Magnetic merging in the magnetospheric tail

Abstract

It is proposed that magnetic merging across the neutral sheet of the geomagnetic tail takes place very near the earth (i.e., between about 10 and 30 RE). The plasma density in the near-earth portion of the tail is usually low because the supersonic solar wind leaves a rarefied wake region extending about 30 RE behind the dipole-like magnetosphere. The merging of oppositely directed field lines occurs when the plasma density is too low to form a current sheet of sufficient strength to prevent merging. (This mechanism is self-limiting in that the plasma energy density is built up by the merging processes to be almost, but not quite, the required level.) The magnetic merging then takes place somewhat as it would in a vacuum. Merging in the neutral sheet, then, depends on the development of vacuum-like conditions immediately behind the earth. This vacuum-merging mechanism preserves the essential features of frozen-in flux that have been so successful in predicting the configuration of both the spiral interplanetary magnetic field and the extended magnetospheric tail. Magnetic merging does not appear to proceed when the plasma density is high enough to produce the currents required to prevent merging.