Ion heating resulting from pickup in magnetic reconnection exhausts

The heating of ions downstream of the x‐line during magnetic reconnection is explored using full‐particle simulations, test particle simulations, and analytic analysis. Large‐scale particle simulations reveal that the ion temperature increases sharply across the boundary layer that separates the upstream plasma from the Alfvénic outflow. This boundary layer, however, does not take the form of a classical switch‐off shock as discussed in the Petschek reconnection model, so the particle heating cannot be calculated from the magnetohydrodynamic, slow‐shock prediction. Test particle trajectories in the fields from the simulations reveal that ions crossing the narrow boundary into the exhaust instead behave like pickup particles: they gain both a directed outflow and an effective thermal speed given by the flow speed v 0 of the exhaust. The detailed dynamics of these particles are explored by taking 1‐D cuts of the simulation data across the exhaust, transforming to the deHoffman‐Teller frame, and calculating explicitly the increment in the temperature, m i v 0 2 /3, with m i , the ion mass. We compare the model predictions with the temperature increment in solar wind exhausts measured by the ACE and Wind spacecraft, confirming that the temperature increment is proportional to the ion mass. The Wind data from 22 high‐shear exhaust encounters confirm the scaling of the proton temperature increment with the square of the exhaust velocity. However, the temperature increments are consistently lower than the model prediction. Implications for understanding the production of high‐energy ions in flares and the broader universe are discussed.