Thin current sheet instabilities relevant to the onset of reconnection in the geomagnetotail: Theory and observations

Two basic families of thin current sheet instabilities responsible for the onset of X- and Y-line reconnection in the tail current sheet of Earth's magnetosphere are discussed. The X-line reconnection onset becomes possible due to the growth of the tearing mode. Its stability is shown to crucially depend on the presence of a transient electron population and as a result the X-line can be formed only far enough from the Earth. Earthward of this critical distance the tail current sheet is tearing-stable and evolves into a thin current sheet (TCS). These results are fully consistent with recent Geotail observations [Asano, 2001], which show that the X-line is initially formed near the tailward edge of the evolved TCS. On the other hand, the formation of TCS creates the free energy source for current-driven instabilities that are not significant in the case of the conventional Harris equilibrium [Daughton, 1999]. This arises due to the bulk flow velocity shear provided by the nonadiabatic motion of ions. We provide the new results of the nonlocal stability analysis of TCS taking into account the effect of bulk flow velocity shear. In contrast to recent results [Shinohara et al., 2001; Daughton, 2002], where the shear appeared as a nonlinear effect resulting from the lower-hybrid turbulence in the pure Harris sheet with zero normal component of the magnetic field, we explore the stability of more realistic self-consistent TCS models with nonzero normal magnetic field that already have the velocity shear [Sitnov et al., 2000]. A distinctive feature of these models is the bean-shaped ion distribution at the center of the sheet. It is consistent with the characteristic ion distributions prior to the onset of the current disruption in the magnetotail [Lui, 2002].