Arase/LEPi Observations of Low‐Energy H ⁺ and O ⁺ Ion Fluxes in the Inner Magnetosphere: Implications for the Relationship Between Warm Plasma Cloak

Abstract

We investigated the spatial distributions and pitch‐angle distributions of 30–300 eV H ⁺ and O ⁺ ion fluxes using data obtained by the low‐energy particle experiments‐ion mass analyzer (LEPi) onboard the Arase satellite. A statistical analysis over 3.7 years reveals that the H ⁺ ion flux is enhanced at L  > 4 from the premidnight sector through dawn to noon, with the peak location shifting inward during geomagnetic disturbances. The pitch‐angle distribution of H ⁺ ions varies with both L and geomagnetic activity, which can be interpreted as the influence of transport processes and the ring current effect. In contrast, the O ⁺ ion flux exhibits a pronounced enhancement in a confined region at L  = 3–5 and 19–9 magnetic local time, with a strong dawn‐dusk asymmetry. The O ⁺ flux increases with geomagnetic activity at L  = 3–5, while remaining nearly unchanged at L  = 6. Its pitch‐angle distribution is consistently bidirectional and field‐aligned, with little dependence on geomagnetic conditions. These distinct spatial and pitch‐angle characteristics indicate that the observed H ⁺ ions correspond to the inner part of the warm plasma cloak, whereas the observed O ⁺ ions represent the high‐energy tail of the oxygen torus. The results further suggest that low‐energy O ⁺ ions are supplied directly from the nightside ionosphere along geomagnetic field lines, rather than being transported inward from the outer magnetosphere. Our findings demonstrate that the warm plasma cloak and the oxygen torus constitute likely independent plasma populations in the inner magnetosphere.