Two steps triggering of magnetic reconnection in a bifurcated current sheet

Carrying out large scale three-dimensional full kinetic simulations, we have found that a quick triggering of magnetic reconnection (QMRT) is possible in the thin current sheet where its characteristic thickness is less than the ion inertia length. In the course of QMRT, the lower hybrid drift instability (LHDI) plays crucial role, and the emergence of the thin embedded electron current layer carried by accelerated meandering electrons boosts up the growth of the collisionless tearing mode. However, reconnection triggering process changes when the thickness of current sheet is more than the ion inertia length. In such cases, the inductive electric field does not permeate into the neutral sheet, and then, the production of accelerated meandering electron decreases. Instead of the thin current layer, the bifurcated current sheet is generated as a result of the LHDI non-linear evolution in thicker current sheets. We found that the speed of reconnection triggering in the bifurcated current sheet is still much faster than that predicted by two dimensional theories and that there are two stages of reconnection triggering. The first stage starts after LHDI has been saturated and electron bifurcated current sheets have been well developed. The growth rate of the tearing mode during the first stage is relatively slow and it is due to the electron temperature anisotropy caused by LHDI. The bifurcated current sheets gradually merge into a single peaked current sheet around the first stage X-line. At the timing of the well-developed X-line emergence, burst trigger of reconnection happens and it makes the second stage reconnection growing. The second stage growth rate is quite fast, similar to QMRT in the thin current sheet. Accelerated meandering electrons are found in the X-line, and they are responsible for the second stage quick growth. These results suggest that not only the electron anisotropy but also the emergence of the accelerated meandering electron are necessary for QMRT in the bifurcated current sheet.