Rayleigh-Taylor Instability

: Two types of fluid instabilities affect cloud-shock interactions. The Rayleigh-Taylor (RT) instability occurs any time a dense, heavy fluid is being accelerated by light fluid such as is the case with a cloud and shock system. Two completely plane-parallel layers of fluid are stable, but the slightest perturbation leads to tangential ``gravity''. Any small local minimum will quickly be amplified by material flowing down under the influence of this force. Dimples are quickly magnified into sets of inter-penetrating ``RT fingers'' as the heavy material moves down and the light material flows up. This process is evident in many terrestrial examples from boiling water to weather inversions.

RT instabilities are quelled by any sort of restoring force. In astrophysical situation, a magnetic field applies a certain amount of tension and can inhibit instability growth.

Theoretical work on RT instabilities and magnetic fields was done recently by Jun, Stone and Norman (1995) and showed that a field of

$$B \geq \frac{[g\lambda(\rho_2-\rho_1)]^{1/2}}{cos \theta}$$

would suppress instabilities where g is the local 'gravity', $\lambda$ is the wavelength of the instability and $\theta$ is the angle between B and the interface. Thus, a field may reduce the small fluff, but large wavelength RT instabilities are harder to suppress. Figure 3 shows the effects of a tangential magnetic field on the formation of turbulence.

RT fingers are especially obvious in the Crab Nebula (Hester et al.1996), Tycho's SNR (Velazquez et al.1998) and several other astronomical objects.