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POYNTING JETS FROM ACCRETION DISKS Authors: R. V. E. Lovelace, H. Li, A. V. Koldoba, G. V. Ustyugova, and M. M. Romanova We
give further consideration to the problem of the
evolution of a coronal, force-free magnetic field that
threads a differentially rotating, conducting Keplerian
disk, extending the recent work of Li and coworkers.
This situation is described by the force-free
Grad-Shafranov (GS) equation for the flux function
Y(r,
z) that labels the poloidal field lines (in cylindrical
coordinates). The GS equation involves a function H(Y)
describing the distribution of the poloidal current, which
is determined by the differential rotation or "twist"
of the disk that increases linearly with time. We
numerically solve the GS equation in a sequence of
volumes of increasing size corresponding to the
expansion of the outer perfectly conducting boundaries
at (Rm, Zm). The outer boundaries
model the influence of an external nonmagnetized plasma.
The sequence of GS solutions provides a model for the
dynamical evolution of the magnetic field in response to
(1) the increasing twist of the disk and (2) the
pressure of external plasma. We find solutions with
magnetically collimated Poynting jets in which there is
a continuous outflow of energy, angular momentum,
and toroidal magnetic flux from the disk into the
external space. This behavior contradicts the commonly
accepted "theorem" of solar plasma physics that
the motion of the footpoints of a magnetic loop
structure leads to a stationary magnetic field
configuration with zero power, angular momentum, and flux
outflows. In addition, we discuss magnetohydrodynamic simulations
that show quasi-stationary collimated Poynting jets similar
to our GS solutions. In contrast with the GS solutions,
the simulations show a steady uncollimated hydromagnetic (nonforce-free)
outflow from the outer part of the disk. The Poynting
jets are of interest for the understanding of the jets
from active galactic nuclei, microquasars, and possibly gamma-ray
burst sources.
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