I’ve had trouble understanding exactly why there is not more mixing of plasma at the core of the sun with the outer layers.
I understand the difference between the radiative zone and the convective zone.
However, even $without$ convection, a plasma should mix at a fairly high degree with everything around it.
So there must be some other physics of high density plasma that keeps it from mixing.
I see no reason to think of this plasma at the core as effectively being some kind of solid, a gel, or even a liquid? It’s always described as a gas, which implies very free mixing with its surroundings.
Maybe someone can answer why, to some order of magnitude, a helium ion in the core does not, on average, migrate 1 million miles outside the core over the course of a billion years or so?
Regardless of temperature gradient or pressure gradient, it seems that helium ions at the core should diffuse to outer layers. I did read one reference to Swartzschild and “negative buoyancy”. It seems to me that the force of negative buoyancy would have to be very strong in order to keep heavier ions from diffusing to outer layers.
EDIT 1: ok I did some more googling and finally found this paper from 1977 on diffusion. It’s going to take me a few days to digest it though :)
It refers to a diffusion rate of ~1000 km every 5 billion years. Now I have to figure out why that estimate is so fantastically slow.
Such a low diffusion rate also seems absolutely ludicrous given that high density tokamak plasma experiments report diffusion rates on the order of 100 meters^2/second. https://www.osti.gov/servlets/purl/6637207
Experiments cite a number 9 or 10 orders of magnitude higher than the rate they “think” is going on inside of stars. Everything I’ve read sounds like handwaving. Physicists don’t seem to know what is going on in the core of stars.
EDIT 2: Here is a paper that discusses at much length, and with much humility, atomic diffusion processes in the core of stars. https://arxiv.org/abs/1707.07454 page65. They reiterate many times that there seems to be unknown process(es) that inhibit diffusion, based on studies gathered from many many different sources, encompassing just about every species of star.