ESCAPE OF MAGNETOSPERIC O⁺ OXYGEN IONS INTO THE MAGNETOSHEATH:MECHANISM AND EFFECTS

During the magnetic storm of January 10, 1997, Geotail spacecraft observed a mgnetospheric O⁺ burst event just on the outside of the dayside magnetopause. Energetic oxygen ions were seen to flow downward in the magnetosheath. The appearance of this O⁺ burst event was in close connection with the strong southward interplanetary magnetic field. The Principle Axis Analysis (PAA) indicates that the dayside magnetopause during this time period was a rotational discontinuity, magnetic reconnection took place in the magnetopause current sheet. Observations showed that reconnection was of the quasi-steady state type and that the O⁺ flow has a southward component. Both normal and inverse energy (velocity) dispersion were seen in the O⁺ flux enhancement process which were caused by the Time Of Fly (TOF) effect of oxygen ions escaping along the reconnected magnetic field lines. Only O⁺ can be transported by the gradient drift from the magnetosphere into the reconnection region, therefore they can be continuously detected in the magnetosheath. The escape rate of O⁺ in this event is estimated to be 0.61 × 10²³ s^-1, which was approximately 33% of the total input rate of the ring current oxygen ions. It is the escape of a great number of oxygen ions from the duskside of the inner magnetosphere (ring current region) that leads to the pronounced asymmetry in the ring current ASY-H index.