Brandenburg, A., Nordlund, A., Stein, R. F., Torkelsson, U. 1995, Astrophys. J. 446, 741-754.

Paper (PostScript). Date of entry: January 4, 1995. Contact: Axel.Brandenburg@ncl.ac.uk

Dynamo generated turbulence and large scale magnetic fields in a Keplerian shear flow

Axel Brandenburg [1], AAke Nordlund [2], Robert F. Stein [3], Ulf Torkelsson [4,5]
  1. Advanced Study Program , High Altitude Observatory,NCAR,P.O. Box 3000,Boulder,CO 80307,USA
  2. Copenhagen University Observatory,Oster Voldgade 3,DK-1350 Copenhagen O,Denmark
  3. Dept. of Physics and Astronomy,Michigan State University,East Lansing,MI 48824,USA
  4. Lund Observatory,Box 43,S-221 00 Lund,Sweden
  5. Sterrekundig Instituut,Postbus 80000,3508 TA Utrecht,The Netherlands

Abstract

The nonlinear evolution of magnetized Keplerian shear flows is simulated in a local, three-dimensional model, including the effects of compressibility and stratification. Supersonic flows are initially generated by the Balbus-Hawley magnetic shear instability. The resulting flows regenerate a turbulent magnetic field which, in turn, reinforces the turbulence. Thus, the system acts like a dynamo that generates its own turbulence. However, unlike usual dynamos, the magnetic energy exceeds the kinetic energy of the turbulence by a factor of 3-10. By assuming the field to be vertical on the outer (upper and lower) surfaces we do not constrain the horizontal magnetic flux. Indeed, a large scale toroidal magnetic field is generated, mostly in the form of toroidal flux tubes with lengths comparable to the toroidal extent of the box. This large scale field is mainly of even (i.e. quadrupolar) parity with respect to the midplane and changes direction on a timescale of about 30 orbits, in a possibly cyclic manner. The effective Shakura-Sunyaev alpha viscosity parameter is between 0.001 and 0.005, and the contribution from the Maxwell stress is about 3-7 times larger than the contribution from the Reynolds stress.