The location of reconnection at the magnetopause: Testing the maximum magnetic shear model with THEMIS observations

Abstract

Reconnection at the Earth's magnetopause is the mechanism by which magnetic fields in different regions change topology to create open magnetic field lines that allow energy and momentum to flow into the magnetosphere. One of the long‐standing open questions about magnetic reconnection is the location of the reconnection line. There are two reconnection scenarios discussed in the literature: (1) antiparallel reconnection where shear angles between the magnetospheric field and the interplanetary magnetic field (IMF) are near 180° and (2) component reconnection where a tilted reconnection line which crosses the magnetopause in the subsolar region at shear angles not near 180°. Early satellite observations were limited to the detection of accelerated ion beams in the magnetopause boundary layer to determine the general direction of the reconnection line location with respect to the satellite. An improved view of the reconnection location at the magnetopause was determined from ionospheric emissions observed by polar‐orbiting imagers which revealed that both scenarios occur. The time‐of‐flight effect of precipitating ions in the cusp in connection with the low‐velocity cutoff method pinpointed reconnection locations and their dependency on IMF conditions. These results are summarized by the maximum magnetic shear model. This study uses confirmed magnetic reconnection locations from the THEMIS mission to test the predictions of this reconnection location model. The results reveal that the maximum magnetic shear model predicts the observed reconnection locations for dominant IMF BY conditions very well but needs further improvement and modifications for dominant southward IMF conditions.