Magnetic field and electric current density distribution in the geomagnetic tail, based on Geotail data

Analysis of Geotail magnetic data enables us to reveal the antisunward elec tric current flowing along the tail axis. To our knowledge, observation of this current has not been reported yet. Distributions of the magnetic field and electric current density (along with their dependencies on the tilt an gle of the Earth's dipole and components of the interplanetary magnetic fie ld) have been derived directly from Geotail data, without any ad hoc assump tions. Analysis of the electric current density distribution shows that, in addition to currents associated with the geomagnetic tail flaring, there i s a current (tentatively identified as the Hall current) flowing antisunwar d along the tail axis. The total strength of this current is of the order o f 1 MA. It closes through the midnight sector of the auroral zone resulting in field-aligned currents in the region of the Harang discontinuity.