Pre-reconnection: Plasma motions preconditioning the magnetotail for topology change

A distinguishing feature of spontaneous reconnection in the magnetotail is that the corresponding plasma instability must start before the magnetic topology change. This is in contrast to reconnection in one-dimensional current sheets where any tearing-type perturbation of the field immediately creates magnetic islands. Such "pre-reconnection" motions may develop as ballooning/interchange modes due to the plasma buoyancy, with the subsequent X-lines being inherently limited in the dawn-dusk direction by the width of the corresponding interchange fingers, i.e., the wavelength of the mode. In this presentation we consider another class of magnetotail instabilities that are capable of creating more global X-lines. Formerly known as the ion tearing instability due to their similar eigenmode structure and the ion Landau dissipation as an energy sink, they may develop initially as an ideal MHD process, leading only to the redistribution of the magnetic flux in the closed field line region. Their development requires an accumulation of the magnetic flux sufficiently far in the tail to relax the sufficient tearing stability criterion (Lembege and Pellat, 1982). The formation of the corresponding ridge in the normal magnetic field is seen in global MHD simulations, Geotail and IMP8 data. The transition from the ideal MHD to kinetic stage of the instability requires that the tail current sheet is sufficiently thin to allow the ion Landau dissipation. We show that such a self-consistent equilibrium with a thin current sheet embedded into a global magnetotail plasma sheet can be formed in case of weak plasma anisotropy. Finally, we discuss the stability of such equilibria and describe their nonlinear evolution using 3D PIC simulations.