Theory of modulational interactions in plasmas in the presence of an external magnetic field

Abstract

In many real physical and astrophysical situations, an external magnetic field is present. The magnetic field results in a variety of specific plasma modes and leads to an anisotropic distribution of plasma oscillations. The present review reflects the present state of the theory of modulational interactions in magnetized plasmas. Modulational interactions are considered on the basis of a simple and universal approach using new methods developed for the description of modulational effects in arbitrary media. This approach enables us to visualize the physical processes occurring in plasmas in the presence of modulational interactions. Emphasis is given to the role of modulational interactions in processes of particle acceleration, generation of current drive, physical phenomena in the Earth's magnetosphere and in active geophysical and space experiments. The influence of boundary conditions and plasma inhomogeneity is considered in detail. These effects can essentially change the dispersion properties of plasma modes, and their influence can be even more significant than effects of plasma anisotropy (caused by the external magnetic field) or thermal corrections to the dielectric permittivity tensor. Modulational interaction is the most important nonlinear process for waves excited as a result of the lower-hybrid drift instability; in particular, the modulational interaction determines the saturation mechanism for this instability which in turn enables us to obtain the effective collision frequency and to estimate the width of the Earth's magnetopause region. The theory of modulational interactions is presented for the explanation of magnetic structures, particle spectra, and the electric field amplitude observed in active geophysical experiments.