Dynamics of Magnetic Loops in the Coronae of Accretion Disks

Abstract

Axisymmetric magnetohydrodynamic (MHD) simulations are used to study the evolution of general magnetic field configurations where a magnetic field B threads different radii of a differentially rotating accretion disk. The differential rotation of the footpoints of B field loops at different radii on the disk surface causes a twisting of the coronal magnetic field, an increase in the coronal magnetic energy, and an opening of the loops in the region where the magnetic pressure is larger than the matter pressure (β 1). In the region where β 1, the loops may be only partially opened. Current layers form in the narrow regions that separate oppositely directed magnetic field lines. Reconnection occurs in these layers as a result of the small numerical magnetic diffusivity of the code. In contrast with the case of the solar coronal magnetic field, the combination of magnetic and centrifugal forces leads to significant matter outflow from the disk. The faster rotation of the inner part of the disk gives a stronger outflow from this part of the disk. The outflow accelerates with increasing distance from the disk up to velocities in excess of the escape speed. The outflows show some collimation within the computational region and have a large power output mainly in the form of a Poynting flux. Thus these outflows are pertinent to the origin of astrophysical jets. We present results of a survey of simulation runs for the behavior of magnetic loops and outflows for a wide range of field strengths B and mass outflow rates j. The model and processes observed are relevant to the coronae of accretion disks around stellar-mass objects, including pre-main-sequence stars, compact stars, and black holes, as well as the coronae of disks around massive black holes in active galactic nuclei. Opening of magnetic field loops may lead to transient and/or steady outflows, while reconnection events may be responsible for X-ray flares in such objects.