Two-scale structure of the current layer controlled by meandering motion during steady-state collisionless driven reconnection

A steady two-scale structure of current layer is demonstrated in the collisionless driven reconnections without a guide field by means of two-dimensional full-particle simulations in an open system. The current density profile along the inflow direction consists of two parts. One is a low shoulder controlled by the ion-meandering motion, which is a bouncing motion in a field reversal region. The other is a sharp peak caused mainly by the electron-meandering motion. The shoulder structure is clearly separated from the sharp peak for the case of a large mass ratio calculation mi/m_e = 200 because the ratio of the ion-meandering orbit amplitude to the electron-meandering orbit amplitude is proportional to (mi/m_e)^1/4. Although the ion frozen-in constraint is broken within a distance of the ion skin depth c/omega_pi, the violation due to the ion inertia is weak compared to the strong violation caused by the ion-meandering motion. The violation of the electron frozen-in constraint caused by the electron-meandering motion is stronger than the violation due to the electron inertia, and thus the electron-meandering motion produces the reconnection electric field in the central region where the current has the sharp peak structure