Parviz

Collisionless shocks are loosely defined as shocks where the transition between pre-and post-shock states happens on a length scale much shorter than the collisional mean free path. In this talk I review our state of knowledge regarding electron heating in astrophysical shocks, mainly associated with supernova remnants (SNRs). Observations of non-radiative SNRs indicate that the ratio of electron temperature, (Te), to ion temperature (Tp), declines with increasing shock speed. Such behavior can be understood on the basis of cosmic-ray driven waves generated by cosmic rays in a shock precursor. The damping of these waves heats electrons to a nearly constant energy, resulting in the observed decline in Te/Tp with shock speed. Similar mechanisms may work in supernova ejecta shocks and solar wind shocks, though with different scaling between Te/Tp and and shock speed.