The Role of a Strong Out-Of-Plane Magnetic Field at the X-Line in Antiparallel Magnetic Reconnection as a Guide Field

The magnetic reconnection process substantially changes the configuration of the magnetic field in plasma and explosively releases magnetic energy. The asymptotic magnetic field lines during magnetic reconnection events can be strictly antiparallel or only have one antiparallel component, and the magnetic reconnection features are distinct under different asymptotic magnetic fields. Here, we provide the first observational evidence demonstrating the existence of a strong out-of-plane magnetic field at the X-line in antiparallel reconnection. The analyses described herein show that this strong out-of-plane magnetic field significantly changes the structure of the diffusion region, thereby altering the reconnection features in terms of particle dynamics and energy dissipation by way of a guide field. A new pattern of the normal electric field near the X-line is responsible for producing this out-of-plane magnetic field. Our findings suggest that, in addition to asymptotic conditions, local processes inside the diffusion region can also determine whether magnetic reconnection events develop as antiparallel reconnection or component reconnection.