Viewing a single comment thread. View all comments

CrustalTrudger t1_jdqg36k wrote

The actual article as opposed to the press release (what you linked) does briefly talk about it (in the first paragraph of their discussion), but mostly it's cited out to prior literature. Specifically, as discussed by papers like Davies et al., 2021, in their words, "Cooling of the liquid core leads to freezing at Earth’s centre and the growth of the solid inner core, which provides additional power to the dynamo through release of latent heat and gravitational energy" and they in turn point to thermodynamic simulations that demonstrate this (e.g., Gubbins et al., 2004). Details of core geodynamics as it relates to the magnetic field is a bit out of my specialty, so I'll leave further discussion/explanation to folks with more domain experience, but it's not as though the articles presenting this data do not discuss the mechanism at all.

625

dukesdj t1_jdqzjci wrote

That quoted line summarises it quite nicely really. Convection is great for dynamo action as not only does it provide the kinds of turbulent motion that is great for inducing magnetic field, but it acts as an energy source. Prior to any freezing of species one would imagine the fluid to be well mixed and essentially only a single phase homogeneous fluid. For a single phase single composition fluid convective instability sets in under the Schwarzschild criterion which essentially says that the instability sets in when the temperature gradient is larger than the adiabatic gradient (the temperature gradient at fixed entropy, we fix the entropy as temperature is a function of density and pressure and so mathematically the gradient is a partial derivative). In physical terms the way I think of this is there is an amount of heat a static fluid can transport through conduction, but if the amount of heat the system is trying to push through the fluid is above this amount then instability sets in, the convection then transports the heat by physically moving it.

If the fluid also has compositional gradient then we fall under the Ledoux criterion for convective instability and it is easier for convection to set in. This is known as compositional convection or double-diffusive convection. Mathematically the compositional gradient is subtracted from the adiabatic thus lowering the actual temperature gradient required to onset the convective instability. The best way to physically understand this is through parcel arguments which really require figures so instead I will refer to Pascal Garauds excellent lecture notes which are more related to astro than geo. The result will be more efficient heat transport and more energy available for dynamo action.

Another process is two phase fluids which is the freezing out of material. This I know a lot less about as I am more concerned with starts than terrestrial planets but I am in the same boat as the rest of the fluids community in this regard as even the hydrodynamic (no magnetic field) problem has only recently begun to be properly worked on. In simplistic terms it can be thought of in a similar way to compositional convection in the sense that it provides an extra source of energy that can power the dynamo.

The bottom line is, convection is a natural way of producing a dynamo and stronger convection will lead to a stronger dynamo. Thus any mechanism which can aid convection or act as a source of energy/entropy will be beneficial for dynamo.

165

TerminationClause t1_jdr251s wrote

So, forgive me if I misunderstand, I'm still on my first cup of coffee... but all those years I spent learning about fluid dynamics (off-handedly learning about chaos theory) can actually be applied to the inner and outer core as well as the mantle? Not only that, but that we understand it as well as you seem to? I really should have gone to college for that stuff instead of just reading it on the crapper.

39

dukesdj t1_jdr6owx wrote

In the words of one of my colleagues "99% of the universe is fluids, the remaining 1% is just details". Fluid dynamics is everywhere, it is actually harder to think of things that do not involve fluid dynamics than otherwise. I extend this not just to geophysical and astrophysical fluid dynamics but all of physics, engineering, biology, medicine, chemistry, and probably more.

85

acfox13 t1_jdrpt6c wrote

>"99% of the universe is fluids, the remaining 1% is just details"

I like that. I live somewhere with huge tides, whirlpools, microclimates, and the fluid dynamics here is stunning to witness.

30

Chalkarts t1_jds7f2j wrote

I was recently contemplating how similar fast moving river water with its swirls and waves looks a lot like atmosphere of a gas giant, or the formation of galaxies. Made me wonder if magnetism and gravity were the only forms of “drag” giving the swirling galaxies their shape since there are no shorelines or rock to provide it in space.

17

Rootriver t1_jdu2vtj wrote

> Made me wonder if magnetism and gravity were the only forms of “drag”

Yes and no (at least according to the current main theories of physics). In a way in that scale gravitation and electromagnetism are the only meaningful forces, but forces (or rather interactions) called strong interaction and weak interaction can have pretty drastic local effects (these forces only work on very short distances, i.e. atomic and subatomic level) that can then affect the things on larger scale.

https://en.m.wikipedia.org/wiki/Standard_Model

Note: my knowledge here is bit rusty (pre observed Higgs boson) and elementary level.

2

Ganondorf_Is_God t1_jdsjgoq wrote

Alright, got any good sources for getting more up to speed on these domains of knowledge?

4

dukesdj t1_jdsowp3 wrote

Depends on exactly what you want to know more about.

Fluid dynamics in general An Introduction to Fluid Dynamics by Batchelor is good. Hydrodynamic and hydromagnetic stability is a classic book on fluid instabilities by Chandrasekhar which includes convective instabilities. Introduction to Modeling Convection in Planets and Stars: Magnetic Field, Density Stratification, Rotation by Glatzmaier is good for the more numerical modelling side but also includes theory. Internally Heated Convection and Rayleigh-Bénard Convection by Goluskin is a good book on convection. An Introduction to Magnetohydrodynamics by Davidson is great for some dynamo theory. Self-Exciting Fluid Dynamos by Moffatt and Dormy is a tough read but focused on all kinds of dynamo theory. Chris Jones lecture notes on Dynamo theory are also great.

For double diffusive convection Pascals notes in the previous post are an excellent place to start.

17

drfarren t1_jdtnnjw wrote

Does the non-spherical shape of the core and mantle layers play into this? Like, does the field become stronger in areas where the friction between layers force mantle material to flow differently.

1