Magnetic field generation in an accretion disk of close binary star

Bisikalo Dmitry, bisikalo@inasan.ru, Institute of astronomy RAS, Russian Fed
Zhilkin Andrey, Institute of astronomy RAS

Abstract
Using results of 3D MHD numerical simulations we investigate the generation of the magnetic field in an accretion disk of a close binary star. The magnetic field in the accretion disk around a compact object can be amplified due to the differential rotation, radial motions, and dynamo action. The diffusion, turbulent dissipation, and magnetic buoyancy can weaken the magnetic field. When these effects work together, a magnetic field with a complex structure occurs, since different effects may dominate in different parts of the disk. Analysis of the numerical simulations shows that the magnetic field, which is mainly toroidal, is intensively generated in the accretion disk. In the disk, we can distinguish three regions: inner region of the intensive toroidal field generation due to the differential rotation; region of the current sheets; and outer region of the magnetic field dissipation. The current sheet is formed in the inner zone due to the variations of the rotation law near the stellar magnetosphere. Further, the formed current sheet is moved outwards because of the decretion of matter. At its place the new current sheet is formed in a while. Thus, in the intermediate zone, several current sheets can exist at the same time. In the outer regions of the accretion disk of a magnetic close binary system two types of the dynamo may exist. One of them is the laminar dynamo that occurs as a result of the nonaxisymmetric motions. The other type is the turbulent alpha-Omega-dynamo. Our simulations show that the mean spirality alphs of the nonaxisymmetric and turbulent motions of gas has no reflection symmetry with respect to the equatorial plane of the disk. Therefore, in the outer regions of the accretion disk the magnetic field is partly generated in the dynamo process. At that, the quadrupole mode grows faster than the others. However, the role of the dynamo is not significant, since, the disk is dominated by the dynamic effects.