Nonhelical inverse transfer of a decaying turbulent magnetic field

Nordita Astrophysics seminars

Wednesday 30 April 2014
from 14:00
to 14:30 at
122:026

Speaker :

Axel Brandenburg (Nordita & Astronomy Department)

Abstract :

In the presence of magnetic helicity, inverse energy transfer from small to large scales is well known in magnetohydrodynamic (MHD) turbulence and has important applications in astrophysics, cosmology, and fusion plasmas. Using high resolution direct numerical simulations, we report a similar inverse transfer even in nonhelical MHD turbulence. We compute for the first time spectral energy transfer rates to show that the inverse transfer is about half as strong as with helicity, but in both cases the magnetic gain at large scales results from velocity at similar scales interacting with smaller-scale magnetic fields. We argue that in both cases, the inverse transfer is a consequence of the universal k^4 and k^2 subinertial range spectra for magnetic and kinetic energies, respectively. The shallower k^2 spectrum forces the magnetic field to attain larger-scale coherence. The inertial range shows a clear k^{-2} spectrum and is the first example of fully isotropic magnetically dominated MHD turbulence exhibiting weak turbulence scaling.