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    Let's Clear Up A Few Things About Geomagnetism
    By Oliver Knevitt | June 20th 2011 02:00 AM | 29 comments | Print | E-mail | Track Comments
    About Oliver

    In a nutshell: I like fossils. But even more than than that, I like arguments about fossils. Which is why my current occupation as a PhD researcher...

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    If you're anything more than an infrequent stumbler to Science 2.0, then you will probably have noticed - if not, read - Helen's article on geomagnetic polarity reversals, which until recently has been floating in the top articles list.

    Whilst it's a gallant attempt to understand the ins and outs of an incredibly complex and poorly understood process, there are a number of misconceptions that I thought would be a good idea to clear up. It's clearly a topic of great interest, because there are over 400 comments on the article.

    Before going into these misconceptions, though, let's start with...

    A crash course in geomagnetism


    Put simply, geomagnetism is a study of the Earth's magnetic field. I guess the guy who kicked this all off was William Gilbert, who, in 1600, made the connection that compasses line up with the Earth as if the Earth was a giant magnet in itself. Previous to this, the concensus was that it was because there was a gigantic magnetic island at the north pole.



    To all intents and purposes, the Earth imagined as a giant magnet is a pretty accurate representation - it really is about 90% dipolar at the surface. And that sort of fitted, because we know that rocks can be magnetic. During the formation of both igneous rocks (those formed from molten rocks) and sedimentary rocks, magnetic minerals can line up in a magnetic field, and the sum total of all these magnetic minerals lining up turns the fluid rock unit into a gigantic magnet. When the temperature of the molten rock cools past a certain temperature (the Curie point) or when the sediments are compacted enough, then these magnetic minerals get frozen in place, and this magnetic field stays in one direction no matter whether the rock is moved into a different field.

    So, the rocks of the Earth could form one gigantic magnet. Gilbert demonstrated this using a big spherical magnet, which he called a terella.



    But there was a problem. As soon as 1635, Henry Gellibrand noted evidence that the declination, the difference between true North, as indicated by the pole star, and magnetic North, slowly changed with time. This doesn't work at all with the Earth being a solid magnet. Edmund Halley, in 1692, came up with the radical suggestion that the Earth could be composed of spheres within spheres, each of which had its own magnetic field and moved relative to the other, but ultimately, this isn't physically feasible due to gravity, which would crush such a set-up. Until relatively recently (1950s), one of the leading theories was that magnetism could be an intrinsic property of a rotating object - after all, if spin and magnetic moment were related in electrons and protons, the same could be true for matter at large. But, unfortunately, field strength doesn't seem to weaken with depth in mines, and spinning gold spheres have been shown to not produce any magnetism.



    The most accepted theories around today are based on fluid dynamos. We've known for over 100 years from global seismic studies that there is a layer in the Earth inpenetrable to S-waves and so must be liquid. Could this be the region responsible for generating the geomagnetic field?

    Building on the observations of inductance by Oersted and Ampere, in the 19th century, Michael Faraday developed the dynamo: a means of generating current by moving a magnet next to a closed circuit. But, he realised, there is another way of inducing a current, and this is by moving the conducting wire relative to the magnet, instead of the magnet relative to the wire. This led to what conventially called a disk dynamo; an admittedly impractical way of making current by spinning a disk of conducting metal next to a magnet - impractical because of the enormous amount of current produced pushed by a low voltage.



    In addition to this, though, he had the insight that the conducting disk needn't be solid, and could instead be a conducting liquid. Faraday actually attempted to measure the induced current in the river Thames by laying a wire across Waterloo bridge*. Its difficult to detect the flow because it is more sparse and less ordered; in normal dynamos we channelize the fields by getting them to flow through wires wrapped in insulators, making them much easier to detect. But the reasoning was sound.



    So, in this example, the wire here needs a preexisting magnetic field in order for current to be induced in it, and this induced current in turn will generate its own magnetic field. But could this new magnetic field, in turn, provide the magnetic field for another wire? In other words, could you link two disk dynamos so that each one generates the other's magnetic field?

