[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-01] Temporal and spatial evolution of cold ions in the inner magnetosphere during large geomagnetic storms

*Khan-Hyuk Kim¹、Junhyun Lee²、Lynn M Kistler³ (1.School of Space Research, Kyung Hee University, Yongin, Gyeonggi, Republic of Korea、2.Korea Astronomy and Space Science Institute, Daejeon, Republic of Korea、3.Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA)

キーワード：Cold ions, Inner magnetosphere, Large magnetic storm, Plasmapause

In order to understand how cold ions (< 10 eV) evolve temporally and spatially within the inner magnetosphere (L < 6.5) during geomagnetic storms, we performed superposed epoch analyses on the partial ion densities of H⁺ (N_H+), He⁺ (N_He+), and O⁺ (N_O+) using flux data from the Van Allen Probes in the 1-10 eV energy range. The analyses were conducted on a set of 14 large storms (Dst < -100 nT) over a five-year period from September 2013 to September 2017. We examined the ion density ratios (N_H+/N_e, N_He+/N_e, and N_O+/N_e) normalized to the electron number density (N_e) and found that the N_O+/N_e ratio dramatically increases during the early recovery phase outside the model plasmapause. The enhanced N_O+/N_e ratio persisted for a period exceeding three days subsequent to the Dst minimum. The variation of the N_He+/N_e ratio observed outside the plasmapause exhibited a trend similar to that of the N_O+/N_e ratio during the recovery phase. However, the N_He+/N_e ratio is more than an order of magnitude smaller than the N_O+/N_e ratio. In contrast to the N_He+/N_e and N_O+/N_e ratios, the N_H+/N_e ratio does not exhibit a significant increase during the recovery phase. The enhanced cold O⁺ ions outside the plasmapause exhibit a bi-directional, predominantly field-aligned population, and the N_O+/N_H+ ratio decreases in the afternoon sector (MLT = 12-18 hr). Based on these observations, we suggest that the enhanced cold O⁺ ions outside the plasmapause drift into the inner magnetosphere from the tail during large storms.