Ion energization in the asymmetric reconnection diffusion region*

We investigate ion energization in the ion diffusion region (IDR) during asymmetric reconnection using particle-in-cell simulations and Magnetosphere Multiscale (MMS) mission observations. In particular, how the ion outflow is built up, the heating characteristics, and the relative importance between the reconnection (E_r) and in-plane (E_in) electric fields are studied. The simulation result shows that E_r and E_in overall have comparable contribution in energizing ions near the time of the peak reconnection rate, while E_in becomes more important later. E_r is most important for accelerating ions entering the central electron diffusion region (EDR). E_in becomes important beyond the central EDR and dominates the net build-up of the ion outflow. Heating of demagnetized ions is mainly due to distributions with counter-streaming populations normal to the current layer. Downstream of the peak outflow, cyclotron turning around the reconnected and Hall magnetic fields leads to further gyrotropization and thermalization. Ion distribution features of acceleration by E_r and E_in are observed by MMS.