>Magnetism or ultra low freqs
Both. Magma is radioactive and has magnetic properties.
>It remains uncertain if silicate liquids at the extreme pressures and temperatures found in this magma ocean would have been electrically conductive enough to drive a dynamo. The researchers plan to test their idea with a more sophisticated model of magnetic field generation.
https://www.livescience.com/41758-earth-magnetic-field-magma-ocean.html