Magnetotail convection in geomagnetically active times .2. Dawn-dusk motion in the plasma sheet

The nature of the dawn-dusk component of the motion of magnetic field lines in the plasma sheet is studied at the distances of 40 to 160 R(e) in active times. In the presence of the B-y component of the magnetic field, even the flow which is purely parallel to the tail axis (which is taken to be the x direction) produces an apparent velocity of field lines in they direction. However, the more substantial cause of the dawn-dusk motion is the rotation of magnetic field lines. In the region of the earthward flow, the magnetic field lines rotate in such a way as to become closer to the plane which contains the earth's dipole moment; that is, the tilting of field lines caused by the presence of B-y becomes smaller as field lines move closer to the Earth. In the region of the tailward flow, the magnetic field lines rotate from northward to southward in a clockwise or counterclockwise sense depending on the sign of B-y as they move tailward. The net convection in the dawn or dusk direction, if it existed, is masked by this rotational motion in the plasma sheet.