Auroral Pulsations From Atmospheric Waves.

A model of the quasiperiodic pulsating aurora (pulsation periods greater than 1s) is proposed in which pressure waves in the neutral atmosphere are invoked as the driver of the brightness fluctuations. If the isodensity surfaces of the local atmosphere become nonplanar, a horizontal movement of the atmosphere can cause the energy deposition by a steady particle distribution to vary with time. When the distortions are wavelike corrugations, and the horizontal motion is steady and also has a component normal to the phasefront, periodic variations in auroral brightness can result. The pulse shape and magnitude of the pulsations depend on both the energy spectrum and pitch angle distribution of the particale and the amplitude and altitude distribution of the atmospheric waves. This model, which attributes variable precipitation to a temporally changing local loss cone, is in sharp contrast to models which require temporal changes in the particle source above the atmosphere. Many of the observed features of pulsating aurorae can be explained by the present model. It is suggested that the required atmospheric oscillations can be provided by acoustic-gravity waves that are generated by active arcs near the poleward edge of the auroral oval. This mechanism can also produce aperiodic pulsations by convection of irregular atmospheric density fluctuations, which may result from interfering acoustic-gravity wave trains. (Author)