    This is known as a self-exciting dynamo, and whilst in principle it entirely permissible, in practise it is hard to produce, although it may be a matter of scale. On top of that, there is a fundamental difficulty in modeling the Earth's dynamo, because of Thomas Cowling's antidynamo theory, namely that any self-sustaining dynamo in the Earth's core cannot have an axis of symmetry, meaning that the maths is pretty bloody tricky. But any, that's the most accepted theory for how the Earth's geomagnetic field is generated.

    Polarity reversals


    Polarity reversals are a very important research area for geophysicists, because its essential for understanding how the Earth's geodynamo works, and, of course, to understand the impact on the Earth system in the event of a reversal.

    We've tackled this problem from two ends. Firstly, from the paleomag perspective. What can we tell about reversals from the trace that they have left in the sedimentary record? Secondly, we analyse it from numerical models and lab fluid dynamos.

    Polarity reversals seem to be an intrinsic feature of these models. Seemingly, in spite of wildly changing any of the parameters by a drastic amount - be it the viscosity, speed of rotation, composition or heat - polarity reversals still occur in the same, spontaneous way. I'll describe our most up to date model of how reversals can occur - remember that these are models and may not necessarily describe precisely what happens in the Earth.

    What seems to happen in these models is that in the long term, the geodynamo is pretty happy in its stable dipolar state. That's until, however, during its churning, it stumbles into a less stable multipolar regime. In this new unsteady regime, it is strongly affected by buoyancy forces, unlike during the more rotationally-influenced dipolar regime.

    The first stage in the reversal process is a dipole strength reduction, leading to a multi-polar field structure on the core-mantle boundary, which is sometimes quite slow. Next is the multi-polar stage, during which the dipole field sometimes undergoes a precursory reversal and then recovers intensity in its original polarity.

    The last stage is the final directional change and intensity recovery in the new polarity. The reversal mechanism initiates with the production and strengthening of reverse magnetic flux in the southern hemisphere of the core. As spots of the reverse flux strengthen and are transported beneath the coreÐmantle boundary, the dipole moment weakens in advance of the actual polarity change, marking the transition from stable dipolar to highly variable multi-polar states. The directional transition is assisted by cross-equatorial meridional circulation.

    That's the most recent model, and as you can see, it's a very complex process and is a very active area of research; some of this could very well change within the next few years. The interesting thing is that some of these events seem to be verified by looking at the record that the previous flip left in the geological record.

    But there still some misconceptions about the magnetic field that need clearing up.

    1. The Strength of the magnetic field is weakening, so we're heading towards a flip



    Since the mid 1850s, we have been monitoring the global magnetic field using spherical harmonic analysis, which gives a quantitative description of the Earth's magnetic field in terms of both its direction and strength. It is certainly true that since this time, the strength of the dominant dipole component of the magnetic field has steadily declined by about 5% per century, the decline accelerating somewhat after 1970.

    However, we are forgetting that the dipole component is just one component of the geomagnetic field; there are many, more complex, smaller spatial scale components, e.g. the quadrupole and octopole components. Even though measurements of the strength of the magnetic field are decreasing, the geomagnetic field is not getting weaker, only more complex. In fact, these more complex parts are gaining in strength.

    It is true that an increase in these complex components is known to be an aspect of reversals. However, the strength of the dipole varies on all sorts of timescales; from centenial to millennial1. The currently observed rate of dipole decay is admittedly quite high, but is still not extraordinarily fast in comparison to the rates of change over the last 7 kyr1. Besides, the present field strength is nearly twice the long-term average field strength2. Indeed, the present field is approximately equal to that during the mid-Cretaceous, when it did not reverse for about 40 million years.

    2. We're well overdue a flip



    I don't know where this concept has come from, because it is not true in the slightest. It may well have been slightly longer than the average for the past geological era, but that's as much as we can say.

    For a start, we have had periods known as superchrons, where the magnetic field didn't flip for over 50 million years. It's only been 780,000 years since the last one. You could argue that we are currently in a "period of increased rate of reversals", but even then, the current chron is barely above average for the Late Cenozoic. The lengths may conform to a Levy distribution, but this doesn't mean that they're not random.

