Asymmetric time‐dependent and stationary magnetic reconnection at the dayside magnetopause

Abstract

Using a two‐dimensional compressible MHD code we have numerically studied the reconnection process at an interface where (1) the magnetic field is higher on one side than on the other and total pressure is in balance by a higher density on the weak field side, and where (2) the magnetic field is identical on both sides, but the temperature is higher on one side than on the other. Reconnection is caused by applying a localized resistivity in the center of the current sheet. The plasma is allowed to enter and exit freely at the boundaries. By using a system size with a large extension parallel to the current sheet, we are able to study time‐dependent as well as steady state effects. Denoting the weak magnetic field or low temperature region as magnetosheath and the high magnetic field or high temperature region as magnetosphere we find the following. Sudden onset of reconnection causes the development of a bulge in the current layer. The bulge moves with about 70% of the magnetosheath Alfvén speed along the magnetopause away from the reconnection region. The growth of the bulge normal to the magnetopause is proportional to the reconnection rate. For a ratio of magnetospheric to magnetosheath magnetic field larger than about 1.5 the bulge is almost exclusively on the magnetosheath side of the magnetopause current layer. The motion of the bulge along the magnetopause leads to signatures in the normal magnetic field component similar to the ones observed in flux transfer events. When the bulge has moved far enough from the reconnection site, stationary reconnection proceeds at the X line. If there exists initially a magnetic field difference across the magnetopause current layer, a slow mode shock is formed in the magnetosphere and possibly a strong intermediate shock is formed in the magnetosheath. The magnetosheath plasma is accelerated to about twice the magnetosheath Alfvén speed and constitutes a boundary layer on the magnetospheric side of the magnetopause current layer.