Controlling Factors of the Local Time Extent of Magnetopause Reconnection: A Statistical Study

Magnetopause reconnection is the dominant mechanism for transporting solar wind energy and momentum into the magnetosphere-ionosphere system. Magnetopause reconnection can occur along X-lines of variable extent in the direction perpendicular to the reconnection plane. Identifying the spatial extent of X-lines using satellite observations has critical limitations. However, we can infer the azimuthal extent of the X-lines by probing the ionospheric signature of reconnection, the antisunward flow channels across the ionospheric Open-Closed Field Line Boundary (OCB). We study 39 dayside magnetopause reconnection events using conjugate in situ and ionospheric observations to investigate the variability and controlling factors of the spatial extent of reconnection. We use spacecraft data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) to identify in situ reconnection events. The width of the antisunward flow channels across the OCB is measured using the concurrent measurements from Super Dual Auroral Radar Network (SuperDARN). Also, the X-line lengths are estimated by tracing the magnetic field lines from the ionospheric flow boundaries to the magnetopause. The solar wind driving conditions upstream of the bow shock are studied using solar wind monitors located at the L1 point. Results show that the magnetopause reconnection X-lines can extend from a few Earth Radii (RE) to at least $\sim$22 RE in the GSM-Y direction. Furthermore, the magnetopause reconnection tends to be spatially limited during high solar wind speed conditions.