Evidence of radial nulls near reconnection fronts

Radial nulls, where magnetic field strength becomes zero and the magnetic field lines point radially in the fan plane, are believed to be crucial for particle acceleration and energy dissipation during magnetic reconnection. Recent simulations have suggested that reconnection fronts (RFs) embedded in the reconnection jet can be host to radial nulls. However, observational evidence for radial nulls near the RFs remains elusive hitherto, owing to the absence of an efficient null-detection method and high-resolution measurements. Here we present such evidence by using the newly developed First-Order Taylor Expansion method and the high-resolution measurements from the recent MMS mission. For the first time, we confirm the existence of radial nulls near the RFs and find that the upstream region ahead of the RFs can preferentially host the radial nulls. The reconstructed topologies of the observed radial nulls are consistent with theoretical models. Associated with these radial nulls, no clear particle and wave activities were found, meaning that they were inactive during the observations.