The distortion of the magnetotail equilibrium structure by a net cross‐tail magnetic field

Abstract

Observations have shown that in addition to a y component associated with the flaring of the tail, a net y component of the magnetic field can persist in the geomagnetic tail, roughly proportional to the interplanetary By. We use the tail equilibrium theory outlined by Birn (1987) to construct self‐consistent three‐dimensional models of the tail that include this effect. We find that the net ByN field varies only weakly along field lines and across the tail. The presence of the net ByN in the neutral sheet implies that a part of the cross‐tail current becomes field‐aligned, flowing from the southern to the northern hemisphere for ByN > 0 and from north to south for Byn < 0. This field‐aligned current is proportional to ByN for small values. For a typical value of 2% of the lobe field strength we find a total current of 4.4×105 A flowing toward one hemisphere at x = −20RE. This current, however, is not expected to reach the Earth, because it decreases strongly along each field line away from the neutral sheet, due to a conversion into perpendicular current. The deformations of magnetic flux surfaces, implied by the models, are not consistent with an entirely closed magnetotail boundary. They suggest that in addition to traditional lobe field lines, interconnected with the solar wind field at large distances or high latitudes, there may be a class of field lines extending from the Earth into the magnetosheath region through the low latitude flanks of the tail. These field lines, which might even be associated with positive Bz at a neutral sheet crossing, effectively reduce the region of closed flux. For typical values of ByN of 2% of the lobe field this effect is small, but for values above about 10%, not extremely unusual, it can be appreciable. For ByN > 0 this opening occurs preferably on the north/dawn and south/dusk sides, producing an asymmetry and an apparent tilt of the closed field line plasma sheet region around the tail axis relative to the neutral sheet. We find also a topological connection of the low latitude region near the magnetopause with the plasma sheet boundary layer, separating closed plasma sheet and open lobe field lines. For ByN > 0 this interconnection takes place on the north/dusk and south/dawn sides of the low latitude boundary region.