A STATISTICAL STUDY OF ACCELERATED FLOW EVENTS AT THE DAYSIDE MAGNETOPAUSE

Accelerated plasma flows in the magnetopause current layer and low-latitude boundary layer are examined using the ISEE 2 spacecraft. Accelerated flow of magnetosheath plasma inside the magnetopause is an indication that reconnection is occurring or has recently occurred. Over 180 inbound and outbound magnetopause crossings of the ISEE 2 spacecraft have been examined for such evidence of reconnection. Of those orbits for which a good determination could be made, a total of 58 statistically independent accelerated flow events were found where only one event is chosen per inbound or outbound magnetopause traversal. Upstream solar wind data are available for 42 of these 58 events. These 42 traversals are examined in relation to the upstream solar wind and magnetosheath parameters. The occurrence of accelerated flows is found to be dependent upon the direction of the upstream interplanetary magnetic field and the local magnetic shear across the magnetopause. The occurrence rate of accelerated flows is greatest when the interplanetary magnetic field has a southward component in solar magnetospheric coordinates and the local magnetic shear across the magnetopause is large. As the magnetosheath beta increases, the occurrence of accelerated flows decreases. Events in which the magnetic shear is small occur only when the beta is low. This study also provides a statistical estimate of the length and tilt of the merging line. The location and direction of the accelerated flows indicate that merging occurs most often near the subsolar point along a line of length approximately 6 R(E) that tilts in response to the direction of the interplanetary magnetic field.