Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998mnras.294..399u&link_type=abstract
Monthly Notices of the Royal Astronomical Society, vol. 294, p. 399
Astronomy and Astrophysics
Astrophysics
13
Accretion Disks, Magnetohydrodynamic Stability, Interstellar Magnetic Fields, Angular Velocity, Boussinesq Approximation, Protoplanets, Vertical Orientation, Linear Equations, Magnetoacoustic Waves
Scientific paper
The stability properties of magnetized disks rotating with angular velocity Omega = Omega(s,z), dependent on both the radial and the vertical coordinates s and z, are considered. Such a rotation law is adequate for many astrophysical disks (e.g., galactic and protoplanetary disks, as well as accretion disks in binaries). In general, the angular velocity depends on height, even in thin accretion disks. A linear stability analysis is performed in the Boussinesq approximation, and the dispersion relation is obtained for short-wavelength perturbations. Any dependence of Omega on z can destabilize the flow. This concerns primarily small-scale perturbations for which the stabilizing effect of buoyancy is strongly suppressed due to the energy exchange with the surrounding plasma. For a weak magnetic field, instability of disks is mainly associated with vertical shear, whilst for an intermediate magnetic field the magnetic shear instability, first considered by Chandrasekhar and Velikhov, is more efficient. This instability is caused by the radial shear, which is typically much stronger than the vertical shear. Therefore, the growth time for the magnetic shear instability is much shorter than for the vertical shear instability. A relatively strong magnetic field can suppress both these instabilities. The vertical shear instability could be the source of turbulence in protoplanetary disks, where the conductivity is low.
Brandenburg Axel
Urpin Vadim
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