can anyone explain how the earths mag field creates and sustains its strength?

Answer 1

The earths core is made of hot liquid metals primarily iron that are spinning. This electrically charged core produces magnetic field. It sustains its strength because of the size and spinning motion.
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Answer 2

Lots of iron in the Earth inner core, which is hot but not molten because of pressure.
The upper layers slide on top the inner core and that creates and maintains the magnetic field, which can change and in fact has changed several times during the Earth's history.
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Answer 3

Circulation of charges in the core create electrical current loops

Answer 4

A large amount is from the earth core.
A small amount is from the currents in the ozone layer.