------------------------------------------------------------------------
 ms.tex MNRAS, 2009, 395, 2183
Date: Fri, 7 Aug 2009 08:04:13 +0800
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%arXiv:0811.2893
%http://www.shao.ac.cn/fyuan/yuan.html

\documentclass[useAMS,twocolumn]{mn2e}
\begin{document}

\title[An MHD model for episodic jets]
{A Magnetohydrodynamical Model for the Formation of Episodic Jets}
\author[F. Yuan, J. Lin, K. Wu, and L. C. Ho]
{Feng Yuan$^{1,2}$\thanks{E-mail: fyuan@shao.ac.cn (F.Y.),
jlin@ynao.ac.cn (J.L.), kw@mssl.ucl.ac.uk (K.W.), and lho@ociw.edu (L.H.)}, Jun
Lin$^{3,4}$, Kinwah Wu$^{5}$, and Luis C. Ho$^{6}$\\
$^{1}$Shanghai Astronomical Observatory, Shanghai 200030, China;\\
$^{2}$Joint Institute for Galaxy and Cosmology (JOINGC) of SHAO and USTC;\\
$^{3}$National Astronomical Observatories/Yunnan Observatory,
Chinese Academy of Sciences, Kunming 650011, China;\\
$^{4}$Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
Cambridge, MA 02138, USA;\\
$^{5}$Mullard Space Science Laboratory, University College London,
  Holmbury St Mary, Surrey RH5 6NT, UK;\\
$^{6}$The Observatories of the Carnegie Institution of Washington,
813 Santa Barbara Street, Pasadena, CA 91101, USA; } \maketitle


\begin{abstract}
Episodic ejection of plasma blobs have been observed
in many black hole systems. While steady, continuous jets are believed
to be associated with large-scale open magnetic fields,
what causes the episodic ejection of blobs remains unclear.
Here by analogy with the coronal mass ejection on the Sun,
we propose a magnetohydrodynamical model for episodic ejections from black
holes associated with the closed magnetic fields in an
accretion flow. Shear and turbulence of the accretion flow deform the
field and result in the formation of a flux rope in the disk corona.
Energy and helicity are accumulated and stored until a threshold is reached.
The system then loses its equilibrium and the flux rope is thrust outward by
the magnetic compression force in a catastrophic way.
Our calculations show that for parameters appropriate for the black hole
in our Galactic center, the plasmoid can attain relativistic speeds
in about 35 minutes.
\end{abstract}
----------------------------------------
Feng Yuan
Shanghai Astronomical Observatory
80 Nandan Road, Shanghai 200030, China
Tel: 86-21-64386191 ext 261
http://www.shao.ac.cn/fyuan
----------------------------------------