Planetary Magnetic Fields: Observational Insights and Challenges (Keynote speaker)

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• Jeremy Bloxham (Harvard University)
• Tuesday 13 September 2022, 09:00-10:00
• Seminar Room 1, Newton Institute.

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DY2W01 - Dynamos in planets and stars - similarities and differences

More than a half century of spacecraft observations of planetary magnetic fields have revealed that no two planets in our Solar System are truly alike magnetically. Mercury has a weak, relatively axisymmetric field that is displaced along the rotation axis; Mars had a dynamo that is now extinct; Venus has no field; Jupiter’s field is different in the two hemispheres, with flux concentrated into a few well-defined regions; Saturn’s field is highly axisymmetric; and Uranus and Neptune have fields that are predominantly non-dipolar, but different. Earth’s field is distinctly different from any of these planets. Is this diversity of magnetic field morphology the result of gross interior differences between the planets or is it simply a result of the sensitivity of dynamos to the relevant governing parameters? In the second part of this talk, we compare analyzing the Earth’s field with those of the gas giants. For Earth, it is a reasonable approximation to consider the mantle to be electrically insulating so the field may be downwardly continued to the core-mantle boundary, where the radial component of the field is continuous across the thin boundary layer at the top of the core. Hence, surface observations can be used to infer the field at the top of the flow field in the core. For the giant planets, the electrical conductivity is a continuous function of depth and so downward continuation is on a less firm footing. Furthermore, even though magnetic flux is frozen in the deep interiors of both the Earth and the gas giants, the Roberts & Scott frozen-flux theory of the secular variation, so useful for Earth, is not so straightforwardly applied to the case of the case giants. For example, the gas giants lack a rigid radial boundary at the top of the convective region so the radial component of the flow does not necessarily vanish. We investigate these complications using a numerical dynamo model with radially-varying electrical conductivity and a stably stratified region. With Hao Cao, Laura Kulowski, Rakesh Yadav and the Juno Science Team

This talk is part of the Isaac Newton Institute Seminar Series series.

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