ION DYNAMICS AT THE EARTHWARD TERMINATION OF THE MAGNETOTAIL CURRENT SHEET

We investigate the dynamics of plasma sheet ions upon interaction with the magnetotail current sheet. During this interaction, ions can be s ubjected to prominent bunching in gyration phase because of enhanced t ailward oriented centrifugal pull. We focus on the inner part of the p lasma sheet and examine the consequences of this centrifugally driven phase bunching effect. In this region of space, as their Larmor radius is locally comparable to or smaller than the field variation length s cale, ions experience nonadiabatic motion in a manner which differs fr om that in the distant tail where particles meander about the field mi nimum. Within the limits of the calculations which do not treat the pa rticle dynamics in a self-consistent manner, we show that phase bunchi ng of plasma sheet ions leads to a distinctive current distribution in the near-Earth tail, which is not found further out. In the equatoria l vicinity, this distribution is characterized by a striated pattern o f current both in the dawn-dusk and in the Earth-tail directions. The latter current leads to field line inclination in the dawn-dusk direct ion at the earthward edge of the magnetotail current sheet, which may be viewed as characteristic of this region of space. We also show that in this region, off-diagonal elements of the pressure tensor appear o f importance to achieve stress balance.