Electron heating at SNR collisionless shocks

The hot gas in supernova remnants is heated by collisionless shocks, genera lly propagating in low density (similar to 1 atom cm(-3) media), where the shock transition occurs on a length scale much shorter than the typical par ticle mean free path to Coulomb collisions. Conservation laws for energy, m omentum, and particle numbers across the shock predict postshock particle t emperatures proportional. to their masses, that is, electron temperature mu ch less than ion temperature. Electrons and ions will, of course, equilibra te by Coulomb collisions, but collisionless processes might bring about fas ter equilibration. I review theoretical and observational work on this impo rtant point and speculate on reasons for discrepancies where they exist.