Evidence for the core field polarity of magnetic flux ropes against the reconnection guide field

Abstract

We present the observational evidence by Time History of Events and Macroscale Interactions during Substorms (THEMIS)-D spacecraft to demonstrate that magnetic flux rope can have a core field polarity opposite to the guide field during asymmetric reconnection with a high magnetic shear across the magnetopause. In the presented event, a bipolar reconnection outflow was observed at the magnetopause, across which the magnetic shear was ~149° corresponding to a predicted guide field BG = −15.6 nT. The ratios for magnetic field strength and ion density between the magnetospheric and magnetosheath sides of the magnetopause were ~1.6 and ~0.15, respectively. During the event, the component of magnetic field along the M axis (the intermediate variance direction) remained negative and stable for most of the time, and its mean value agreed with the BG. Two magnetic flux ropes with a strong core field were identified, but one flux rope had a core field polarity against the guide field BG. Our finding differs from current theoretical simulations that the core field polarity of the flux ropes inherits from the guide field.