A 2.5 dimensional MHD simulation of multiple‐plasmoid‐like structures in the course of a substorm

Abstract

As well known, the magnetic cross‐tail component By in the magnetotail is well correlated with the interplanetary magnetic field (IMF) By component, and its variation is significant during substorms. A 2.5‐dimensional MHD simulation has been carried out on the basis of equilibrium solutions of the quiet magnetotail. Two types of distributions of the By component and the plasma density (Type I and Type II) are used as the initial states of the simulation studies. The results of all cases with different initial conditions illustrate that the formation of plasmoids occurs intermittently and repeatedly. Also, all the plasmoids are high‐density and high‐temperature regions in comparison with the ambient environment; that means the plasma with high‐temperature is embedded in plasmoids formed repeatedly. These results are in line with features of multiple‐plasmoid‐like structures observed on January 15, 1994, with Geotail at 96 RE in the tail. Thus it can be concluded that a large amount of the energy stored in the magnetotail is gradually dissipated by ejecting multiple plasmoids in the course of substorms. In this simulation it is shown that the lasting inflow caused by the electric field E imposed on the boundary of the simulation box controls the recurrent formation of plasmoids. Further, as an additional evidence, the multiple‐plasmoid‐like structures have been detected by Geotail in conditions of high‐speed solar wind streams and southward IMF. Therefore our simulation suggests that the solar wind and the IMF have close control over the magnetotail dynamic process. The features of a classic plasmoid (bipolar Bz) and a travelling compression region (TCR) in the (X, Z) plane can be seen in all cases. Taking the time evolution of the By component into account, two kinds of plasmoid‐like structures with a flux rope core and with both By and Bz bipolar signatures can be reproduced for the Type I and Type II conditions, respectively. Therefore the occurrence of various magnetic structures in the magnetotail might be related to nonsteady driven reconnection with different initial distributions of the By component.