into the inner magnetosphere, ii) first to the near-Earth plasma sheet and then into the inner magnetosphere, iii) farther down the tail and then to the near-Earth plasma sheet and finally into the ring current. Despite the recognized importance of O+, and the proposed mechanisms to reach the inner magnetosphere, the relative contribution of each of these mechanisms during geomagnetic storms remains unclear. We present here a superposed epoch analysis of magnetic field and plasma data from the Magnetospheric Multiscale Mission (MMS) for times when the MMS constellation apogee was located in the nightside, and during geomagnetic storms. We show trends of different ion properties (average energy, number density, and energy density) as they evolve during the development of a storm in order to understand the path and energization that these ions follow on their way to populate the ring current.