    It could be currently flipping. Or it could be on an excursion and not flip for millions of years. Basically, we just can't tell whether we're going to flip or not, and there's no requirement that we do. So, really, it's pointless worrying about it.

    3. Because the pole is wandering, we might flip soon


    The pole wanders because that's what it does. It drifts westwards, it wanders, and it jerks from time to time. In fact, there are many aborted reversals, when the magnetic poles are observed to move toward the equator for a while, but then move back and align closely with Earth's spin axis.

    4. Reversals are linked to extinctions and to large scale biospheric effects



    The Earth indeed experiences periods when the surface field strength is puny compared to today, especially during the 5000-10000 year or so transition, and during these periods the Earth's atmosphere (note: not its surface!) is presumably more vulnerable to energetic cosmic particles. The effect of this would most likely result in increased UV-B stress, and increased cloud cover, which would have some effect on the biosphere. And so this is a fair enough question to ask.

    The problem is, despite looking closely at very detailed microfossil records, no conclusive link has ever been found3**. Whilst we can't rule out that the biosphere and atmosphere are adversely affected during a reversal, historically, life seems to have got through them OK.

    A couple of things, though. As for mammoths, what reversal? There was no reversal 11 Ka when the mammoths died out, and so they were not killed by a geomagnetic reversal. More likely is climate change and man killing them en masse.

    I also hope it's clear that flips will not affect plate tectonics or anything on that scale. Why would it? The magnetic field is only strong enough to rotate small magnetic minerals suspended in a liquid, and that is the extent of its strength. On the flip side, it's certainly possible that sinking tectonic plate slabs could sink to the core and affect the circation, but I've never seen any convincing seismic tomography scans that clearly show that slabs can reach that far without being recycled.

    Finally, it is certainly possible that large scale astronomical events could cause a reversal - even though, historically, they haven't. But gigantic ion colliders, and other man made activities? If you remember what the magnetic field is being bombarded continually in terms of solar wind, you will realise that man's influence really is puny, particularly on a large scale like this.



    5. A self sustaining fluid dynamo is the only way to create a magnetic field


    The geodynamo model is our theory for the Earth's geomagnetism, but it needn't be the only way.  The fields of Europa and Callisto are produced by induction within Jupiter's magnetic field. And its certainly possible that magnetism can be remnant as well.

    In the Sun, the story is a bit simpler, because it is essentially fluid as a whole. I'll describe this in a bit more detail, because its very interesting and demonstrates an important bit of physics. The equatorial region of the Sun rotates more quickly than the poles, which is what drives the dynamo that produces sun spots. Joseph Larmor was the first to apply the idea of a dynamo to the sun's magnetic field, and tentatively suggested that the moving aspect of the dynamo was the convecting plasma. The most major breakthrough after that came with the realisation that, wierdly, magnetic field lines get literally frozen into superconducting plasma particles, and remain aligned even after flow has separated them - or even after they have been catapaulted away from the sun in a flare***. In this way, magnetic field lines are less of a visualization aid and an actual physical construct. They are the direction in a field in which ions and electrons (and their accompanying electric fields and heat) travel most easily; a bit like the grain in a piece of wood that splits easily, and of course can be moved.

    Ganymede is an oddity, because has a strong geomagnetic field, and its high degree of central condensation seems to suggest that dynamo action within a metallic core generates the field. But unfortunately, tidal heating is not pervasive enough to melt the core, so our best hypothesis is that it hasn't cooled yet. Or that we don't know enough about generating geomagnetic fields - which is certainly possible.

    So, I hope that's cleared up a few things about geomagnetism. As you can see, there's nothing simple about it. However, it's a very active research area, and so, who knows where we will stand in 10 years or so.

    ----

    *He was thwarted from measuring this by small voltages due to chemical processes. Faraday actually speculated that the Gulf Stream could generate a voltage, and if its circuit were closed through the high atmosphere, it could generate there the light of the aurora. Unfortunately, the atmosphere is too poor a conductor of electricity to complete the circuit.

