A preliminary exploration of the mechanism for the occurrence of two typesof various magnetic structures in the magnetotail

As well known, the magnetic cross-tail component B-y in the magnetotail is in direct proportion to the interplanetary magnetic field (IMF) B-y compone nt. And the polarity of IMF and plasmoid/flux rope B-y components do indeed agree. This results indicate that the IMF B-y penetrates plasmoids and the magnetic structures must therefore be three-dimensional. In this note, the dynamical processes of magnetotail in the course of a substorm are studied using a MHD code with two-dimensions and three components on the basis of two types of initial equilibrium solutions of the quiet magnetotail. The nu merical results of two cases illustrate various features of time evolution of B-y component that correspond to two kinds of plasmoid-like structures: one is associated with a flux rope core and the other resembles a "closed l oop" plamoid. Therefore, the occurrence of various magnetic structures in t he magnetotail might be related to nonsteady driven reconnection with diffe rent distributions of the B-y component.