Zonal Flow Generation by Modulational Instability

This paper gives a pedagogic review of the envelope formalism for excitation of zonal flows by nonlinear interactions of plasma drift waves or Rossby waves, described equivalently by the Hasegawa–Mima (HM) equation or the quasigeostrophic barotropic potential vorticity equation, respectively. In the plasma case a modified form of the HM equation, which takes into account suppression of the magnetic-surface-averaged electron density response by a small amount of rotational transform, is also analyzed. Excitation of zonal mean flow by a modulated wave train is particularly strong in the modified HM case. A local dispersion relation for a coherent wave train is calculated by linearizing about a background mean flow and used to find the nonlinear frequency shift by inserting the nonlinearly excited mean flow. Using the generic nonlinear Schrödinger equation about a uniform carrier wave, the criterion for instability of small modulations of the wave train is found, as is the maximum growth rate and phase velocity of the modulations and zonal flows, in both the modified and unmodified cases.