Electron bulk heating in magnetic reconnection at Earth's magnetopause: Dependence on the inflow Alfven speed and magnetic shear

We surveyed 79 magnetopause reconnection exhausts detected by the THEMIS spacecraft to investigate how the amount and anisotropy of electron bulk heating produced by reconnection depend on the inflow boundary conditions. We find that the amount of heating, T-e, is correlated with the asymmetric Alfven speed, V-AL,V-asym, based on the reconnecting magnetic field and the plasma density measured in both the high-density magnetosheath and low-density magnetospheric inflow regions. Best fit to the data produces the empirical relation T-e=0.017 m(i)V(AL,asym)(2), indicating that the amount of heating is proportional to the inflowing magnetic energy per proton-electron pair, with similar to 1.7% of the energy being converted into electron heating. This finding, generalized to symmetric reconnection, could account for the lack of electron heating in typical solar wind exhausts at 1AU, as well as strong heating to keV energies common in magnetotail exhausts. We also find that the guide field suppresses perpendicular heating.