Ion dynamics in the ion diffusion region of symmetric magnetic reconnection

We investigate ion dynamics in the ion diffusion region of an antiparallel symmetric reconnection event observed by the Magnetospheric MultiScale mission in Earth's magnetotail. Through detailed investigation of the ion velocity distribution functions, we show that the ion dynamics are initially (upon first entering the current sheet) dominated by the acceleration from the Hall electric and magnetic fields. The Hall electric field ballistically accelerates the ions toward the neutral sheet. The subsequent magnetic forces associated with the reconnecting and Hall magnetic fields have two distinct effects. First, the magnetic force due to the reconnecting magnetic field upstream of the X line delays the Speiser acceleration by gyroturning the ions in the direction opposite to the reconnection electric field. Second, the magnetic force due to the Hall magnetic field gyroturns the ions toward the outflow, kick-starting the outflow jet over short distances. These dynamics are consistent with fast reconnection rates.