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تسجيل مستخدم جديدInverse cascade and magnetic vortices in kinetic Alfven-wave turbulence
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A Hamiltonian two-field gyrofluid model for kinetic Alfven waves (KAWs) in a magnetized electron-proton plasma, retaining ion finite-Larmor-radius corrections and parallel magnetic field fluctuations, is used to study the inverse cascades that develop when turbulence is randomly driven at sub-ion scales. In the directions perpendicular to the ambient field, the dynamics of the cascade turns out to be nonlocal and the ratio $chi_f$ of the wave period to the characteristic nonlinear time at the driving scale affect some of its properties. For example, at small values of $chi_f$, parametric decay instability of the modes driven by the forcing can develop, enhancing for a while inverse transfers. The balanced state, obtained at early time when the two counter-propagating waves are equally driven, also becomes unstable at small $chi_f$, leading to an inverse cascade. For $beta_e$ smaller than a few units, the cascade slows down when reaching the low-dispersion spectral range. For higher $beta_e$, the ratio of the KAW to the Alfven frequencies displays a local minimum. At the corresponding transverse wavenumber, a condensate is formed, and the cascade towards larger scales is then inhibited. Depending on the parameters, a parallel inverse cascade can develop, enhancing the elongation of the ion-scale magnetic vortices that generically form.
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