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DYNAMICS OF
MAGNETIC LOOPS IN THE CORONAE OF ACCRETION DISKS
Authors:
M.M. Romanova, G.V. Ustyugova, A.V. Koldoba, V.M. Chechetkin, and R.V.E. Lovelace
Axisymmetric magnetohydrodynamic (MHD) simulations are used to study
the evolution of general magnetic field configurations where a B
field threads different radii of a differentially rotating accretion disk. The
differential rotation of the foot-points 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 (b
< 1). In the region where (b > 1), the loops may be only
partially opened. Current layers form in the narrow regions which separate oppositely
directed magnetic field. 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 surface. The faster rotation of the inner part of
the disk gives a stronger outflow from this part of the disk. The outfow 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 B field
strengths and mass outflow rates. 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.
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