A type of instability that often occurs in fluids which are thermally
stably stratified, but have an inhomogeneous composition.
A well-known example, found in upper layers of the Earth’s oceans, is
→ salt fingers. Similar fingering instabilities
can occur in any other thermally stably stratified solution, provided the concentration
of the slower-diffusing solute increases with height. The saturated state of this instability,
→ fingering convection, takes the form of
tightly-packed, vertically-elongated plumes of sinking dense fluid and rising
light fluid, and significantly enhances the vertical transport of both heat and
chemical composition.
The fingering instability occurs in stars within radiation zones that
have an unstable mean → molecular weight
→ gradient (μ gradient).
This situation is often found as a result of material
accretion onto a star by anything from a single or multiple planets, to material from a
dust-enriched or debris accretion disk, or
material from a more evolved companion. It also naturally arises in the vicinity of
the → hydrogen shell burning
in → red giant branch
(RGB) stars, and in thin element-rich layers
near the surface of intermediate-mass stars. The fingering instability initially takes the
form of thin tubes, hence the name “finger,” within which the fluid
moves vertically. The tubes rapidly break down, however, as a result
of parasitic shear instabilities that develop inbetween them, and the fingering instability
eventually saturates into a state of homogeneous fingering convection
where the typical aspect ratio of the eddies is closer to one
(P. Garaud et al., 2015, arXiv:1505.07759).
See also: → finger; → -ing;
→ instability.
