Whistler propagation and modulation in the presence of nonlinear Alfvén waves

Abstract

This paper presents the modulation of whistler waves due to linear interactions between a weak whistler signal and a pump kinetic Alfvén wave (KAW) in intermediate -β(mₑ/mᵢ ≪ β ≪ 1) plasmas (where β is the ratio of the ion sound speed to the Alfvén speed). As a consequence of ponderomotive nonlinearity, the pump KAW becomes filamented when its power exceeds the threshold for the filamentation instability. The periodic enhancements of the whistler amplitude occur in the locations where the beam width is narrower and are thus produced by the focusing of the whistler beam. Our results reveal that KAW has a (magnetic) power spectrum with an approximate scaling of k⁻⁵/³ (where k is ∣k∣). The amplified whistler has electric field power spectra that are steeper than k⁻⁸/³ (where k is ∣k∣). These results are consistent with recent observations by the Cluster spacecraft in the magnetotail region of the Earth's magnetosphere.