Bursts of fast magnetotail flux transport Original Research Article

On the basis of several years of Geotail data we performed a comprehensive statistical study of rapid plasma transport in the near- and midtail central plasma sheet. We chose a new approach by using flux transport and not ion bulk velocity as the threshold parameter for the identification of rapid flows, thus defining rapid flux transport events (RFTs). We found that the occurrence rate of earthward rapid flux transport events is constant at radial distances greater than 15 RE and that it starts to drop earthward of that distance. Tailward rapid flux transport events with BZ < 0 are extremely rare inside of 20 RE, but their occurrence rate increases strongly beyond that distance. Tailward directed events with BZ > 0 can be seen at all distances, but are of different nature in the regions earthward and tailward of about 20 RE. Using a superposed epoch analysis we found that rapid flux transport events are associated with a reconfiguration of the plasma sheet. While earthward events lead to non-adiabatic heating and a dipolarization of the plasma sheet, tailward events beyond a radial distance of about 20 RE are related to the ejection of a plasmoid. Tailward RFTs with a northward BZ observed inside of 20 RE could possibly be related to rebouncing flows or vortices in the plasma sheet. Earthward RFTs show characteristics of plasma sheet bubbles, i.e., an ion density lower than the surrounding plasma but a higher magnetic field pressure. By replacing the velocity criterion in the definition of bursty bulk flows by a convection electric field criterion, we defined rapid convection events and could show that they are responsible for 30 to 50% of the observed total transport of mass, energy and magnetic flux past Geotail in the central plasma sheet between 10 and 50 RE.