Electron acceleration by parallel and perpendicular electric fields during magnetic reconnection without guide field

Electron acceleration due to the electric field parallel to the background magnetic field during magnetic reconnection with no guide field is investigated by theory and two-dimensional electromagnetic particle-in-cell simulations and compared with acceleration due to the electric field perpendicular to the magnetic field. The magnitude of the parallel electric potential shows dependence on the ratio of the plasma frequency to the electron cyclotron frequency as (ω_pe/Ω_e)^-2 and on the background plasma density as nb-1/2. In the Earth's magnetotail, the parameter ω_pe/Ω_e∼9 and the background (lobe) density can be of the order of 0.01 cm^-3, and it is expected that the parallel electric potential is not large enough to accelerate electrons up to 100 keV. Therefore, we must consider the effect of the perpendicular electric field to account for electron energization in excess of 100 keV in the Earth's magnetotail. Trajectories for high-energy electrons are traced in a simulation to demonstrate that acceleration due to the perpendicular electric field in the diffusion region is the dominant acceleration mechanism, rather than acceleration due to the parallel electric fields in the exhaust regions. For energetic electrons accelerated near the X line due to the perpendicular electric field, pitch angle scattering converts the perpendicular momentum to the parallel momentum. On the other hand, for passing electrons that are mainly accelerated by the parallel electric field, pitch angle scattering converting the parallel momentum to the perpendicular momentum occurs. In this way, particle acceleration and pitch angle scattering will generate heated electrons in the exhaust regions.