Detecting Reconnection Sites Using the Lorentz Transformations for Electromagnetic Fields

Abstract

We take a pragmatic definition of reconnection to find locations where a reconnection electric field causes an E × B drift that carries two components of the magnetic field toward their elimination. With this in mind as our target, we observe that such locations can be found using a new indicator: the velocity of the Lorentz transformation that eliminates two components of the local magnetic field. Serendipitously, the indicator naturally becomes subluminal in the close proximity of a point where two components of the magnetic field vanish and it is hard zero at the vanishing location. Everywhere else the velocity of this Lorentz frame change far exceeds the speed of light. This property can be quickly applied in practice because computing the frame change is a local operation that requires only the knowledge of the local magnetic and electric field: it can be applied in a simulation or in observational data from a field instrument. We further show that the points identified can be classified in six categories that extend the usual types of magnetic nulls to the case of 3D reconnection in the presence of a guide field. The approach is used to identify secondary electron-scale reconnection sites in a turbulent outflow from a primary reconnection site in a highly resolved massively parallel fully kinetic particle-in-cell simulation. Numerous points are found, and their detailed analysis is reported.