Solar Wind Dynamic Pressure during Magnetic Storms and Its Implications on the Dayside Ring Current Particle Loss

It has been known that (untrapped) ring current particles can be lost through the dayside MP(magnetopause) into the magnetosheath. However, details of the loss mechanism of this process have not received much attention. In this paper, we show that the solar wind dynamic pressure can play a significant role in the dayside loss. In order to show that, we have first analyzed the average characteristics of the 95 geomagnetic storm events selected from the period 1997 to 2002. We find that the dynamic pressure overall enhances during the magnetic storm. The enhancement is most significant during the storm main phase and it is higher for stronger storms. Using one of the most recent Tsyganenko models, T01_s, we show that this enhanced dynamic pressure not only pushes MP to move inward but also sets an enhanced gradient of the magnetic field intensity along the MP. On the basis of the test particle calculation, we explicitly show that the increased gradient of the magnetic field intensity along the MP can be a significant factor for the particles to effectively cross the MP by the gradient-drift. We argue that this can often apply to the majority of the ring current particles.