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

We surveyed 87 magnetopause reconnection exhausts detected by the THEMIS spacecraft to investigate how the amount and anisotropy of ion bulk heating depend on the inflow boundary conditions. We find that the heating, T-i, is correlated with the asymmetric Alfven speed, V-AL,V-asym, based on the reconnecting magnetic field and the plasma number density measured in both inflow regions. Best fit to the data produces the empirical relation T-i = 0.13 m(i)V(AL,asym)(2), where m(i) is the proton mass, indicating that the increase in the ion internal (thermal) energy, 3T(i)/2, is 20% of the available magnetic energy per proton-electron pair. The observed parallel heating generally exceeds perpendicular heating (by a factor of similar to 2), and there are some indications that the heating is reduced in the presence of a strong guide field. Finally, the ratio of ion to electron bulk heating is similar to 8 on average.