Low-latitude dayside magnetopause and boundary layer for high magnetic shear .2. Occurrence of magnetic reconnection

Quantitative comparisons of the flow velocity change across the magnetopause (MP) with the prediction from local tangential stress balance ill a one-dimensional time-stationary rotational discontinuity, that is, with the Walen relation, have been performed on 69 Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) low-latitude (<30 degrees) dayside (0800-1600 LT) high-shear (>45 degrees) MP crossings. It is found that in 61% of the crossings the observed how changes agree with the prediction tb better than 50%. No dependence of the occurrence of reconnection Bows on local magnetic shear, local time/latitude, local tangential magnetosheath flow speed, and local magnetosheath Alfven Mach number is found. We confirm an earlier result that the agreement with the Walen relation becomes worse with increasing magnetosheath plasma beta(beta is the ratio of plasma pressure to magnetic pressure) but find that the velocity change itself, predicted by the walen relation, decreases with increasing beta. Moreover, the motion and thickness of the boundary also depend on beta: the higher the beta value, the faster the speed and the smaller the thickness. These effects combine to make velocity changes in high beta events more difficult to measure accurately, which may contribute to the poor agreement with the Walden relation in these events. The 42 events which exhibit plasma flows in reasonable agreement with the Walen relation include 21 cases where the flow direction is inconsistent with a single X line hinged at the subsolar point. The discrepancies between the former result and dayside X line locations-reported earlier may be due to a bias in selection of reconnection events in earlier studies. An average (dimensionless) reconnection rate that is substantially lower than 0.1 is inferred for-the 42 events.