The magnetosphere under the radial interplanetary magnetic field: A numerical study

We investigate the magnetosphere under radial interplanetary magnetic fields (IMF) by using global magnetohydrodynamic simulations. The magnetosphere-ionosphere system falls into an unexpected state under this specific IMF orientation when the solar wind electric field vanishes. The most important features that characterize this state include (1) magnetic reconnections can still occur, which take place at the equatorward of the cusp in one hemisphere, the tailward of the cusp in the other hemisphere, and also in the plasma sheet; (2) significant north-south asymmetry exists in both magnetosphere and ionosphere; (3) the polar ionosphere mainly presents a weak two-cell convection pattern, with the polar cap potential valued at approximate to 30 kV; (4) the whole magnetosphere-ionosphere system stays in a very quiet state, and the AL index does not exceed -70 nT; and (5) the Kelvin-Helmholtz instability can still be excited at both flanks of the magnetosphere. These results imply the controlling role of the IMF direction between the solar wind and magnetosphere interactions and improve our understanding of the solar wind-magnetosphere-ionosphere system.