Earthward plasma sheet flows during substorm phases

Magnetic reconnection in the Earth's magnetotail converts open magnetic flux to closed, producing fast flows in the plasma sheet. Earthward fast flows can thus be used as a proxy for energy and momentum transfer into the closed magnetosphere during the course of a substorm. Characteristics of flows observed before substorm onset can provide information on the role of the flows in substorm onset triggering as well as the sequence of substorm-related processes. We use 15 years of data obtained by Geotail, Cluster, and THEMIS to study statistically the distribution of earthward plasma sheet flows during substorms. Substorm phases are determined from time series of the AL index and the z-component of the interplanetary magnetic field. We find that the occurrence frequency of medium-speed (100 km/s < V < 500 km/s) flows increases slightly toward the end of the substorm growth phase. At substorm onset there is a sharp increase of medium-speed flows, and during substorm recovery there is a gradual decrease of such flows. The occurrence frequency of high-speed (V > 500 km/s) flows, on the other hand, does not clearly increase during the substorm growth phase. The increase at substorm onset is more gradual than for the medium-speed flows. The occurrence frequency of high-speed flows peaks at the beginning of substorm recovery. These results indicate that any effect of the flows on substorm onset triggering would be caused by medium rather than very high speed flows.