Global structure of self-excited magnetic fields arising from the magnetic shear instability

Details Global structure of self-excited magnetic fields arising from the magnetic shear instability Global structure of self-excited magnetic fields arising from the magnetic shear instability

Sep 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000mnras.317...45d&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 317, Issue 1, pp. 45-54.

Astronomy and Astrophysics

Astronomy

12

Accretion, Accretion Discs, Instabilities, Magnetic Fields, Mhd

Scientific paper

The global structure of a self-excited magnetic field arising from the magnetic shear instability has been simulated in spherical geometry by a 3D fully non-linear approach. In order to model the structure of an accretion disc we prescribe a rotation profile of the Brandt type which is Keplerian in the outer regions but yields rigid rotation at the inner core. We performed a whole series of runs at different dynamo numbers with an increasing number of modes in spectral space, thereby checking the influence of the numerical resolution in our simulations. Starting from arbitrary small perturbations, the magnetic and kinetic energies grow by several orders of magnitude as soon as a certain azimuthal resolution of at least m=15 was used at a dynamo number of order CΩ=105. Several phases of the transition to turbulence are realized and interpretations are given for the respective effects occurring at each stage. The resulting magnetic field is highly non-axisymmetric and possesses a pronounced inhomogeneous vortex structure of twisted flux tubes. The flow is almost axisymmetric but shows a Kolmogorov-like behaviour for small scales. The outer surface of the shell is penetrated by magnetic field lines in spot-like regions, which are located mainly in the equatorial plane. For very high dynamo numbers we find a cyclic behaviour of the averaged magnetic field amplitude. The problem of angular momentum transport is discussed in terms of the Shakura-Sunyaev viscosity alpha, which depends exponentially on the radial distance and adopts values in the range 10-3-10-5.

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