The physical processes that control the partition of released magnetic energybetween electrons and ions during reconnection is explored throughparticle-in-cell simulations and analytical techniques. We demonstrate that thedevelopment of a large-scale parallel electric field and its associatedpotential controls the relative heating of electrons and ions. The potentialdevelops to restrain heated exhaust electrons and enhances their heating byconfining electrons in the region where magnetic energy is released.Simultaneously the potential slows ions entering the exhaust below theAlfv\'enic speed expected from the traditional counterstreaming picture of ionheating. Unexpectedly, the magnitude of the potential and therefore therelative partition of energy between electrons and ions is not a constant butrather depends on the upstream parameters and specifically the upstreamelectron normalized temperature (electron beta). These findings suggest thatthe fraction of magnetic energy converted into the total thermal energy may beindependent of upstream parameters.
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