Identification of Magnetic Null on Reconnection 19 September 2015 Event using Magnetospheric Multiscale Mission

Magnetic nulls referred to a region with vanishing magnetic fields play an essential role in energy conversion and particle acceleration during magnetic reconnections. Moreover, they are also useful tools to characterize the complex magnetic topologies and are essential in three-dimensional (3D) magnetic reconnection. In this paper, we apply the linear interpolation methods to derive the null positions of the dayside reconnection Electron Diffusion Region (EDR) on 19 September 2015 using Magnetospheric Multiscale (MMS) mission spacecraft. The number of the signs of the real part of eigenvalues determines the magnetic field topology nearby this null. The change of the topology can be related to the occurrence of bifurcation or magnetic reconnection. Here, we report the first in situ measurement that the field topology of magnetic nulls changes from B type to A type at around 07:43:30.4 associated with the reported EDR event.