SURVEY OF ELECTRON AND ION BULK FLOWS IN THE DISTANT MAGNETOTAIL WITH THE GEOTAIL, SPACECRAFT

A histogramic survey is reported for the electron and ion bulk flows in Earth's magnetotail at geocentric radial distances in the range of about 10 to 210 R(E) from the Comprehensive Plasma Instrumentation on board the Geotail spacecraft. Specifically the X-components of the bulk flows are examined for high ion temperatures, > 5 x 10(6) K, and for the lower ion temperatures. This separation of plasmas provides a reasonably effective criterion for identifying the hot plasmas associated with the plasma sheet and its boundary layers and the folder plasmas found in the magnetotail lobes and magnetopause boundary layer. The survey shows that the distributions of earthward bulk flows of ions and electrons are similar for each of the two temperature regimes, respectively. However, average speeds for the ions and electrons as a function of radial distance are dissimilar in that the electron bulk speeds are greater than those for the ions. All average electron and ion bulk flows are directed tailward at geocentric radial distances > 30 R(E). The most striking dissimilarity in the occurrence frequencies of the bulk flows is the large number with high tailward speeds for the electrons, > 500 km/s, relative to those for the ions. One such example which also exhibits a plasmoid-like magnetic signature is presented in detail. It is found that the disparity in bulk flows is due to the presence of two ion velocity distributions, one with high temperatures and a bulk speed similar to that of the electrons and the second, colder distribution with higher number density and considerably lesser bulk flow speed. Thus a current is associated with the high-speed electron flow. An assumption of co-moving ions and electrons is not valid for this event.