Effect of a localized minimum in equatorial field strength on resistive tearing instability in the geomagnetotail

Abstract

A two‐dimensional, resistive‐MHD computer code is used to investigate the spontaneous reconnection of magnetotaillike configurations. The initial conditions adopted in our simulations are of two types: (1) in which the equatorial normal magnetic field component Bze declines monotonically down the tail, and (2) in which Bze exhibits a deep minimum in the near‐earth plasma sheet. Configurations of the second type have been suggested by Erickson (1984, 1985) to be the inevitable result of adiabatic, earthward convection of the plasma sheet. To represent the case where the earthward convection stops before the X line forms, i.e., the case where the interplanetary magnetic field turns northward after a period of southward orientation, we impose zero‐flow boundary conditions at the edges of the computational box. The initial configurations are in equilibrium and stable within ideal MHD. The dynamic evolution of the system starts after the resistivity is turned on. The main results of our simulations basically support the neutral‐line model of substorms and confirm Birn's (1980) computer studies (which used a different numerical scheme and type 1 initial configurations). Specifically, we find spontaneous formation of an X‐type neutral point and a single O‐type plasmoid with strong tailward flow on the tailward side of the X point. In addition, the results show that the formation of the X point for the configurations of type 2 is clearly associated with the assumed initial Bz minimum. Furthermore, the time interval from the turning on of the resistivity to the formation of a plasmoid is much shorter in the case where there is an initial deep minimum. A simple analytic calculation is also carried out to demonstrate why the configuration with a deep minimum is more susceptible to the development of the neutral point.