A self‐consistent fully turbulent theory of auroral E region irregularities

Abstract

We present a nonlinear theory of high‐latitude E region irregularities that is based solely on the effects of mode coupling on the turbulent state of the plasma. We have been using a nonlinear equation based on a two‐fluid model governing the evolution of ion acoustic turbulence in the absence of k∥ = k · B0/B0 effects. We have first shown that the linear eigenmodes cannot be used as a zeroth‐order approximation to describe the often‐seen strongly turbulent high‐latitude E region plasma waves. However, with the assumption of weakly growing or decaying nonlinear modes with broad frequency spectra (quasi‐steady state turbulence), we have been able to obtain a set of constraints on the mean frequency and spectral width of both linearly stable and linearly unstable modes. With the use of an “eddy damped quasi‐normal Markovian” closure scheme for the spectral density and a self‐consistent expression for the frequency broadening and the nonlinear growth rate, we have been able, as a result, to relate the Doppler width of radar spectra to the broadband density fluctuation level. Our estimates have led us to believe that strongly turbulent conditions are associated with broadband fluctuation levels of the order of 5 to 10%, no more, in the situations of interest. Our closure scheme also produces a specific set of nonlinear coupled equations that could be solved numerically in order to derive spectral laws pertinent to high‐latitude E region turbulence.