Electron heating and electron-ion temperature ratio at shocks
In plain words
Behind fast shocks in supernova remnants, electrons end up much colder than ions, while behind slow shocks the two are similar. Which processes heat electrons inside a collisionless shock is not known.
Precise statement
Balmer-line observations of supernova remnant shocks show downstream $T_{\mathrm{e}}/T_{\mathrm{i}}$ near 1 at low shock speed and well below 1 at high speed. Identify the electron heating mechanism (cross-shock potential, wave heating by Buneman or whistler instabilities, reconnection in the shock transition) and derive $T_{\mathrm{e}}/T_{\mathrm{i}}$ versus Mach number and obliquity. An answer is a derived relation matching observations and kinetic simulations.
What would settle it
Kinetic simulations at realistic mass ratio reproducing the observed $T_{\mathrm{e}}/T_{\mathrm{i}}$ trend, with the mechanism identified.