### magnetorotational instability

**(MRI, Velikhov-Chandrasekhar Instability, Balbus-Hawley Instability)**
(instability in the fluid of a disk from MHD effects)

**Magnetorotational instability**
(**MRI** or **Velikhov-Chandrasekhar instability** or **Balbus-Hawley instability**)
is an instability in a body of fluid, e.g., a disk,
that occurs in a conducting fluid within a magnetic field
when the **angular velocity** decreases with radial distance.
In such conditions, small disturbances receive positive feedback,
resulting in the instability.
(Disk simulations need sufficiently high resolution to model
the initial small disturbance.)
The result can be turbulence, termed **magnetoturbulence**
or **magnetorotational turbulence**.

It is thought to occur in accretion disks and
to be a contributor to Type I planetary migration.

The criteria for magnetorotational instability in a Keplerian disk
in an axial magnetic field is:

*d*Ω²
———— > 0
*d*ln*R*

- Ω - angular momentum
*R* - radius

Modeling *MRI* is more difficult if the disk is not fully
ionized.

(*physics,fluid dynamics,MHD,accretion disk,magnetism,disturbance,migration*)
http://en.wikipedia.org/wiki/Magnetorotational_instability

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