Ring current oxygen ions escaping into the magnetosheath

Storm-related magnetospheric oxygen bursts were observed in the dayside mag netosheath during the coronal mass ejection on January 10, 1997. These sing ly charged oxygen ion events exhibited a clear antisunward flow. The oxygen ions are associated with a strong negative interplanetary magnetosheath fi eld (IMF). The average B-z 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 degrees relative to the Earth-Sun axis. The magnetopause was i dentified as a rotational discontinuity by using the principal axis analysi s (PAA) method. The three-dimensional polar versus azimuthal angle distribu tion of the oxygen ions showed that the oxygen flow has a north to south ve locity component. The observations suggest that the dayside reconnection pr ocess is generally steady. The energy dispersion can be explained with the time-of-flight (TOF) effect assuming oxygen ions are escaping from the magn etosphere along the reconnected field lines. The lack of hydrogen and heliu m ions during the observed oxygen bursts can be explained, as only oxygen i ons are resupplied by the gradient drift in the inner magnetosphere because of their larger bounce periods with respect to hydrogen and helium ions. T herefore only oxygen ions are observed continuously in the magnetosheath. T he estimated oxygen escape rate amounts to 0.61 x 10(23) ions/s, about 33% of the input rate of the ring current. The observations imply that the stor mtime ring current is asymmetric. A large amount of ring current oxygen ion s escape from the magnetosphere into the magnetosheath.