Observational investigation of Hall effect in asymmetric magnetic reconnection

Using MMS observation, we do a series of studies to investigate Hall effect in multiple reconnection cases at dayside magnetopause. The studies show the following interesting results. First, the hexapolar Hall magnetic field was observed in collisionless magnetic reconnection for the first time. And the associated electric field and electron dynamic display the different features from previous study. Second, it was found that the middle polar of hexapolar Hall magnetic field can play the role of guide field. Thus, hexapolar Hall magnetic field can make antiparallel reconnection to bear the features of guide field reconnection. Furthermore, the coexistence of hexapolar Hall magnetic field and magnetic flux rope provides the first evidence that the Hall effect in quasi-antiparallel magnetic reconnection can generate the core field inside a magnetic flux rope. Finally, for three reconnection events at the magnetopause, the quadrupolar Hall magnetic fields in them display their respective properties on the intensity asymmetry and the distributing location. The analyses indicate that the different density asymmetry inside the Hall region, but not the asymptotic density asymmetry, is an exact indicator that explains the different observed Hall patterns. This series of studies offer a new insight into the role of Hall effect in collisionless magnetic reconnection, and how the Hall effect works with the evolution of asymmetry during reconnection.