GEOTAIL observation of magnetospheric convection in the distant tail at 200 RE in quiet times

Magnetic field and plasma observations by GEOTAIL in the distant tail at x = −200 R E are studied for a geomagnetically quiet interval when IMF Bz is predominantly northward and | By | is larger than B on average. In the distant tail during this interval, Bz is northward and Ey is directed dusk‐to‐dawn on average. This combination of Bz and Ey does not seem to represent the tailward convection of the closed field lines, because Ey is much weaker in the lobe than in the plasma sheet so that the field lines would be piled up at the plasma sheet boundary if they were closed. The observations suggest instead that the plasma and field lines are convected parallel to the neutral sheet across the tail at the same time as they flow tailward. The direction of this cross‐tail convection depends on the polarity of IMF By and is antisymmetric with respect to the neutral sheet, which can be twisted by tens of degrees under the influence of IMF By . A consistent picture is obtained from observations both inside the tail and at the tail magnetopause. This convection profile agrees in topology with the cusp reconnection model, but it occurs mainly in the plasma sheet while only the lobe field lines are expected to be involved according to this model. Observations show that not only the cold‐dense ions which can be linked directly with the entrant solar wind plasma but also the hot‐tenuous ions in the plasma sheet take part in the convection.