Statistics of plasma sheet convection

Abstract

Determining the characteristics of plasma sheet convection and their response to changes in various solar wind parameters is important for understanding the energy and mass transport, as well as disturbance propagation, through geospace. We use 15 years of data obtained by Geotail, Cluster, and THEMIS to study statistically the characteristics of plasma sheet flows and the effect of the interplanetary magnetic field (IMF) on the convection. We find that plasma sheet convection is dominated by slow speed (<100 km/s) flows that circulate around Earth on both sides toward the dayside. With increasing flow speed the sunward component of the flow velocity becomes more pronounced such that flows with V > 500 km/s are directed almost purely sunward. Both IMF B_y and IMF B_z are observed to penetrate the plasma sheet. During southward IMF conditions, a channel of increased B_z is created in the nightside around the aberrated midnight axis. We suggest that the channel is caused by dipolarization and magnetic flux pileup related to fast flows. The nightside region of highest mean flow speed is located more duskward during dawnward IMF conditions than during duskward IMF conditions. For plasma sheet flows with speeds higher than 100 km/s, we find that the orientation of IMF (clock angle) controls the speeds, while the magnitude of the solar wind electric field plays a minor role. The increasing speed indicates that energy transfer per unit length of the nightside X line increases as IMF turns southward.