Free access article
| Issue |
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A&A
Volume 444,
Number 2,
December III 2005
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Page(s)
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337 - 346 |
| Section |
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Astrophysical processes |
| DOI |
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10.1051/0004-6361:20042591 |
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A&A 444, 337-346 (2005)
DOI: 10.1051/0004-6361:20042591
Magneto-rotational overstability in accretion disks
J. W. S. Blokland1, 2, E. van der Swaluw1, R. Keppens1, 2 and J. P. Goedbloed1, 2 1
FOM-Institute for Plasma Physics Rijnhuizen, PO Box 1207, 3430 BE Nieuwegein, The Netherlands
e-mail: blokland@rijnh.nl
2
Association EURATOM-FOM, Trilateral Euregio Cluster
(Received 22 December 2004 / Accepted 13 April 2005)
Abstract
We present analytical and numerical studies of magnetorotational
instabilities occuring in magnetized accretion disks. These calculations are
performed for general radially stratified disks in the cylindrical limit. We elaborate
on earlier analytical results and confirm and expand them with numerical computations
of unstable eigenmodes of the full set of linearised compressible MHD equations. We
compare these solutions with those found from approximate local dispersion
equations from WKB analysis.
In particular, we investigate the influence of a nonvanishing toroidal magnetic field
component on the growth rate and oscillation frequency of magnetorotational instabilities in
Keplerian disks. These calculations are performed for a constant axial
magnetic field strength. We find the persistence of these instabilities in
accretion disks close to equipartition. Our calculations show that these
eigenmodes become overstable (complex eigenvalue), due to the presence of a toroidal
magnetic field component, while their growth rate reduces slightly.
Furthermore, we demonstrate the presence of magneto-rotational overstabilities in
weakly magnetized sub-Keplerian rotating disks. We show that the growth rate scales
with the rotation frequency of the disk. These eigenmodes also have a nonzero oscillation
frequency, due to the presence of the dominant toroidal magnetic field component.
The overstable character of the MRI increases as the rotation frequency of the disk decreases.
Key words: accretion, accretion disks
-- instabilities
--
magnetohydrodynamics (MHD)
-- plasmas
© ESO 2005
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