Inverse cascade of hybrid helicity in $B\Omega$-MHD turbulence

Abstract

We investigate the impact of a solid-body rotation ${\Omega }_0$ on the large-scale dynamics of an incompressible magnetohydrodynamic turbulent flow in presence of a background magnetic field ${\mathbf{B}}_0$ and at low Rossby number. Three-dimensional direct numerical simulations are performed in a periodic box, at unit magnetic Prandtl number and with a forcing at intermediate wavenumber $k_f=20$. When ${\Omega }_0$ is aligned with ${\mathbf{B}}_0$ (i.e. $\theta \equiv \widehat{\left({\Omega }_0,{\mathbf{B}}_0\right)}=0$), inverse transfer is found for the magnetic spectrum at $k<k_f$. This transfer is stronger when the forcing excites preferentially right-handed (rather than left-handed) fluctuations; it is smaller when $\theta >0$ and becomes weak when $\theta \ge 35^o$. These properties are understood as the consequence of an inverse cascade of hybrid helicity which is an inviscid/ideal invariant of this system when $\theta =0$. Hybrid helicity emerges, therefore, as a key element for understanding rotating dynamos. Implication of these findings on the origin of the alignment of the magnetic dipole with the rotation axis in planets and stars is discussed.

arXiv comment9 pages, 10 figures

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Cited by in corpus (3)

• Wave turbulence: the case of capillary waves (a review)
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• Rotating magnetohydrodynamic turbulence