Magnetic flux concentration and zonal flows in magnetorotational instability turbulence

Journal Article · Thu Nov 20 00:00:00 EST 2014 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/796/1/31· OSTI ID:22370197

Bai, Xue-Ning ^[1]

---

1. Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-51, Cambridge, MA 02138 (United States)

Accretion disks are likely threaded by external vertical magnetic flux, which enhances the level of turbulence via the magnetorotational instability (MRI). Using shearing-box simulations, we find that such external magnetic flux also strongly enhances the amplitude of banded radial density variations known as zonal flows. Moreover, we report that vertical magnetic flux is strongly concentrated toward low-density regions of the zonal flow. Mean vertical magnetic field can be more than doubled in low-density regions, and reduced to nearly zero in high-density regions in some cases. In ideal MHD, the scale on which magnetic flux concentrates can reach a few disk scale heights. In the non-ideal MHD regime with strong ambipolar diffusion, magnetic flux is concentrated into thin axisymmetric shells at some enhanced level, whose size is typically less than half a scale height. We show that magnetic flux concentration is closely related to the fact that the turbulent diffusivity of the MRI turbulence is anisotropic. In addition to a conventional Ohmic-like turbulent resistivity, we find that there is a correlation between the vertical velocity and horizontal magnetic field fluctuations that produces a mean electric field that acts to anti-diffuse the vertical magnetic flux. The anisotropic turbulent diffusivity has analogies to the Hall effect, and may have important implications for magnetic flux transport in accretion disks. The physical origin of magnetic flux concentration may be related to the development of channel flows followed by magnetic reconnection, which acts to decrease the mass-to-flux ratio in localized regions. The association of enhanced zonal flows with magnetic flux concentration may lead to global pressure bumps in protoplanetary disks that helps trap dust particles and facilitates planet formation.

• OSTI ID: 22370197
• Journal Information: Astrophysical Journal, Vol. 796, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ACCRETION DISKS
AMBIPOLAR DIFFUSION
ANISOTROPY
AXIAL SYMMETRY
CONCENTRATION RATIO
CORRELATIONS
DENSITY
ELEMENT ABUNDANCE
FLUCTUATIONS
INSTABILITY
MAGNETIC FIELDS
MAGNETIC FLUX
MAGNETIC RECONNECTION
MAGNETOHYDRODYNAMICS
MASS
NMR IMAGING
PLANETS
PROTOPLANETS
SIMULATION
TURBULENCE