Observations of reconnection of interplanetary and lobe magnetic field lines at the high‐latitude magnetopause

Abstract

Measurements made with the Fast Plasma Experiment on ISEE 2 in the vicinity of the high‐latitude, dusk magnetopause near the terminator plane, at a time when the local magnetosheath and tail lobe magnetic fields were nearly oppositely directed, provide direct evidence for the reconnection of the open field lines of the tail lobes with the interplanetary magnetic field (IMF). The evidence consists primarily of observations of accelerated magnetosheath plasma flowing both tailward and sunward within the high‐latitude magnetopause current layer. Observed speed changes at the magnetopause were of the order of twice the magnetosheath Alfven speed and were quantitatively consistent with the predictions of reconnection models. At times when the newly entering magnetosheath plasma observed at ISEE 2 was accelerated sunward a secondary beam of ions, presumably mirrored at low altitudes, was occasionally present. At times when the newly entering magnetosheath plasma observed by ISEE 2 was accelerated tailward a secondary beam of largely unaccelerated mantle plasma was occasionally present. Small plasma accelerations observed on reconnected field lines in the magnetosheath were associated with the presence of ions reflected at the magnetopause and moving at a speed of approximately twice the Alfven speed relative to the remainder of the magnetosheath plasma. Although previous work has anticipated that the re‐reconnection of the open field lines of the tail lobes with the IMF would be associated with northward IMF in the magnetosheath, the present reconnection event was associated with a local magnetosheath IMF which had a small southward component.