Magnetic Personalities – Saturn’s Magentic Field

Magnetic-Field-on-SaturnThe latest in our paper discussions took us away from our usual  territory- and out to Saturn. Cao et al’s paper Saturn’s high degree magnetic moments: Evidence for a unique planetary dynamo looks at modelling the magnetic field of Saturn, which is very different to the field of the Earth. Parts of the magnetic field have been measured by Pioneer 11, and both Voyager missions, and more recently, Saturn has been visited by the Cassini mission, which also recorded magnetic data, and essentially confirmed what was already known. Analysis of the data from Pioneer and Voyager(s) and modelling of low spherical harmonic degrees (g01, g02 and g03) show Saturn’s field to be highly axisymmetric (i.e. symmetrical around the spin axis, with little difference as you move along the equator but poles of opposite sign) and also much weaker than the field of the Earth, being around 30,000nT at the surface, compared to 50,000nT here. The field axis is the same as the rotational axis, to within 0.06o. There has also been very little change in the field measured between the Voyager missions and Cassini’s arrival. The magnetic moments of the lower spherical harmonic degrees having being well modelled with the available data, in this paper the authors try to model higher degrees, g04 and g05 with the new data from Cassini.

Cassini probe approaching Saturn

Cassini probe approaching Saturn

The results they find agree with what was previously found – that Saturn’s field is highly axisymmetric, with most of the magnetic flux confined to polar regions, and higher degrees of the field are relatively small contributions to the overall measured field – hence not being seen on some more distant orbits of the satellite. They also suggest a similarity to certain types of dynamo models – such as spherical Couette dynamos, where the dynamo is driven by differential rotation between outer and inner layers of a shell. This is a very different type of dynamo to the Earth, with its convection driven dynamo – and indeed may not even be physically possible for Saturn! In this case, the different rotations could be due to a transition in the phase of helium (such as the gaseous atmosphere ‘raining’ liquid helium at pressure), a metallic layer, or the solid core of the planet. This was certainly the part of the paper that provoked most discussion amongst us – and several sci-fi type visualisations of what helium rain might look like!

Cao, H, Russell, C, Wicht, J, Christensen, U, and Dougherty, M.  Icarus 221, 2012, 388–394: Saturn’s high degree magnetic moments: Evidence for a unique planetary dynamo.

kellyToday’s post was guest written by Kelly Barker. Kelly is a PhD student in the geomagnetism research group at the Department of Earth, Ocean and Ecological Sciences at the Univeristy of Liverpool. Her research project is entitled:Improved application of remote referencing data in aeromagnetic processing: insights and applications from global geomagnetic modelling.

About LauraR

I am a Final Year Phd student at the geomagentism laboratory at Liverpool University. My research project: A Palaeomagnetic Study of 3.2-3.5 Billion Year Old Rocks from South Africa has the overall aim to shed light on early Earth processes, particularly those relating to the early Earth’s magnetic field. My work is both field and laboratory based and concentrates on samples collected by myself and others, from the Barberton Greenstone Belt, in South Africa. So far, I have collected Archaean aged, palaeomagnetic samples in the field from which I intend to produce reliable measurements of palaeomagnetic directions and (if possible) intensities, supplemented by detailed magnetic and microscopic analyses. My field area (The Barberton Greenstone Belts) is located in Northeastern South Africa. The Greenstone Belt is unique because Archaean supercrustal rocks are well preserved here, have suffered low grade metamorphism and relatively small amounts of deformation (see De Wit et al. 2011); which is very encouraging for palaeomagnetic studies, given the age of the rocks. Recent work by Biggin et al. (2011) and Usui et al. (2009) indicate that the rocks from the Barberton Greenstone Belt record a magnetic field dating back to at least 3.5Ga and Biggin et al (2011) further argue that the rocks may also record a polarity reversal. These results are encouraging, but more data is required to further support these claims. I aim to add to the existing data sets and hope to support the claims of the early research. Prior to starting my Phd (in September 2011) I had no experience in palaeomagnetism. I graduate from Liverpool University with a MESci in Geology in 2007. I undertook a 4th year masters research project in near vent processes related to phreatomagmatic volcanic eruptions. After my degree I spent 3 years working for an environmental consultancy as an environmental consultant, working on contaminated land site investigations.
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One Response to Magnetic Personalities – Saturn’s Magentic Field

  1. says:

    One reason may be that the saturnian perfectly axially symmetric magnetic field fails to impose a strict corotation on the magnetospheric plasma making it slip relative to the planet.

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