Role of magnetic flux function and geometrical analysis of field line topology in magnetic reconnection events

Abstract This study examines the structural and dynamic properties of flattened tail current sheets during intervals of weak substorm activity, based on observations from the magnetospheric multiscale mission. Using the nonlinear magnetic field gradient algorithm and geometrical invariant techniques, the analysis focuses on two distinct events characterized by opposite signs of the B y component. In both cases, a prominent B y is observed at the neutral current sheet’s center. Fast magnetic reconnection near the neutral sheet generates intense electron jets, with electron velocities significantly exceeding ion velocities, confirming electrons as the principal charge carriers. Magnetic field lines within the neutral sheet display clockwise rotation around the northward normal, forming left-handed spiral configurations. The curvature and torsion of these field lines differ notably between events: the first event ( B y > 0) shows maximum curvature and zero torsion at the sheet’s center, while the second event ( B y < 0) exhibits minimal curvature and elevated torsion. The helix angle averages approximately 45° in the first event and shifts toward 90° in the second. Geometrical invariants reveal varying magnetic topologies, with flux tube-like structures dominating the first event and a combination of flux tubes and flux ropes emerging in the second. The prevalence of flux tubes in the first event suggests a weaker magnetic field transition, as flux ropes are more effective in transporting magnetic flux. These findings highlight the significance of current sheet geometry in governing reconnection dynamics.