Nonlinear coupling of whistler wave turbulence with magnetosonic perturbations

Abstract

The coupling between the short scale whistler turbulence and the long scale magnetosonic excitations are studied in both weak and strong turbulence regimes. The coupling occurs via the following mechanism. The nonlinear interactions among the whistler waves can lead to low frequency perturbations involving ion motion. The low frequency perturbation changes the background plasma density and the magnetic field, thereby altering the whistler wave trajectories. Wave kinetic description and Fourier representation are adopted for the whistler turbulence and the magnetosonic excitations, respectively, keeping in view the disparity in their scales. It is shown that the turbulent spectrum of whistler waves is modulationally unstable to magnetosonic excitations. For the weakly turbulent case, it is shown that the long scale magnetosonic scales backreact on the whistler spectrum, leading to a diffusion of the whistler spectrum towards shorter scales. However, for a strongly turbulent case, the whistler wave trajectories can become trapped inside the magnetosonic waves, thereby leading to the formation of a variety of nonlinear coherent structures. These structures are of relevance for understanding the phenomena of intermittency in magnetofluids.