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, forcefree^{ }magnetic field that
threads^{ }a differentially rotating, conducting^{ }Keplerian
disk, extending the^{ }recent work of Li^{ }and coworkers.
This situation^{ }is described by the^{ }forcefree
GradShafranov (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 (R_{m}, Z_{m}). 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 quasistationary collimated^{ }Poynting jets similar
to^{ }our GS solutions. In^{ }contrast with the GS^{ }solutions,
the simulations show^{ }a steady uncollimated hydromagnetic^{ }(nonforcefree)
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^{ }gammaray
burst sources.

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