Poloidal Field Line Resonances Driven by a Fast Wave

We present numerical simulations of the excitation of resonant poloidal Alfvén waves. The resulting Alfvén waves could be loosely described as “high-$m$” ($m$ is the azimuthal wave number) in as much as the azimuthal scale of the wave is much less than the scale in the direction normal to $L$-shells. Such waves are generally excited by wave-particle interactions. In this article we show how resonant poloidal Alfvén waves can be excited by a fast mode (of large azimuthal scale) in a cold plasma. The key property that enables this is a three-dimensional equilibrium, which facilitates the process of phasemixing in the azimuthal direction. We show that the classification of resonant Alfvén waves as high-$m$ and low-$m$ has limited applicability in 3D inhomogeneous media and suggest an alternative classification be based on the excitation mechanism.