Three Dimensional MHD Simulations of Accretion Disks

Details Three Dimensional MHD Simulations of Accretion Disks Three Dimensional MHD Simulations of Accretion Disks

May 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aps..may.k1401h&link_type=abstract

American Physical Society, APS/AAPT Joint Meeting, May 2-5, 1996, abstract #K14.01

Astronomy and Astrophysics

Astrophysics

Scientific paper

Using three-dimensional numerical simulations, we have found that the weak-field magnetorotational instability constitutes a hydromagnetic dynamo in astrophysical disks. The magnetorotational instability rapidly generates anisotropic turbulence within the disk, producing significant Maxwell and Reynolds stresses that transport angular momentum outward. The magnetic energy is amplified and maintained far longer than the magnetic decay time. For comparison we consider simple, nonshearing fluids stirred by imposed velocity fields. A magnetic dynamo can be produced, for velocity fields with and without net helicity. Lorentz forces limit the growth of the magnetic field. The dynamo occurs only because the turbulence is artificially sustained by turbulence. Next we consider a simple nonrotating shear flow. Such flows are hydrodynamically unstable to finite amplitude instabilities and this leads to tubulence; however, the combination of turbulence and shear are by themselves insufficient to produce a dynamo. In contrast, a differentially rotating system is hydrodynamically stable to finite amplitude perturbations, but MHD linearly unstable. This is the crucial difference. This field amplification mechanism lies outside the scope of kinematic dynamo theory, which is thus inapplicable to differentially rotating systems.

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