A review of the physics of electron heating at collisionless shocks

The current understanding of the increase of electron temperature across fast and slow magnetosonic collisionless shocks observed in space is reviewed. The concept advanced by Goodrich and Scudder /1/ that the electron temperature increase is essentially caused by the inflation of the phase space in the presence of the DC deHoffmann-Teller (HT) /2/ electric field within the shock layer has achieved wide acceptance. This review reiterates the essential basis of that work: that with their relatively small inertia the electrons remain magnetized while the ions do not. It is this fact. that makes the HT frame the relevant one for electron energetics. Subsequent discussions of the electron issues and its corollaries always come back to the same issue: m/M much less than 1 makes collisionless shocks the way they are observed. The predicted corollaries of this understanding in terms of the diagnostic signatures in the magnetic field within the shock structure have been challenged in the intervening years, but nevertheless confirmed by independent groups. This understanding has also been transferred to the relatively rare slow shocks. These corollaries concern the geometry of magnetic tubes of force that underlie the one-fluid J x B force. This geometry clarifies that the tubes of force that pierce the shock do not lie in a single coplanarity plane, but meander in a staircase-like fashion between distinct asymptotic coplanarity planes perpendicular to the tangential electric field direction, E(T). It is also developed that the relative size of the deHoffmann-Teller and Normal Incidence cross shock electrical potential is now established, with the Normal Incidence potential jump bigger (smaller) than the deHoffmann-Teller jump in observed fast (slow) shock waves. A geometrical discussion has been made in this review that these relations are consistent with the required tangential torques needed for the ion deflections required by Hugoniot, being opposite for the fast and slow shocks. Computer simulations with particle electrons have just recently been able to verify the experimental picture summarized above, including the self-consistent generation of the strong shock flat-topped electron distribution functions and the relatively unimportant role of instabilities in electron heating as opposed to assimilation or scattering. The diagnostics of this code have supported the overwhelming importance of the DC effects of the coherent forces in the electron shock heating physics.