Strength of Linear and Nonlinear Accelerations in MHD Turbulence: Global versus Local Measures

Abstract:

Magnetized turbulent plasmas are generally characterized as weakly or strongly turbulent based on the relative strength of the linear and nonlinear terms on average. While this description is useful, it does not represent the full picture and can be misleading. We investigate the variation of the linear and nonlinear accelerations in the Fourier space of an MHD system with a mean magnetic field and broad selection of initial states and plasma parameters. We show that the local Fourier space ratios of nonlinear-to-linear accelerations can show significant departures from what is expected from the general global measure. We find that high cross helicity systems that are traditionally believed to be weakly nonlinear can show strong nonlinearities in parts of the Fourier space. These nonlinearities can exceed in strength the level of nonlinearities recovered from low cross helicity systems. If these Fourier space regions are energetically significant then nonlinear effects can govern parts of the spectrum while the other parts remain subject to linear effects.