On temporal variation of reconnection rate and X line electric field structure

We have inspected how the reconnection rate and electric field structure around an X line vary in response to macroscopic circumstances surrounding the X line. In a very simple simulation of two-dimensional collisionless magnetic reconnection, wherein reconnection is initiated in a thin current sheet with antiparallel magnetic field bounded by a periodic boundary, the reconnection rate increases explosively, hits a peak, slowly declines, and then reconnection terminates. This temporal variation of the reconnection rate at an X line is affected by the condition outside the X line, but it has not been clear how the X line senses the external conditions. By inspecting the associated variation of the spatial structure of the out-of-plane electric field, we found that the out-of-plane electric field at the outer edge of the outer Electron Diffusion Region (oEDR) plays a key role in regulating the reconnection rate by communicating the conditions beyond the oEDR to the inner EDR via the electric field profile. We also show that the behavior of the reconnection rate in more complicated situations can be explained from the same perspective.