On statistical distribution of characteristics of chorus element generation

A generation mechanism for VLF chorus was suggested by V. Yu. Trakhtengerts [1, 2] on the basis of the backward wave oscillator (BWO) regime of magnetospheric cyclotron maser. Up to now many predictions of the BWO regime were supported by observations. In this report we discuss a statistical distribution of the dimensionless parameter q, quantifying an excess of the electron flux over the absolute-instability threshold. Typically the q parameter values deduced from VLF chorus elements observed by WBD instrument onboard the CLUSTER spacecraft exhibit significant scatter, while their mean values were ~10. We suppose that so large excess of the instability threshold cannot be permanently supported in the magnetospheric plasma. On the basis of the discrete numerical model we demonstrate that if the noised “on-off” intermittency regime generation is realized, then the observed q values deduced from chorus elements should be extreme ones, but the average value over the entire event can be much smaller. We stress an importance of taking the noise-induced type of chorus generation into account.