Probing the boundary between antiparallel and component reconnection during southward interplanetary magnetic field conditions

One of the major outstanding questions about magnetic reconnection is where reconnection will occur at the Earth's magnetopause for specific conditions of the solar wind and the interplanetary magnetic field (IMF). There are two scenarios discussed in the literature: (1) antiparallel reconnection, which occurs where the magnetospheric magnetic field and the IMF are antiparallel (shear angle of approximately 180 degrees) and (2) component reconnection, where shear angles between the magnetospheric field and the IMF as low as 50 degrees have been reported. The distinction between the two reconnection scenarios is important for the energy and momentum transfer from the solar wind to the magnetosphere. Here we report on a method using three-dimensional plasma observations from the Toroidal Imaging Mass-Angle Spectrograph instrument on the Polar spacecraft as it passes through the northern magnetospheric cusp to calculate the distance to the reconnection line and subsequently trace the distance along model magnetic field lines back to the magnetopause. Results from 130 events reveal that in general, magnetic reconnection occurs along an extended line across the dayside magnetopause (i. e., consistent with the component reconnection scenario). During strong sunward or antisunward IMF conditions (B-X), however, the reconnection location resembles the antiparallel reconnection model at high latitudes and does not cross the dayside magnetopause as a single tilted reconnection line. These results show that either reconnection scenario can occur at the magnetopause, depending on the specific IMF conditions.