Magnetorotational instability

*Magnetorotational instability*

The classical instability of a differentially rotating fluid takes place
according to Rayleigh's criterion, i.e. when there is a radially outward
decrease of angular momentum (per unit mass). In 1978, however, I showed
that (i) a tiny radial decrease of *angular velocity* is sufficient to give
instability if a small azimuthal magnetic field is present, and (ii) the fastest-growing
modes then amplify very fast indeed, at a rate determined solely by the
differential rotation.

This phenomenon - now generally known as the *magnetorotational* instability - arises when the magnetic energy is
small compared with the rotational energy, provided that the electrical
conductivity of the fluid or gas is not too weak. Non-axisymmetric waves
then grow and propagate around the rotation axis.

*Phil. Trans. Roy. Soc. A Vol 289, pp 459-500, 1978*

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