Evidence of Magnetic Nulls in Electron Diffusion Region

Theoretically, magnetic reconnection—the process responsible for solar flares and magnetospheric substorms—occurs at the X-line or radial null in the electron diffusion region (EDR). However, whether this theory is correct is unknown, because the radial null (X-line) has never been observed inside the EDR due to the lack of efficient techniques and the scarcity of EDR measurements. Here we report such evidence, using data from the recent MMS mission and the newly developed First-Order Taylor Expansion (FOTE) Expansion technique. We investigate 12 EDR candidates at the Earth's magnetopause and find radial nulls (X-lines) in all of them. In some events, spacecraft are only 3 km (one electron inertial length) away from the null. We reconstruct the magnetic topology of these nulls and find it agrees well with theoretical models. These nulls, as reconstructed for the first time inside the EDR by the FOTE technique, indicate that the EDR is active and the reconnection process is ongoing.