GALILEO OBSERVATIONS OF THE MOTIONS OF ION AND ELECTRON PLASMAS IN THE MAGNETOTAIL

We report the first fully three-dimensional determinations of the bulk motions of ion and electron plasmas in Earth's magnetotail at geocentric radial distances near the lunar orbit, The two regions that are examined are the plasma sheet and the plasma mantle. The magnitudes and directions of the bulk velocities are the same for the ion and electron plasmas in the plasma mantle. However, in the plasma sheet the electron bulk speeds are often substantially greater than the ion bulk speeds. This situation arises because the component of electron flow along the magnetic field is greater than the ion parallel flow. Most of the bulk flow of the electrons is often attributable to the presence of field-aligned currents, not E x B convection of plasmas. Hence previously reported two-dimensional electron velocity distributions do not give a reliable determination of mass transport and convection in the plasma sheet at these distances in the magnetotail.