# Why is "at least a small tilt" between rotation and magnetic axis required by models of magnetic field formation?

I saw something I don't understand in the Gizmodo article Cassini's First Grand Finale Images Are Stunning—But What Are We Really Looking At?. In a quote of Caltech Professor of Planetary Science Andrew P. Ingersoll, it is suggested that a it necessary for there to be at least a small tilt between a planet's rotation axis and magnetic axis.

Ingersoll mentioned the final Grand Finale observation he was looking forward to most at the very end of our conversation, which could solve an ongoing mystery of the planet’s formation. Saturn’s magnetic field has a very unusual property in that it’s “axisymmetric”—it has no tilt. That’s weird, because the math behind planetary magnetic fields requires a bit of tilt.

“How does a planet generate a magnetic field? The off-axis tilt is sort of part of that whole theory and if there’s no tilt at all, well, it will be interesting,” he said with a chuckle.

Is there a way to understand why a tilt is required in current theories of planetary magnetic fields?

It sounds like Ingersoll was referring to Cowling's Theorem -- that flow in a planet's core that is purely symmetric around the planet's rotation axis (that would give a magnetic dipole field with no tilt) cannot produce a self-sustaining dynamo.

So a magnetic field that is purely symmetric around the rotation axis, as looks to be the case for Saturn, should not be possible by known dynamo theory. As best we know, all other self-sustaining planetary dynamos (and those of stars) have some degree of asymmetry that leads to a self-sustaining dynamo.

• Indeed it does sound exactly like that, thank you!
– uhoh
Feb 11, 2019 at 9:53

"Tilt" is a consequence of a convecting metal inner core....not a "necessity". In the earth's case, that tilt has, over time, wandered from 0 to 180 degrees, sometimes with some temporal regularity, other times completely randomly. The science writer likely poorly paraphrased Professor Ingersoll's curiosity regarding the existence of a molten metal core deep within Saturn.

• By "...wandered from 0 to 180 degrees, sometimes with some temporal regularity... " do you mean reversal events, or do you really mean a tilt that wanders? I don't really know much about either, but I thought reversals were more like a complete loss of the dipole shape, then appearance again, but with opposite sign, rather than a wandering of orientation. If you can add a link or two to highlight the concepts, that would be great!
– uhoh
Apr 29, 2017 at 16:51