Ring current oxygen ions escaping into the magnetosheath

Abstract

Storm‐related magnetospheric oxygen bursts were observed in the dayside magnetosheath during the coronal mass ejection on January 10, 1997. These singly charged oxygen ion events exhibited a clear antisunward flow. The oxygen ions are associated with a strong negative interplanetary magnetosheath field (IMF). The average Bz was almost − 50 nT, and the field projection in the x – y plane (GSE) was nearly constant in the Sun/dawn sector forming an angle of 45° relative to the Earth‐Sun axis. The magnetopause was identified as a rotational discontinuity by using the principal axis analysis (PAA) method. The three‐dimensional polar versus azimuthal angle distribution of the oxygen ions showed that the oxygen flow has a north to south velocity component. The observations suggest that the dayside reconnection process is generally steady. The energy dispersion can be explained with the time‐of‐flight (TOF) effect assuming oxygen ions are escaping from the magnetosphere along the reconnected field lines. The lack of hydrogen and helium ions during the observed oxygen bursts can be explained, as only oxygen ions are resupplied by the gradient drift in the inner magnetosphere because of their larger bounce periods with respect to hydrogen and helium ions. Therefore only oxygen ions are observed continuously in the magnetosheath. The estimated oxygen escape rate amounts to 0.61 × 1023 ions/s, about 33% of the input rate of the ring current. The observations imply that the stormtime ring current is asymmetric. A large amount of ring current oxygen ions escape from the magnetosphere into the magnetosheath.