Hall Fields and Current Systems of Magnetic Reconnection under Asymmetric Conditions

Abstract

The Hall effect is a key component of collisionless reconnection. A previous study showed that a quadrupolar Hall pattern under symmetric conditions degenerates into a bipolar pattern under highly asymmetric conditions. We study the properties of Hall magnetic fields and current systems during three reconnection events observed by the Magnetospheric Multiscale mission at the dayside magnetopause. Although the asymptotic density ratio between the magnetosheath and magnetospheric plasmas is very high for all three events, clear quadrupolar Hall field patterns are observed in all three events. The quadrupolar Hall magnetic fields in the three events display their respective properties on the intensity asymmetry and the distributing location. Among these events, a quadrupolar Hall field pattern is observed for the first time in in situ observations: the magnetosheath Hall pattern occupies the whole midplane region, while the magnetosphere Hall pattern still exists under highly asymmetric conditions. Observations show that the plasma mixture modulates the density asymmetry in the Hall region, which can be very different from the asymptotic density asymmetry in the magnetosheath and magnetospheric inflow plasma. 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. Based on the observed facts, we suggest that the three reconnection events studied here are in different phases of their development after they are triggered under highly asymmetric conditions. Our results provide new insights into how the Hall effect works with the evolution of asymmetry during reconnection.