skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: LARGE-SCALE AZIMUTHAL STRUCTURES OF TURBULENCE IN ACCRETION DISKS: DYNAMO TRIGGERED VARIABILITY OF ACCRETION

Abstract

We investigate the significance of large-scale azimuthal, magnetic, and velocity modes for the magnetorotational instability (MRI) turbulence in accretion disks. We perform three-dimensional global ideal MHD simulations of global stratified protoplanetary disk models. Our domains span azimuthal angles of {pi}/4, {pi}/2, {pi}, and 2{pi}. We observe up to 100% stronger magnetic fields and stronger turbulence for the restricted azimuthal domain models {pi}/2 and {pi}/4 compared to the full 2{pi} model. We show that for those models the Maxwell stress is larger due to strong axisymmetric magnetic fields generated by the {alpha}{Omega} dynamo. Large radial extended axisymmetric toroidal fields trigger temporal magnification of accretion stress. All models display a positive dynamo-{alpha} in the northern hemisphere (upper disk). The parity is distinct in each model and changes on timescales of 40 local orbits. In model 2{pi}, the toroidal field is mostly antisymmetric with respect to the midplane. The eddies of the MRI turbulence are highly anisotropic. The major wavelengths of the turbulent velocity and magnetic fields are between one and two disk scale heights. At the midplane, we find magnetic tilt angles around 8 Degree-Sign -9 Degree-Sign increasing up to 12 Degree-Sign -13 Degree-Sign in the corona. We conclude that an azimuthalmore » extent of {pi} is sufficient to reproduce most turbulent properties in three-dimensional global stratified simulations of magnetized accretion disks.« less

Authors:
; ; ; ;  [1]
  1. Max Planck Institute for Astronomy, Koenigstuhl 17, 69117 Heidelberg (Germany)
Publication Date:
OSTI Identifier:
22004198
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 744; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCRETION DISKS; ASTROPHYSICS; AXIAL SYMMETRY; COMPUTERIZED SIMULATION; MAGNETIC FIELDS; MAGNETOHYDRODYNAMICS; NORTHERN HEMISPHERE; ORBITS; PROTOPLANETS; THREE-DIMENSIONAL CALCULATIONS; TURBULENCE

