New Insights on the Dominant Acceleration Mechanism and Formation of Power-Law Energy Distribution during Anti-parallel Relativistic Magnetic Reconnection

We present new insight on the mechanisms for dominant acceleration and formation of power-law particle energy distributions in an anti-parallel magnetic reconnection layer in the magnetically dominated regime $\sigma = B^2/(4 \pi \rho c^2) \gg 1$. Through first principles kinetic simulations for the evolution of a force-free current sheet, we show that the dominant acceleration mechanism is a Fermi acceleration process in the motional electric field induced from plasma motion in the reconnection layer. By adding a test-particle component that does not experience any non-ideal electric field, we show that the test-particle component can still generate a power-law distribution with the spectral index similar to that of the electrons self-consistently evolved in the system. We conclude that the formation of power-law distribution does not rely on non-ideal electric field at the X-points.