Gents:
I was there when 'weewads' came up with the question, and all of us in the room could not come up with a valid answer. Yes, the a/c IR's do get their sensing from earth's movement, as I do not know of any flux valves on the plane or connections to any other magnetic source, but the meaning of the actual question was from a cold start while the IR's are aligning, how does the systems know the magnetic heading while in align. The heading will show up in "ARC" and the bottom of the PFD even while the align is completing it's cycle.
Cheers all...