Bursty energetic oxygen events in the dayside magnetosheath: GEOTAIL observations

Two energetic oxygen ion enhancements with a clear anti-sunward flow were o bserved in the dayside magnetosheath when GEOTAIL was located at (4.3, 16.3 , 0.7) R-E on 26 January 1994. The oxygen ions peaked at energies of about 300 keV. The commencement of the two oxygen enhancements at 1405 and 1440 U T were in coincidence with two substorm injections at 1349 and 1422 UT. The oxygen events are characterized by strong impulsive increases in the flux whereas energetic protons and helium ions were essentially absent. The magn etosheath field projection in the x-y plane (GSE) was almost constant and p ointed into the dusk/tail sector forming an angle of 135 degrees to the Sun -Earth axis. The field magnitude was 35 nT with rather little fluctuations and the Bz component was negative. These observations show that energetic o xygen ions can leak out of the magnetosphere into the magnetosheath during isolated substorm activity (storm conditions are not required). Enhanced so lar wind dynamic pressure compressed the magnetopause inward and may have h elped to observe energetic oxygen ions to leave from the magnetopause curre nt sheet thus escaping into the magnetosheath.