[Lewis Thomson (OP)]

Donald sent in this question to The Naked Scientists

"Iron magnets lose ALL their magnetic properties if they get hot enough, yet the molten iron in the Earth's core produces a protective magnetic field around the Earth, HOW?  And why is it so stable day-to-day yet change so dramatically over time, even disappear completely?  And why doesn't life on Earth's surface suffer when the field is absent?"

Discuss your thoughts in the comments below...

[evan_au]

Each magnetic material has a "Curie temperature", where it loses any previous external magnetic field.
- Earth's nickel-iron core is though to have a temperature of around 5000C (similar to the surface of the Sun), which is well above the Curie temperature of iron (around 770C). So magnetic iron cannot explain the Earth's magnetic field.
- The Sun also has a strong magnetic field, but being made primarily of non-magnetic hydrogen & helium, natural magnetism cannot explain this magnetism either.

It is thought that the Earth's magnetic field is produced by electric currents circulating in the electrically conductive liquid iron core of the Earth.
- Similarly, the Sun's magnetic field is produced by electric currents circulating in the electrically conductive plasma core of the Sun.
- These currents are driven by convection due to heat leaking from the center of the Earth towards the surface (and similar for the Sun).
- The Earth's iron core is much more viscous than the Sun's hydrogen plasma, so magnetic reversals on the Earth take around 700,000 years, while magnetic reversals on the Sun take about 11 years.

Modeling turbulent airflow around an aeroplane is now quite practical on a supercomputer. But throw in a conductive fluid, electric currents and magnetic fields, and it becomes extremely difficult, even on the largest supercomputers.
- There have been some laboratory experiments successfully simulating this effect with spinning vats of molten sodium (sodium is not a magnetic metal).

See: https://en.wikipedia.org/wiki/Dynamo_theory

[Mckenzie]

Since Earth core is liquid and is everything is in continuous movement which stabilize and create natural equilibrium. The huge glacial mountains must have role in it.
Also what evan_au said.