Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

We investigate MHD turbulence in Keplerian disk flows threaded by a non-zero net azimuthal magnetic flux. In order to gain a deeper insight into the underlying dynamical balances and sustaining mechanism, we performed a set of numerical simulations in the shearing box model and based on the simulation data, analyzed in detail the turbulence dynamics in Fourier/wavenumber space. In the case of a net azimuthal field, classical exponential/modal instabilities are absent in the disk flow and linear growth of perturbations (shearing waves) is transient, also referred to as nonmodal growth. Namely, in the presence of disk rotation, radial shear and azimuthal field, the magnetorotational instability (MRI), being only available source of energy for turbulence, takes on transient character and, therefore, by itself, cannot ensure a long-term sustenance of the perturbations, i.e., it is “imperfect” in this sense. A necessary positive nonlinear feedback is required to regenerate nonmodally growing modes. In other words, the role of nonlinearity becomes crucial in this case: it lies at the heart of the sustenance of turbulence. The detailed analysis of the dynamics in Fourier space, allows us to demonstrate existence of the positive feedback. Specifically, the main novelties of our findings are the following:

1. The nonmodal MRI growth process is strongly anisotropic in Fourier space that, in turn, leads to anisotropy of nonlinear processes in this space. As a result, the main nonlinear process appears to be not a usual and well-known direct/inverse, but rather a new type of transverse/angular redistribution of perturbation modes in Fourier space, when their wavevector changes mostly its orientation (and not its magnitude) during nonlinear interactions. We refer to this angular transfer as the nonlinear transverse cascade.
2. Both the linear nonmodal growth of MRI and nonlinear transverse cascade mainly operate/dominate at large length scales, comparable to the box/system size. Consequently, the corresponding central, small wavenumber area of Fourier space is crucial in the turbulence sustenance process and is thus called the vital area.
3. We demonstrate that the turbulence in disks with net azimuthal field is sustained by a subtle interplay of the linear nonmodal growth of MRI and the nonlinear transverse cascade. Analyzing this interplay, we revealed the basic subcycle of the sustenance scheme that clearly shows synergy of the linear and nonlinear processes in the self-organization of the magnetized flow system.