  High Mach number collisionless shocks are found in planetary systems and
supernova remnants (SNRs). Electrons are heated at these shocks to the
temperature well above the Rankine-Hugoniot prediction. However processes
responsible for electron heating are still not well understood. We use a set of
large-scale Particle-In-Cell simulations of non-relativistic shocks in high
Mach number regime to clarify the electron heating processes. The physics of
these shocks is defined by ion reflection at the shock ramp. Further
interaction of the reflected ions and the upstream plasma excites electrostatic
Buneman and two-stream ion-ion Weibel instabilities. Electrons are heated via
shock surfing acceleration, the shock potential, magnetic reconnection,
stochastic Fermi scattering and the shock compression. The main contributor is
the shock potential. Magnetic field lines are tangled due to the Weibel
instability, which allows the parallel electron heating by the shock potential.
The constrained model of the electron heating predicts the ion-to-electron
temperature ratio within observed values at SNR shocks and in Saturn's bow
shock.
