Testing of the backward wave oscillator model by using the spectral characteristics of VLF chorus elements

A generation mechanism for chorus was suggested by Trakhtengerts (1995) on the basis of the backward wave oscillator (BWO) regime of the magnetospheric cyclotron maser. According to this model, step-like deformation on the electron distribution function is the most important factor of chorus generation, but such a feature is very difficult to observe. By measuring the frequency sweep rates in chorus elements detected by the Cluster spacecraft we determine the mean values and distributions of a dimensionless parameter characterizing the step feature. These values are in agreement with the results of numerical simulations of chorus elements based on the BWO model.