Having glanced through the FE Wiki on Form and Magnitude I can't find any mention let alone explanation for how the Earths molten core was formed. This was discovered back in 1936 of course following a study of Earthquakes in New Zealand. The molten core is what produces the Earths magnetic field which in turn produces the aurora displays that we all know and love.
Surely if the Earth was flat there couldn't be 'core' as such?
Actually recent experiments have shown the inner core is not actually molten, but solid. The outer core however is molten apparently!
It’s not the core that produces the Auroras, rather its the interaction of charged particles from the sun with the earth’s magnetic field, which as you say is produced as a result of the earth’s metallic core.
But you are correct it’s just one of the many natural phenomena that go to prove the spherical nature of the earth.
Though while we have all these facts the FE movement do have a book from 1922, so let’s not forget that!