Instabilities in pitch angle diffusion and their possible relation to a 3 Hz modulation in pulsating aurora

Abstract

The Coroniti-Kennel theory of electron precipitation pulsations has been reviewed, and an additional restriction of differential relaxation time has been introduced, making the model unstable under most conditions. The solution to the model was found to be nonlinear, and a very crude numerical solution has been attempted. Although suffering from several nonrealistic approximations, this model has confirmed the idea that a 3 Hz modulation in pulsating auroral forms is related to short bursts of Coroniti-Kennel type strong pitch angle diffusion. Strong pitch angle diffusion will fill the loss cone and is therefore not consistent with a self-excited steady state diffusion, i.e. it must be of a transient nature. The repetition frequency of such bursts depends on the size of the diffusion region along the magnetic field line combined with the parallel velocity of precipitating particles. Estimates based on reasonable diffusion region and velocity give a modulation frequency comparable to observations. The suggested mechanism is consistent with self-excited pitch angle diffusion only; it excludes the possibility that the diffusion is parasitic. The numerical model also indicated that the abnormal pitch angle distribution generated as a result of a magnetic disturbance might survive for hundreds of seconds and thus acts as a ‘memory’ on the fieldline, making it more susceptible for further disturbances.