Contribution to charge exchange to the ring current decay: TWINS observations

We examine the contribution of charge exchange to the decay of the ring current during the recovery phase of magnetic storms, using energetic neutral atom (ENA) data obtained by the Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission. Loss of ring current ions through charge exchange has been believed to be a major loss process. However, previous studies using modeling and/or ENA data have found that its contribution to the rapid ring current decay is small. For example, the estimates of the contribution from ENA measurements by IMAGE/HENA [Keika et al., 2006] suggested that the rapid ring current decay cannot be explained by charge exchange alone. The present study follows the IMAGE/HENA-based estimates, paying attention to the loss for the energy range of 1-20 keV, which was not covered by IMAGE/HENA and is expected to make a considerable contribution to the total energy loss through charge exchange. Comparison between IMAGE/HENA and TWINS at shared energy ranges (27-100 keV) will be performed to validate the non-overlapping energy range (1-27 keV). By taking advantage of TWINS simultaneous ENA imaging from two widely separated locations, we also attempt to reduce the uncertainties that are associated with the assumptions used to interpret ENA images from the single spacecraft IMAGE mission: isotropic/anisotropic ENA emission and radial dependence of ENA flux.