Citation Formats

Flock, M., Dzyurkevich, N., Klahr, H., Turner, N., and Henning, Th. LARGE-SCALE AZIMUTHAL STRUCTURES OF TURBULENCE IN ACCRETION DISKS: DYNAMO TRIGGERED VARIABILITY OF ACCRETION. United States: N. p., 2012. Web. doi:10.1088/0004-637X/744/2/144.
Flock, M., Dzyurkevich, N., Klahr, H., Turner, N., & Henning, Th. LARGE-SCALE AZIMUTHAL STRUCTURES OF TURBULENCE IN ACCRETION DISKS: DYNAMO TRIGGERED VARIABILITY OF ACCRETION. United States. doi:10.1088/0004-637X/744/2/144.
Flock, M., Dzyurkevich, N., Klahr, H., Turner, N., and Henning, Th. 2012. "LARGE-SCALE AZIMUTHAL STRUCTURES OF TURBULENCE IN ACCRETION DISKS: DYNAMO TRIGGERED VARIABILITY OF ACCRETION". United States. doi:10.1088/0004-637X/744/2/144.
@article{osti_22004198,
title = {LARGE-SCALE AZIMUTHAL STRUCTURES OF TURBULENCE IN ACCRETION DISKS: DYNAMO TRIGGERED VARIABILITY OF ACCRETION},
author = {Flock, M. and Dzyurkevich, N. and Klahr, H. and Turner, N. and Henning, Th.},
abstractNote = {We investigate the significance of large-scale azimuthal, magnetic, and velocity modes for the magnetorotational instability (MRI) turbulence in accretion disks. We perform three-dimensional global ideal MHD simulations of global stratified protoplanetary disk models. Our domains span azimuthal angles of {pi}/4, {pi}/2, {pi}, and 2{pi}. We observe up to 100% stronger magnetic fields and stronger turbulence for the restricted azimuthal domain models {pi}/2 and {pi}/4 compared to the full 2{pi} model. We show that for those models the Maxwell stress is larger due to strong axisymmetric magnetic fields generated by the {alpha}{Omega} dynamo. Large radial extended axisymmetric toroidal fields trigger temporal magnification of accretion stress. All models display a positive dynamo-{alpha} in the northern hemisphere (upper disk). The parity is distinct in each model and changes on timescales of 40 local orbits. In model 2{pi}, the toroidal field is mostly antisymmetric with respect to the midplane. The eddies of the MRI turbulence are highly anisotropic. The major wavelengths of the turbulent velocity and magnetic fields are between one and two disk scale heights. At the midplane, we find magnetic tilt angles around 8 Degree-Sign -9 Degree-Sign increasing up to 12 Degree-Sign -13 Degree-Sign in the corona. We conclude that an azimuthal extent of {pi} is sufficient to reproduce most turbulent properties in three-dimensional global stratified simulations of magnetized accretion disks.},
doi = {10.1088/0004-637X/744/2/144},
journal = {Astrophysical Journal},
number = 2,
volume = 744,
place = {United States},
year = 2012,
month = 1
}
  • The well-known bluer-when-brighter trend observed in quasar variability is a signature of the complex processes in the accretion disk and can be a probe of the quasar variability mechanism. Using a sample of 604 variable quasars with repeat spectra in the Sloan Digital Sky Survey-I/II (SDSS), we construct difference spectra to investigate the physical causes of this bluer-when-brighter trend. The continuum of our composite difference spectrum is well fit by a power law, with a spectral index in excellent agreement with previous results. We measure the spectral variability relative to the underlying spectra of the quasars, which is independent ofmore » any extinction, and compare to model predictions. We show that our SDSS spectral variability results cannot be produced by global accretion rate fluctuations in a thin disk alone. However, we find that a simple model of an inhomogeneous disk with localized temperature fluctuations will produce power-law spectral variability over optical wavelengths. We show that the inhomogeneous disk will provide good fits to our observed spectral variability if the disk has large temperature fluctuations in many independently varying zones, in excellent agreement with independent constraints from quasar microlensing disk sizes, their strong UV spectral continuum, and single-band variability amplitudes. Our results provide an independent constraint on quasar variability models and add to the mounting evidence that quasar accretion disks have large localized temperature fluctuations.« less
  • Here, we study the dynamo generation (exponential growth) of large-scale (planar averaged) fields in unstratified shearing box simulations of the magnetorotational instability (MRI). In contrast to previous studies restricted to horizontal (x–y) averaging, we also demonstrate the presence of large-scale fields when vertical (y–z) averaging is employed instead. By computing space–time planar averaged fields and power spectra, we find large-scale dynamo action in the early MRI growth phase – a previously unidentified feature. Non-axisymmetric linear MRI modes with low horizontal wavenumbers and vertical wavenumbers near that of expected maximal growth, amplify the large-scale fields exponentially before turbulence and high wavenumbermore » fluctuations arise. Thus the large-scale dynamo requires only linear fluctuations but not non-linear turbulence (as defined by mode–mode coupling). Vertical averaging also allows for monitoring the evolution of the large-scale vertical field and we find that a feedback from horizontal low wavenumber MRI modes provides a clue as to why the large-scale vertical field sustains against turbulent diffusion in the non-linear saturation regime. We compute the terms in the mean field equations to identify the individual contributions to large-scale field growth for both types of averaging. The large-scale fields obtained from vertical averaging are found to compare well with global simulations and quasi-linear analytical analysis from a previous study by Ebrahimi & Blackman. We discuss the potential implications of these new results for understanding the large-scale MRI dynamo saturation and turbulence.« less
  • The influence of dynamical friction on stars on the global distribution of giant molecular clouds in galaxies is calculated in different model approximations. It is shown that in their Galaxy dynamical friction alone cannot lead to the formation of the observed ring-shaped distribution of the giant molecular clouds during a time comparable with the age of the disk of the Galaxy.
  • Activity of the nuclei of galaxies and stellar mass systems involving disk accretion to black holes is thought to be due to (1) a small-scale turbulent magnetic field in the disk (due to the magnetorotational instability, MRI), which gives a large viscosity enhancing accretion, and (2) a large-scale magnetic field, which gives rise to matter outflows and/or electromagnetic jets from the disk which also enhances accretion. An important problem with this picture is that the enhanced viscosity is accompanied by an enhanced magnetic diffusivity, which acts to prevent the buildup of a significant large-scale field. Recent work has pointed outmore » that the disk's surface layers are nonturbulent, and thus highly conducting (or nondiffusive) because the MRI is suppressed high in the disk where the magnetic and radiation pressures are larger than the thermal pressure. Here, we calculate the vertical (z) profiles of the stationary accretion flows (with radial and azimuthal components) and the profiles of the large-scale magnetic field, taking into account the turbulent viscosity and diffusivity due to the MRI and the fact that the turbulence vanishes at the surface of the disk. We derive a sixth-order differential equation for the radial flow velocity v{sub r} (z), which depends mainly on the midplane thermal to magnetic pressure ratio {beta}>1 and the Prandtl number of the turbulence P= viscosity/diffusivity. Boundary conditions at the disk surface take into account a possible magnetic wind or jet and allow for a surface current in the highly conducting surface layer. The stationary solutions we find indicate that a weak ({beta}>1) large-scale field does not diffuse away as suggested by earlier work. For a wide range of parameters {beta}>1 and P{>=}1, we find stationary channel-type flows where the flow is radially outward near the midplane of the disk and radially inward in the top and bottom parts of the disk. Channel flows with inward flow near the midplane and outflow in the top and bottom parts of the disk are also found. We find that Prandtl numbers larger than a critical value (estimated to be 2.7) are needed in order for there to be magnetocentrifugal outflows from the disk's surface. For smaller P, electromagnetic outflows are predicted.« less