The storm‐time access of solar wind ions to the nightside ring current and plasma sheet

We investigated the storm‐time entry of solar wind ions into the magnetosphere by carrying out a large‐scale kinetic particle tracing calculation for the 24–25 September 1998 and the 17 April 2002 geomagnetic storms. For each storm, we simulated the magnetosphere by using a global magnetohydrodynamic (MHD) code that was driven by solar wind and interplanetary magnetic field (IMF) data from spacecraft upstream of Earth. We then launched ions in the solar wind in the global time‐dependent fields obtained from the MHD simulation, beginning prior to storm onset and extending into the main phase. We collected those ions that successfully reached the nightside plasma sheet and ring current, and then determined the mechanisms of entry and transport responsible for the injection of these ions into the magnetotail. We found that, in agreement with quiet time entry results, the IMF B y component strongly influenced whether ions entered through the dawn or dusk flanks, and changes in IMF B y were immediately reflected on the ion entry pattern. Also, in addition to the usual dayside merging source, ion entry into the magnetosphere was facilitated by dynamic pressure enhancements, during which entry occurred over a wide swath of latitudes and extended from the dayside to locations far downtail. More significantly, because of the warmer storm‐time ion population in the magnetosheath, ions were more readily affected by inhomogeneities and rapid changes in the IMF, which caused solar wind ions to gradient drift onto open field lines and reach the inner magnetosphere.