Earth Planets Space, 58, 633–643, 2006 A study of the storm event on October 21–22, 1999 by the MHD simulation

We carried out a high resolution three-dimensional magnetohydrodynamic (MHD) simulation of the interac-tion between the solar wind and the Earth’s magnetosphere during a strong magnetic storm on October 21–22, 1999. The input to the simulation was from WIND solar wind observations. As the IMF is strongly south-ward (−20 nT to −30 nT) for 6 hours, the geomagnetic field lines in the dayside magnetopause are eroded to the geosynchronous orbit (GEO) region by reconnection. The associated magnetic flux is transferred from the dayside magnetosphere to the tail. The reconnection region still appears near GEO region on the dayside magne-topause, even though the IMF Bz component becomes small or northward, because of the influence of the strong IMF By (30 nT). IMF lines can successively reconnect with the naked and large geomagnetic field line in the dayside flank regions. Thus, the cross polar cap potential is maintained to be large value and convection in the ionosphere is enhanced. The cross polar cap potential is governed by IMF By as well as Bz (φ ∼ 250 kV for Bz ∼ −20 nT and φ ∼ 300 kV for Bz ∼ −30 nT), and it saturates during the strong southward IMF. A large energy flux enters the ionosphere at very low latitudes (50◦) and the inner edge of the plasma sheet becomes very close to the Earth (X = −3.2 RE) for a strong magnetic storms. The open-closed boundary extends to 60 ◦ latitudes on the nightside, 72 ◦ on the dayside, 62 ◦ on dawn, and 66 ◦ on dusk. Enhanced energy flux appears at low latitudes (50◦) on the nightside in simulation. Moreover, the energy flux in the dusk region (19 MLT) appears down to 55