Inverse cascade of hybrid helicity in BΩ-MHD turbulence
Résumé
We investigate the impact of a solid-body rotation Ω0 on the large-scale dynamics of an incompressible magnetohydrodynamic turbulent flow in presence of a background magnetic field B0 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 wave number kf=20. When Ω0 is aligned with B0 (i.e., θ≡(Ω0,B0)=0), inverse transfer is found for the magnetic spectrum at k0 and becomes weak when θ≥35∘. These properties are understood as the consequence of an inverse cascade of hybrid helicity which is an inviscid/ideal invariant of this system when θ=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.