Modulational interaction between drift waves and trapped ion convective cells: A paradigm for the self‐consistent interaction of large‐scale sheared flows with small‐scale fluctuations

The linear and nonlinear dynamics of modulational interaction between small‐scale drift waves and large‐scale trapped ion convective cells are investigated. This example is a paradigm of the more general problem of describing the self‐consistent interaction of small‐scale fluctuations with mean sheared flows. The growth rate of modulational instability is determined by spectral properties of drift waves and can exceed the linear growth rate of the trapped ion mode. An anisotropic spectrum of drift waves is always modulationally unstable. The spatial orientation of the convective cell pattern and structure (i.e., shear strength) is determined by drift wave spectrum anisotropy and propagation direction. In the presence of a sheared magnetic field, which pins small‐scale drift waves to mode rational surfaces, the modulational growth rate becomes intrinsically anisotropic, on account of the modified radial structure of drift waves.