Electron distribution functions and collisionless resistivity during asymmetric reconnection

In the electron diffusion region (EDR) in asymmetric reconnection, crescent-shaped electron distributions are formed due to the finite gyroradius effect, and understanding how they are related with the reconnection rate is an important issue. We investigate the impact of electric fields on crescent electron distributions and the generation of collisionless resistivity in asymmetric reconnection, by means of theory and 2-D particle-in-cell (PIC) simulations. Electrons are accelerated by the reconnection electric field, and we predict that the thickness of the crescent distribution is broadened because of the acceleration. Those accelerated electrons eventually exit the EDR owing to the gyro-turning in the magnetic field normal to the current sheet. In this process, effective resistivity is generated because of the finite acceleration time in the EDR, even without the existence of scattering by waves in the EDR. First, we discuss electron acceleration during meandering motion by the reconnection electric field, and its effect on the crescent distributions. The crescent thickness is a function of the number of meandering across the current sheet, and we compare the theoretical prediction with simulation results. Next, we further model the electron distribution function in the EDR to compute collisionless resistivity in asymmetric reconnection. The predicted magnitude of the collisionless resistivity as well as the reconnection electric field is in remarkable agreement with PIC simulations. We will apply this theoretical model to estimate collisionless resistivity and a reconnection electric field in magnetopause reconnection observed by MMS. Finally, we will study the impact of wave fluctuations on electron motion within the EDR by PIC simulations.