Response of the magnetospheric convection to sudden interplanetary magnetic field changes as deduced from the evolution of partial ring currents

Abstract

We examined the response of the partial ring current (PRC) to sudden changes in the interplanetary magnetic field (IMF) Bz for two PRC events, in order to investigate the response of the convection in the near‐Earth magnetotail to IMF changes. We found that ground magnetic changes due to the PRC started to develop simultaneously at all local times with a time lag of 5–11 min from the growth of the ionospheric convection as detected by the polar cap magnetometers. The ground magnetometers furthermore showed that the asymmetric ring current changed into a symmetric ring current with no significant time lag (<1 min) when the convection electric field started to decay because of the northward turning of the IMF. The temporal and spatial evolution of the plasma pressure and the ground magnetic perturbations due to the PRC are simulated for the events involved using a particle simulation model. The simulation results agree well with the observations in the temporal variations and their local time dependence. The quick development and reconfiguration of the observed PRC suggest that the ionosphere played a crucial role in driving the convection in the near‐Earth magnetotail and in the inner magnetosphere. In particular, the immediate reconfiguration of the PRC caused by the northward turning of the IMF implies nearly instantaneous propagation of the ionospheric convection electric field to the inner magnetosphere and subsequent quick change in the ring current configuration. We discuss the propagation of the convection electric field to the near‐Earth magnetotail through the ionosphere after its propagation along the magnetic field lines from the generator in the dayside magnetosphere.