Extension of the coherence function to quadratic models

It is shown how the use of higher coherence functions can recover some of the lost coherence due to nonlinear relationship between two fluctuating quantities whose degree of mutual coherence is being measured. The relationship between the two processes is modeled with the aid of a linear term and a quadratic term. As a specific example, the relationship between plasma density and potential fluctuations in a plasma is considered. The fraction of power in the auto-power spectrum of the potential fluctuations due to a linear relationship and to a quadratic relationship between the density and potential fluctuations is estimated.