Field-Aligned Low-Energy O^+ Flux Enhancements in the Inner Magnetosphere Observed by Arase

The present study examines the low-energy ion flux variations observed by the Arase satellite in the inner magnetosphere. From the magnetic field and ion flux data obtained by the fluxgate magnetometer and the low-energy particle experiments–ion mass analyzer onboard Arase, we find 55 events of the low-energy O^+ flux enhancement accompanied with magnetic field dipolarization in the periods of April 1–October 31, 2017 and July 1, 2018–January 31, 2019. The low-energy O^+ flux enhancements (a) start a few minutes after the dipolarization onset, (b) have energy-dispersed signatures with decreasing energy from a few keV down to ∼10 eV, (c) are observed in both storm and non-storm periods, (d) have a field-aligned distribution (α ∼ 0° in the southern hemisphere and α ∼ 180° in the northern hemisphere), (e) are accompanied by the low-energy H^+ flux enhancements that have lower energies than O^+ by a factor of 3–10, and (f) increase the O^+ density and the O^+/H^+ density ratio by ∼10 times and 4–5 times, respectively. We perform a numerical simulation to trace ion trajectories forward in time from the Arase positions. It is revealed that both H^+ and O^+ ions drift eastward and reach the dawn-to-morning sector without being lost in the ionosphere, if the pitch angle scattering effect is considered near the equatorial plane. This result suggests that these low-energy field-aligned ions can contribute to formation of the warm plasma cloak.