Plasma velocity distributions in the near-Earth plasma sheet: A first look with the Geotail spacecraft

An initial examination of the three-dimensional velocity distributions of positive ions and electrons as observed with the Geotail spacecraft in the near-Earth plasma sheet at radial distances of similar to 10 R(E) near the magnetic equator has revealed several fascinating features of this region. The velocity distributions of the hot ions in the energy range of about 1-25 keV are not isotropic as a function of pitch angle but exhibit complex features that appear to be the ''memory'' of their previous acceleration and transport in the more distant magnetotail. The composition of the plasma is primarily H+, with traces of He++, He+, and O+, and thus its sources are both the solar wind and the ionosphere. Field-aligned beams of ionospheric ions with energies in the range of 3-5 keV are observed to be streaming into the central plasma sheet and are presumably due to a field-aligned potential drop of similar magnitude which is positioned at lower altitudes. The pitch angle distributions of hot electrons are generally more isotropic than those of the positive ions with one major exception. This exception is the presence of counterstreaming electron beams, magnetically field-aligned, with energies in the range of 100-300 eV. These electron beams suggest the presence of a second field-aligned potential drop of 100-300 V at altitudes above the acceleration region for the ions. It is likely that the field-aligned electron beams are associated with the Birkeland currents to the ionosphere that provide the closure of currents in the equatorial plane associated with the distributions of the hot equatorial plasmas.