Quantitative study of the nonlinear formation and acceleration of plasmoids in the Earth's magnetotail

Abstract

The formation and dynamical evolution of plasmoids are investigated by two‐dimensional resistive MHD computations. It is shown that a magnetotail equilibrium has to include a distant neutral line for a quantitative evaluation of the relevant dynamical properties of plasmoids. Special attention is given to the acceleration mechanism, which exhibits qualitatively a universal pattern but depends quantitatively on the details of the initial field structure and on the type of the microscopic dissipation. In addition to pressure and magnetic forces, mass and momentum transfer from the surrounding plasma to the plasmoid contributes to the acceleration process. These contributions arise in the presence of nonidealness, i.e., resistivity in the system. In the beginning of the plasmoid formation process, a unique feature of the acceleration is the dominant contribution of momentum transfer and pressure forces. For the cases considered, the pressure forces strongly depend on the chosen initial equilibrium state. Although magnetic forces increase before the plasmoid detaches from the near‐Earth X line, the contribution of mass transfer to the plasmoid becomes dominant, which leads to rapid growth and small deceleration of the plasmoid. When the detachment has occurred, mass and momentum transfer is small, and the relative importance of magnetic forces and pressure forces depends on the chosen initial state and the chosen resistivity. Larger Lundquist numbers and/or a current‐dependent resistivity similar to the one used in the computations seem to enhance magnetic forces. At rather late times an enhanced electric field at the distant neutral line might lead to growing mass transfer by which a plasmoid may be decelerated. The qualitative picture of a slingshot effect, i.e., plasmoid acceleration by the tension of open interplanetary magnetic flux, is shown to be misleading for the cases considered in this paper. For magnetotail configurations that are more realistic than those considered so far, the results confirm that spontaneous magnetic reconnection resulting in plasmoid formation and acceleration represents a sufficiently fast process in order to explain magnetotail dynamics during substorms.