Simulations of two-dimensional magnetic electron drift vortex mode turbulence in plasmas

Abstract

Abstract. Simulations are performed to investigate turbulent properties of nonlinearly interacting two-dimensional (2D) magnetic electron drift vortex (MEDV) modes in a nonuniform unmagnetized plasma. The relevant nonlinear equations governing the dynamics of the MEDV modes are the wave magnetic field and electron temperature perturbations in the presence of the equilibrium density and temperature gradients. The important nonlinearities come from the advection of the electron fluid velocity perturbation and the electron temperature, as well as from the nonlinear electron Lorentz force. Computer simulations of the governing equations for the nonlinear MEDV modes reveal the generation of streamer-like electron flows, such that the corresponding gradients in the direction of the inhomogeneities tend to flatten out. By contrast, the gradients in an orthogonal direction vary rapidly. Consequently, the inertial range energy spectrum in decaying MEDV mode turbulence exhibits a much steeper anisotropic spectral index. The magnetic structures in the MEDV mode turbulence produce nonthermal electron transport in our nonuniform plasma.

Two Dimensional Inhomogeneous Magnetic Electron Drift Modes

“…The spectral and statistical properties [21], as well as the generation of large scale magnetic fields [22] by the MEDV mode turbulence, have been investigate both theoretically and numerically. The generation of steep, nonKolmogorov spectra of streamer-like structures by MEDV turbulence has been observed in recent simulation studies [23].…”

“…(1) can be contrasted with our previous work [23] on the small amplitude case that was studied in the absence of a background magnetic field gradient. In Ref [23], we have shown that mode coupling interactions during the nonlinear stage of evolution leads to the formation of streamer-like structures in the magnetic field fluctuations associated with ky ^0,kx^ 0. These structures were similar to the zonal flows but contained a rapid kx variations, thus the corresponding frequency is relatively large.…”

“…Consequently, the energy stored in these gradients excite linear as well as nonlinear instabilities in a different manner, than that described in [23], because the nonlinear mode coupling interactions are significantly modified by the presence of both plasma and magnetic field gradients. Our numerical code employs a doubly periodic spectral discretization of magnetic field and temperature fluctuations in terms of its Fourier components, while nonlinear interactions are de-convoluted back and forth in real and Fourier spectral spaces.…”

“…These structures were similar to the zonal flows but contained a rapid kx variations, thus the corresponding frequency is relatively large. The temperature fluctuations in Fig (1) of [23], on the other hand, depict an admixture of isotropically locahzed turbulent eddies and a few stretched along the direction of the background inhomogeneity. Clearly, the presence of a background magnetic field gradient reduces the kx variation in the steady-state flow.…”

“…We have adapted the nonlinear fluid code that was developed to study the evolution of MEDV modes in [23] in the presence of equilibrium electron density and temperature gradients, to also include a background gradient in the magnetic field [24]. Consequently, the energy stored in these gradients excite linear as well as nonlinear instabilities in a different manner, than that described in [23], because the nonlinear mode coupling interactions are significantly modified by the presence of both plasma and magnetic field gradients.…”