Theoretical models based upon the concept of self-modulated VLF wave-electron interactions have been proposed to explain pulsating aurorae. These models incorporate the idea of a trapping limit, above which strong diffusion into the loss cone rapidly removes any excess electrons. At flux values near the trapping limit, perturbations of the trapped electron distribution can result in cyclic wave growth and electron precipitation. The trapping limit is thus related to the energy deposited and the characteristic energy of electrons precipitated in pulsating aurorae. Photometric measurements of the total energy deposit and of the mean energy parameter made at Chatanika, Alaska (invariant geomagnetic latitude, 65 ¿) indicate that the well-developed pulsations are caused mainly by a modulation of the mean energy parameter. Thus, a nearly constant ''limiting'' value for the precipitating flux is measured F=7¿108 el/cm2 sec from which a trapped flux limit of J?3¿109 el/cm2 sec can be inferred.