Pulsating auroras of very thin vertical extent have recently been observed. It is shown that if the primary electrons are stopped in a region as thin as that reported, extremely strong electric fields, ac or dc (or both), are implied. This consequence could be tested by in situ measurements. If the stopping is due to dc electric fields, a potential drop of the order of 10 kV or more in a thin region is required. The local field strength, several mV/m, has an order of magnitude comparable to that estimated for so-called electric double layers. If the stopping were due to ac fields, the rms electric field strength would have to be even larger. Alternatively, the reported thinnes of the luminosity distribution might result without a correspondingly abrupt stopping of the primary electrons. Two conceivable mechanisms for this are discussed and shown to be inadequate. However, there is at least one, involving excitation of the N2+ Meinel band by keV electrons, that cannot be definitely rejected on the basis of existing observations. This question could, however, be resolved by TV observations using suitably band-limited filters.