Observations of magnetic reconnection at the lobe magnetopause

Measurements made by the Fast Plasma Experiment on ISEE 2 provide direct evidence for the rereconnection of the open field lines of the tail lobes with the interplanetary magnetic field at the lobe magnetopause. The evidence, obtained on seven different dates at locations well removed from the noon-midnight plane, consists primarily of observations of accelerated magnetosheath plasma within the lobe magnetopause current layer. These accelerations were either sunward or tailward, depending on the position of the satellite relative to the rereconnection site. Such accelerations have been observed in both the northern and the southern hemispheres, on both the dawn and the dusk sides of the magnetosphere, and both sunward and tailward of the dawn-dusk terminator. Except for one event, all of the lobe magnetopause reconnection events examined here occurred at times of large local field shears at the magnetopause ; these large field shears were associated primarily with large x and y magnetic field components rather than with the z component. In each case the magnetosheath plasma crossing into the magnetopause current layer was heated as well as accelerated. Often, this newly heated and accelerated plasma, which was confined to the current layer where the field rotated from its magnetosheath orientation to its magnetospheric orientation, coexisted as a double-ion beam with unaccelerated mantle plasma that had entered the magnetosphere earlier upstream from the rereconnection site. The change in velocity experienced by the newly entered plasma within the current layer was usually in reasonable accord with stress balance at the magnetopause. Apparent accelerations and decelerations observed upstream from the magnetopause in two of these events were associated with secondary, field-aligned ion populations that appeared to result from reflection of magnetosheath ions at the magnetopause.