What is meant by “stable/unstable to convection” in the context of the solar atmosphere?

Question

In studying the Schwarzschild stability criterion (for the solar atmosphere); $$\left\lvert\frac{dP}{dr} \right\rvert\lt\frac{\gamma P}{\rho}\left\lvert\frac{d\rho}{dr}\right\rvert$$ From my notes are three quotes that I am trying to understand:

A steep temperature gradient is potentially unstable to convection if the gradient is steeper than the temperature gradient that would be produced by matter rising adiabatically. If this happens then convection will take place.

From the above quote I am getting the sense that for convection to take place there must be an unstable temperature gradient. But what is an unstable temperature gradient?

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If the fluid element is denser than its new surroundings it will sink back down, and the material is stable with respect to convection and heat transport will be by radiation. If the fluid element is less dense than its surroundings it will continue to rise, so the material is unstable to convection.

Again, it is the same words that are in bold text which I just cannot understand, this time I would like to know what it means to say that a "material is stable with respect to convection"?

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Convection is a very efficient process for transporting heat, with only marginal instability capable of transporting the solar energy output. This means that in convectively unstable regions we can change the Schwarzschild instability criterion from an inequality to an equality $$\left\lvert\frac{dP}{dr} \right\rvert = \frac{\gamma P}{\rho}\left\lvert\frac{d\rho}{dr}\right\rvert.$$

I have no idea what justifies the change of the inequality to an equality (I thought the inequality was required for convection to take place). Now the words "convectively unstable regions" have been used and I have simply no idea what this means.

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The questions are basically all regarding the same thing and are summarized in the title. The word convection is constantly being used in conjunction with the words stable/unstable, therefore, until someone please explains what this means in simple English I'm probably never going to understand the Schwarzschild stability criterion in the solar atmosphere.

Basically, this question is not really about the physics taking place here, it is confusion over the use of certain words. If someone would kindly re-phrase "stable/unstable to convection" in a way that is more intelligible to humans, I would be most grateful.

Answer 1

You are right: stable to convection is a bit misleading. A better term is stable to disturbances. Disturbances here means minimal fluctuations, e.g. produced by molecular motions of the gas particles that in the case of stability will be dampened and in the case of instability are amplified until they grow to macroscopic scales leading to convection.

However, there are situations where stable to convection makes sense because there exist different kind of instabilities. In stars these are e.g. semi-convection and thermohaline mixing. Without going into details, those can occur when material with different molecular weight (e.g. helium enriched material) is stacked on top of each other. The molecular weight gradient can have a stabilising or destabilising effect. Thus, zones that are stable/unstable to convection can be unstable/stable to semiconvection or thermohaline mixing.

Answer 2

Consider a "radiative zone" in a star, where all the heat transport is via photons and the gas is stably stratified. That means that a lump of gas at a particular density and temperature stays where it is. If it is displaced upwards, it expands, becomes less dense but is still denser than its surroundings and so immediately falls back. We would say this situation is stable against convection.

However, if the temperature gradient increases (perhaps because the opacity of the gas increases), then the stability of this situation is disrupted. If a parcel of gas now rises, it still expands and becomes less dense, but the density of the surrounding gas falls more quickly. Therefore the parcel of gas remains buoyant and rises further. This is what is meant by being "unstable to convection" and is reached when the Schwarzschild criterion is exceeded.

As your notes say - convection is very efficient and so once it is established, the temperature gradient stays very close (but just above) the Schwarzschild criterion value, which is also known as the "adiabatic temperature gradient". In some circumstances, radiative transport is still a significant contributor and the temperature gradient can become " super adiabatic" (but not by much).