On the auroral double‐layer criterion

It is shown that the minimum electron streaming speed required in the Bohm‐Block criterion for the existence of one‐dimensional double‐layer solutions can be relaxed by the presence of a sufficient population of trapped electrons with energies in the parallel motion ranging up to the double‐layer potential energy. The resulting auroral double‐layer criterion is found to be relatively independent of the forms of the particle distribution functions. The trapped electrons can be identified with the energy‐degraded primary electrons and the secondary electrons on auroral field lines. The magnetic mirror due to the converging geomagnetic field and the electron‐neutral collisions in the upper atmosphere are necessary elements in building up the trapped electron population.