The Geodynamo Mechanism (Electrodynamics focus)
The core of the earth is made up iron and nickel, in an outer liquid core and an inner solid core. The outer core is in constant movement churned through the Carnot heat engine that is called earth. Because it is liquid iron flowing, magnetohydrodynamics come into play.
Magnetohydrodynamics combines the intuitive nature of Maxwell's equations with the easy solvability of the Navier-Stokes equations. It's so straightforward physicists add "relativistic" or "quantum" just to keep it from getting boring.
Generating the Magnetic Field
Why it isn't simple
The core also generates the magnetic field, but this cannot do it like a normal stick magnet, because of the Curie point. After a certain temperature atoms move so violently that they cannot retain a fixed direction, which in turn shatters magnetic alignment and therefore the material loses permanent magnetic properties.
So the magnetic field has to be constantly generated in an ongoing process. Else the magnetic field would die down because of ohmic dissipation, in about 20'000 years.
Magnetohydrodynamic Generation of the Magnetic Field
We start with an existing seed magnetic field, maybe coming from the original formation of the earth or a fluctuation in the plasma. Now when a conductor moves through it, it induces a current and this in turns generates its own magnetic field. In a fluid the magnetic field lines move with the fluid, which in an ever churning sea of liquid iron feeds mechanical energy back into this electromagnetic system.
The velocities of the fluid in the outer core are 10 - 20 km per year. But why does the chaotic movement of the fluid not cancel out the magnetic effects or generate a violently changing magnetic field? After Cowling's anti-dynamo theorem, a symmetric flow cannot generate a magnetic field that will not decay over time.
The spinning Ball to the rescue
The spin of the earth changes the rules for the fluid dynamics. The liquid iron has a surprising low viscosity. For a spinning fluid the dimensionless Ekman number is most important, it compares the strength of the viscous forces to the strength of the Coriolis forces.
For the outer core the Ekman number is small in the range of
But the heat engine must move fluid outwards in all directions, so the fluid tries to do both and organizes itself into columnar convection. A ring of tall tornados aligned with the pol stretching from the bottom of the outer core to the boundary to the mantle.
These structures allow the generation of the magnetic field.
Mean Field Electrodynamics
The flow inside the small tornados is helical, because of the Coriolis force acting on them, in the Northern Hemisphere the helicity is generally negative and opposite in the Southern Hemisphere.
The Alpha Effect
This leads to the alpha effect. Imagining a horizontal magnetic field line going around the equator of earth, a toroidal field. The small tornados catch these lines and because of the helical motion twist it upwards to be a vertical loop. A vertical, north-south running field is called a poloidal field.
But when just all horizontal field get turned vertical, this does not close the loop to be a valid sustainable magnetic field. So the core needs a way to strengthen the horizontal toroidal fields.
The Omega Effect
Because of the inner core rotates on a slightly different speed that the outer core and at different latitudes the fluid flow at different speeds, there are massive shear flows.
This shear flows grabs the poloidal field lines and shears them around so they get more and more horizontal and toroidal again. This also drastically amplifies the magnet energy.
The Flip
Why does the magnetic field flip around on a semiregular basis?
The geodynamo, generated by the alpha-omega-loop, is not per se stable. It is a statistical equilibrium. Magnetic reversals are born from extreme internal turbulence. This happens about 4 times every million years.
We can see this because of paleomagnetism, the magnetic field is recorded in rocks. Because tectonic plates move apart, and new lava turns into rocks constantly we have a nearly continuous recording of the magnetic field.
The flipping itself takes between 2000 and 12000 years.
But why
The fluid creates the magnetic field, but the magnetic field exerts a massive physical force back into the fluid (Lorentz force). The Lorentz force is quadratic in respect to the magnetic field, and therefore invariant to a flip of the magnetic field. Because neither polarity is preferred, and the outer core is a high Rayleigh number fluid (violently, chaotically turbulent), intrinsic chaotic movement can trigger a flip.
The reason why it is semiregular, on the scale of hundreds of thousands of years, is coherent energy that is needed to overcome the inertia of a fully established large-scale field.
During a Flip
The large-scale north-south magnetic field can drop to 10% of its normal strength. The energy cascades down to smaller chaotic scales. Which could mean multiple different north and south poles, spread over the earth.
Because this is random, it can also happen that in the end the field is in the same direction as before, this is a failed reversal or polarity excursion.
The Rate of Flips
About 83 million years ago, a Cretaceous Normal Superchron ended. During this time, about 120 million years, the field did not reverse a single time.
The mantle comes into play here, it moves at the speed fingernails grow, but it controls the heat flow. Because the heat flow through the D'' layer is not even, it destabilizes the tornados below and therefor generate more chaos and increase the amount of flips.
In the Cretaceous period the mantle blanked shifted into a regular layer and this leads to no flips in this time.