Asymmetric Hall Magnetic Field Structures in Oppositely Directed Jets During Guide‐Field Magnetopause Reconnection

Abstract Symmetric Hall magnetic field structures, such as bipolar or quadrupolar patterns, are commonly expected around a single magnetic X‐line during asymmetric guide‐field reconnection. In such cases, oppositely directed reconnection jets are generally associated with the same number of Hall magnetic field polarities. Here, we report for the first time Magnetospheric Multiscale (MMS) observations of asymmetric Hall magnetic field structures in oppositely directed magnetopause reconnection jets. We validate a method for estimating Hall magnetic fields from in‐plane electric currents using virtual spacecraft measurements from particle‐in‐cell (PIC) simulations of symmetric reconnection. We then examine three magnetopause reconnection events (G1, G2, and G3) with substantial guide fields (26%, 48%, and 79% of the reconnecting field). The G1 and G2 events exhibit northward and southward jets, respectively, while G3 shows a bipolar jet. Their upstream magnetic field and density ratios (magnetosphere to magnetosheath) are G1: (1.4, 0.17), G2: (1.3, 0.19), and G3: (1.6, 0.09), where the first value in each pair represents the magnetic field ratio and the second the density ratio. Despite comparable upstream conditions for G1 and G2, their Hall magnetic field structures differ: the southward jet exhibits a tripolar structure (, , ), whereas the northward jet shows a bipolar structure (, ), with the negative polarity located on the magnetospheric side. In G3, a tripolar structure (, , ) is also observed in the southward jet, whereas the northward jet shows only positive Hall magnetic field polarity. These observations agree with predictions from previous PIC simulations of asymmetric guide‐field reconnection.