CENTRIFUGALLY DRIVEN PHASE BUNCHING AND RELATED CURRENT SHEET STRUCTURE IN THE NEAR-EARTH MAGNETOTAIL

We examine the role of centrifugal effects during nonadiabatic interac tions of charged particles with the magnetotail current sheet. It is s hown that when the parameter kappa (defined as the square root of the minimum curvature radius- to- maximum Larmor radius) is of the order o f unity, as is the case for ions traveling in the near-Earth plasma sh eet, enhanced centrifugal effects lead to prominent bunching of the pa rticles in gyration phase. As a result of this bunching effect we demo nstrate that a thin current sheet develops in the vicinity of the tail midplane. When average values of the plasma density (a few tenths of ions per cubic centimeter) and temperature (several keV) in the near-E arth tail are used, the current sheet obtained has a characteristic th ickness of the order of a few tenths of an Earth radius and leads to s ignificant stretching of the local magnetic field lines. A further con sequence of phase bunching is the buildup of a substantial current in the Earth-tail direction at law latitudes, which leads to field line i nclination in the dawn-dusk direction. This phase bunching mechanism, which maximizes when the bulk of the ion distribution nears kappa = 1, is of potential importance for the dynamics of the inner plasma sheet during the growth phase of substorms.