Evidence for collisionless conduction fronts in impulsive solar flares

"Conduction fronts" feature in this medium-early cartoon. A correct treatment of this problem remains perplexing, when the hot loop-top region was often considered to be the origin of an electron beam that meets no other obstacles than Coulomb collisions. Return currents, electrodynamics, turbulence, kinetic/inertial Alfvén waves, PIC simulations, double layers... all remained in the future here. The motivation of the authors is that the hard X-rays could be explained as thermal bremsstrahlung. See also the Colgate cartoon.