2000ApJS..127..409L

The hot gas in supernova remnants is heated by collisionless shocks, generally propagating in low density (∼1 atom.cm^–3 media), where the shock transition occurs on a length scale much shorter than the typical particle mean free path to Coulomb collisions. Conservation laws for energy, momentum, and particle numbers across the shock predict postshock particle temperatures proportional to their masses, that is, electron temperature {Lt} ion temperature. Electrons and ions will, of course, equilibrate by Coulomb collisions, but collisionless processes might bring about faster equilibration. I review theoretical and observational work on this important point and speculate on reasons for discrepancies where they exist.