Axisymmetric shearing box models of magnetized disks

Xiaoyue Guan, Charles F. Gammie

Research output: Contribution to journalArticlepeer-review

Abstract

The local model, or shearing box, has proven a useful model for studying the dynamics of astrophysical disks. Here we consider the evolution of magnetohydrodynamic (MHD) turbulence in an axisymmetric local model in order to evaluate the limitations of global axisymmetric models. An exploration of the model parameter space shows the following: (1) The magnetic energy and a-decay approximately exponentially after an initial burst of turbulence. For our code, HAM, the decay time τ ∝ Res, where Res/2 is the number of zones per scale height. (2) In the initial burst of turbulence the magnetic energy is amplified by a factor proportional to Res3/4 λR, where λR is the radial scale of the initial field. This scaling applies only if the most unstable wavelength of the magnetorotational instability is resolved and the final field is subthermal. (3) The shearing box is a resonant cavity and in linear theory exhibits a discrete set of compressive modes. These modes are excited by the MHD turbulence and are visible as quasi-periodic oscillations (QPOs) in temporal power spectra of fluid variables at low spatial resolution. At high resolution the QPOs are hidden by a noise continuum. (4) In axisymmetry disk turbulence is local. The correlation function of the turbulence is limited in radial extent, and the peak magnetic energy density is independent of the radial extent of the box LR for L R > 2H. (5) Similar results are obtained for the HAM, ZEUS, and ATHENA codes; ATHENA has an effective resolution that is nearly double that of HAM and ZEUS. (6) Similar results are obtained for 2D and 3D runs at similar resolution, but only for particular choices of the initial field strength and radial scale of the initial magnetic field.

Original languageEnglish (US)
Pages (from-to)145-157
Number of pages13
JournalAstrophysical Journal, Supplement Series
Volume174
Issue number1
DOIs
StatePublished - Jan 2008

Keywords

  • MHD
  • Methods: numerical

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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