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This answer to Why is the magnetic axis of Uranus and Neptune off center? in Astronomy SE perplexes me, partly because I can't understand the block quotes and partly because they constrast those planets with Earth and Earth's dipole is off center as well and it's field also flips back and forth.

Question: Is it possible to write a simplified, plain-language summary of what this explanation for why Uranus and Neptune are different than Earth in having large offsets of their dipole field?


From the linked answer, which is sourced from Nellis 2017 Magnetic fields of Uranus and Neptune: Metallic fluid hydrogen:

Earth's axisymmetric magnetic field

The most researched magnetic field in our Solar System is, of course, Earth. Nellis 2017 explains how coupling between the rotational motion and convective dynamo motions in the Earth's outer core cause the magnetic axis to drive towards alignment to the spin axis:

Because rotational motion (RM) of Earth is strongly coupled into convective dynamo (CD) motions of its fluid-Fe outer core, planetary RM stabilizes convective motions that generate a dipolar magnetic field. If a convective fluctuation occurs which tends to destabilize a given dipolar axis, then strong RM-CD coupling either drives convective motions that essentially restore the initial orientation or CD fluctuations that drive the initial magnetic axes out of orientational equilibrium are so strong that RM-CD coupling eventually drives the dipolar axis into an alignment anti-parallel to its initial one.

and

Uranus/Neptune non-axisymmetric magnetic fields

The magnetic generation near the surface, as well as the H-HE envelope (again according to Nellis) means that:

local convective dynamo motions of fluids that produce the magnetic fields are essentially decoupled from global rotational motions [...]. The dynamos of U/N would then be relatively free to wander as local convective fluctuations dictate. Thus, tilt angles and center-offsets of their fields would vary slowly over the age of the Solar System.

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Earth's rotation is an important factor in contributing to the generation of the field, and helping to stabilise the form of it. The rotation influences the flow of compositionally convecting material deep within the outer core, forming cylindrical rolls that align along the rotation axis (a Coriolis effect).

While Earth's field polarity may reverse occasionally, it is to a good approximation (and we believe predominantly has always been) a roughly centred dipole, aligned roughly with the rotation axis of the Earth because of the influence of Earth's rotation.

For Uranus and Neptune, this is not the case. The centre of their dipoles are significantly offset from their centres of mass, the axes of the dipoles are significantly offset from the rotation axes.

For both Uranus and Neptune, magnetic field generation is believed to occur in flowing material much closer to the planets' surfaces (due to the very different composition of ice giants versus the terrestrial Earth). With flow constrained in a relatively thin layer near the surface, planet rotation does not have a strong influence on the form of the convective motion, so the field that results is not particularly tied to how the planet rotates.

This article gives a good idea of how the geometry of the convecting flow region can lead to Earth-like or Neptune/Uranus-like magnetic fields.

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  • $\begingroup$ Okay this is helpful! I'll give it another read now, and I'll see if I can figure out why "Near the surface, planet rotation does not have a strong influence on the form of the flow..." should be true. Thanks! $\endgroup$ – uhoh May 26 at 20:49
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    $\begingroup$ I've edited to clear up my own misleading line that you noted there, sorry! It's the flow being in a thin shell that really constrains the form of the flow, not that it is near the surface per se. $\endgroup$ – WJB May 26 at 21:03
  • $\begingroup$ Oh, yes that helps a lot, thanks! $\endgroup$ – uhoh May 26 at 21:07

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