Acceleration and Loss Processes During CME-driven and CIR-driven Storms

Two categories of magnetic storms have been identified based upon their solar origin: a) recurrent, associated with corotating interaction regions (CIRs) formed by the interaction of high-speed streams from coronal holes with the dense, slow-speed solar wind plasma, or b) transient, associated with huge plasma eruptions from the Sun called coronal mass ejections (CMEs). We use our kinetic ring current--atmosphere interactions model (RAM) to simulate ring current evolution during two geomagnetic storms of similar strength representative of each solar origin, respectively, the 10 March 1998 and the 15 May 1997 storms, and compare the mechanisms responsible for energizing particles and for causing their loss. Ring current intensification due to enhanced plasma inflow from the magnetotail and both convective and diffusive transport and acceleration is considered. Using an electric potential model driven by interplanetary parameters, we find that the ring current injection rate calculated with RAM is in good agreement with Dst index during May 1997, however, it underestimates Dst during March 1998. Additional intensifications by radial diffusion during March 1998 reproduce better its long lasting recovery phase. Charge exchange is the dominant loss process during the storms' recovery phase; the net convective losses are larger than charge exchange losses at Dst minima. The regions of strong EMIC wave instability occur in the postnoon local time sector and along the plasmapause; these waves cause ~5% decrease of the total ring current energy by proton precipitation during May 1997. These results suggest that the increased injection during the recovery phase combined with the smaller losses may be the cause for the slower ring current decay during recurrent storms.