Test for Wavevector Anisotropies in Plasma Turbulence Cascades

Abstract

The frequency and wavevector matching conditions in nonlinear three-wave coupling are used to test whether the forward cascade of plasma turbulence may lead to wavevector anisotropies in a homogeneous, collisionless, magnetized plasma. Linear kinetic theory at β _p = 0.01, 0.10, and 1.0 is used to determine the frequency-wavenumber dispersion of three normal modes: long-wavelength Alfvén-cyclotron waves, long-wavelength magnetosonic waves, and intermediate-wavelength magnetosonic-whistler waves. Using linear dispersion in the nonlinear matching conditions, the test predicts with one exception that forward cascades are favored by fluctuations propagating nearly perpendicular to the background magnetic field B _o . This is consistent with the typical development of wavevector anisotropies with k Gt k _∥ (subscripts refer to directions perpendicular and parallel to B _o , respectively) in computer simulations of the forward cascade of various types of plasma turbulence. The exception is that, at β _p = 1.0, the test predicts that the cascade of long-wavelength magnetosonic waves should be favored by modes at k ~ k _∥.