On the nature of magnetic turbulence in rotating, shearing flows

Justin Walker , Geoffroy Lesur (1) , Stanislav Boldyrev

1 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble

The local properties of turbulence driven by the magnetorotational instability (MRI) in rotating, shearing flows are studied in the framework of a shearing-box model. Based on numerical simulations, we propose that the MRI-driven turbulence comprises two components: the large-scale shear-aligned strong magnetic field and the small-scale fluctuations resembling magnetohydrodynamic (MHD) turbulence. The energy spectrum of the large-scale component is close to k^-2, whereas the spectrum of the small-scale component agrees with the spectrum of strong MHD turbulence k^-3/2. While the spectrum of the fluctuations is universal, the outer-scale characteristics of the turbulence are not; they depend on the parameters of the system, such as the net magnetic flux. However, there is remarkable universality among the allowed turbulent states - their intensity v₀ and their outer scale λ₀ satisfy the balance condition v₀/λ₀ ∼ dΩ/dln r, where dΩ/dln r is the local orbital shearing rate of the flow. Finally, we find no sustained dynamo action in the Pm = 1 zero net-flux case for Reynolds numbers as high as 45 000, casting doubts on the existence of an MRI dynamo in the Pm ≤ 1 regime.

Mots clés: accretion; accretion discs; dynamo; MHD; plasmas; Astrophysics - Solar and Stellar Astrophysics; Astrophysics - High Energy Astrophysical Phenomena; Physics - Fluid Dynamics; Physics - Plasma Physics

Domaines: Planète et Univers [physics]; Astrophysique [astro-ph]

Justin Walker, Geoffroy Lesur, Stanislav Boldyrev. On the nature of magnetic turbulence in rotating, shearing flows. Monthly Notices of the Royal Astronomical Society: Letters, 2016, 457, pp.L39-L43. ⟨10.1093/mnrasl/slv200⟩. ⟨insu-03691562⟩