[freebeard]

It's all https://en.wikipedia.org/wiki/Magnetohydrodynamics

Dynamo theory suggests that convection in the outer core, combined with the Coriolis effect, gives rise to Earth's magnetic field. The solid inner core is too hot to hold a permanent magnetic field (see Curie temperature) but probably acts to stabilize the magnetic field generated by the liquid outer core. The average magnetic field strength in Earth's outer core is estimated to be 25 Gauss (2.5 mT), 50 times stronger than the magnetic field at the surface.

https://en.wikipedia.org/wiki/Struct...the_Earth#Core

A dynamo trapped between the solid core and a crust that attenuates the leakage to about 2%.