Comparative Study of Bipolar Hall Magnetic Field Structures at Earth's Dayside Magnetopause Using MMS Observations

Abstract Hall magnetic fields at Earth's dayside magnetopause provide key diagnostics for collisionless reconnection and the associated Hall‐current closure. Using observations from Magnetospheric Multiscale (MMS) mission, we present a unified framework to classify out‐of‐plane Hall fields by peak composition—central unipolar ( C ), sunward bipolar ( S + C ), earthward bipolar ( C + E ), and tripolar ( S + C + E ) ‐ and by their displacement relative to the magnetopause midplane. We apply this framework to seven bipolar events (H1–H7) and relate Hall‐field displacement to the Hall‐region density asymmetry parameter. For the first five earthward‐type events (H1–H5), Hall‐density asymmetry covaries with the observed displacement of the bipolar Hall field structure, indicating that asymmetric Hall‐density produces a larger offset of the peak amplitude toward the magnetosphere. Previously, the sunward‐type event H6 was displaced toward the magnetosheath with moderate Hall‐field asymmetry. On 16 October 2015 (H7), we find a sunward bipolar ( S + C ) signature but displaced deep into the magnetosphere, reaching a nearly symmetric Hall‐field ratio despite high Hall‐density asymmetry. In H7, the absence of a magnetospheric E peak, together with the complete traversal of the Hall‐region into the magnetosphere, rules out a tripolar configuration previously proposed for H6. In addition, sequential electron jets and a new pattern in the bipolar normal electric field coincide with the magnetospheric Hall interval in H7. This suggests that displaced Hall electron dynamics, together with extreme asymptotic density and high temperature asymmetry, reshaped the bipolar Hall magnetic field. The findings of this study provide observational constraints for kinetic Hall models of asymmetric guide‐field reconnection.