Effects of magnetic shear and flow shear on magnetopause reconnection: Simultaneous observations from MMS and THEMIS

Magnetic shear and flow shears form across Earth’s magnetopause when shocked solar winds flow around Earth. Previous studies have shown that these two kinds of shears can similarly affect magnetopause reconnection. However, a direct investigation to evaluate their relative importance is lacking. In this study, we focus on simultaneous magnetopause reconnection observed by Magnetospheric Multiscale mission (MMS) and Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft at different magnetopause locations to quantitatively evaluate the magnetic shear and flow shear effects. In these observations, the overall effect of magnetic shear (the normalized guide field < 1) is limited unless the guide field is large enough to suppress reconnection, while the flow shear can significantly affect the observed reconnection outflow speed primarily by introducing non-zero X-line motion. Finally, we propose a new relationship combining magnetic and flow shear effects by assuming the X-line drift motion is independently affected by these two effects, which shows that X-line drift speed is dominated by the magnetosheath flow, and the suppression of reconnection is more likely to occur under strong guide field conditions. This study deepens our understanding on magnetopause reconnection occurrence and reconnection behaviors in large scales.