On the structure of the tail magnetic field

The configuration of the magnetic field in the Earth's geometric tail is studied. The results in the lobe region and in the plasma sheet boundary layer region are compared. A systematic enhancement of the By component of the field is detected in the plasma sheet boundary layer with respect to the lobe; this enhancement leads to a lower rate of flaring of the magnetic field lines in the lobe with respect to the plasma sheet boundary layer. This observation is a signature of field aligned currents at the boundary. The direction of such field aligned currents agrees with the nightside region two current system.