    **It has been suggested that there may be a link to the Earth's obliquity, (I can't find a citation) but it's tenuous.

    ***As the sun rotates, this causes the field lines to become curved round. Solar wind particles on the same field line will therefore be on different meridians than the "roots" of that line on the Sun. The longer these particles have traveled through space, the more have their "roots" rotated and therefore the greater the difference in solar longitude from those roots, suggesting that the magnetic field lines curve to form a spiral, which ultimately become wrapped tighter and tighter.

    ----

    Citations

    1. http://onlinelibrary.wiley.com/doi/10.1111/j.1365-246X.2006.03088.x/full
    2. http://geology.geoscienceworld.org/cgi/content/abstract/8/12/578
    3. http://www.sciencedirect.com/science/article/pii/S0019103508002807

    Handy links

    http://www.igpp.ucla.edu/people/mkivelson/Publications/ICRUS1572507.pdf
    http://www.eolss.net/ebooks/Sample%20Chapters/C01/E6-16-04-01.pdf
    http://www.iki.rssi.ru/mirrors/stern/earthmag/mill_5.htm
    http://measure.igpp.ucla.edu/solar-terrestrial-luminaries/2000RG000097.pdf
    http://www.psc.edu/science/glatzmaier.html

    Comments

    rholley
    I like this one.  Electromagnetism is a big field!
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Oliver Knevitt
    Hah - I'm really funny and I didn't even know it! Reminds me of my math's teacher's mantra: "Every couple has its moment in a field"
    Bonny Bonobo alias Brat
    Great article Oliver, however I still managed to get a bit lost with this statement :-
    On top of that, there is a fundamental difficulty in modeling the Earth's dynamo, because of Thomas Cowling's antidynamo theory, namely that any self-sustaining dynamo in the Earth's core cannot have an axis of symmetry, meaning that the maths is pretty bloody tricky.
    Any chance of elaborating a bit more as to why it can't have an axis of symmetry and what that means? Also you said :-
    It's only been 780,000 years since the last one. You could argue that we are currently in a "period of increased rate of reversals", but even then, the current chron is barely above average for the Late Cenozoic. The lengths may conform to a Levy distribution, but this doesn't mean that they're not random.
    But wasn't it you that pointed out that some 30,000 year old clay baked aboriginal fireplaces showed a polar reversal to what we have at present? Doesn't polar reversal seem to occur pretty randomly and therefore the averages are irrelevant, isn't there also a 50/50 chance as to whether the poles even reverse when they become unstable, a bit like flipping a coin?

    Anyway Oliver, thanks for clearing up what is considered known and not known about a lot of these areas. Also, I'm interested to know what you think about Dr Herndon's hypothesis of the core of the Earth possibly being a nuclear reactor which was mentioned on my blog? The article is at DISCOVER at http://discovermagazine.com/2002/aug/cover and Dr Herndon's site is at http://www.nuclearplanet.com/ Is this at all possible?
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    Oliver Knevitt
    OK, well the antidynamo is a mathematical disproof really, but I'll try and put it bluntly. Basically, a simple, axisymmetric dynamo model would be a cylindrical tube of magnetic flux, which produces an electromotive force driving a current in the azimuth direction, which in turn enhances the magnetic flux in the cylinder. Cowling proved that such a set up breaks down rapidly at the ring of neutral points. If you imagine the Ohmic decay of the axisymmetric field as the steady diffusion of closed field lines into the neutral point, the moving liquid drags field lines around meridian planes (remember what I was saying about freezing field lines into plasma), but crucially, does not create new ones to replace the loops that disappear. It would need liquid to emerge from the neutral point with infinite speed, which of course is impossible. This means that topological solutions are not simple, although it doesn't say that they don't exist. I'll got to disappear somewhere, so I'll reply to the rest in a bit....
    Bonny Bonobo alias Brat
    No need to hurry Oliver, it could take me quite a while to digest this information. Thank god for wikipedia and also for English men, who I miss quite a lot out here in Australia (well actually Bali at present). I'm assuming you are English? Hopefully I will still be alive when you post the next part, as I am currently in the company of a 12 year old pyromaniac and tonight on the beach he and I are letting off the biggest fireworks I have ever set eyes upon! Hopefully they won't adversely affect the geomagnetic force :)
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    Oliver Knevitt
    ...back again!
    Hopefully they won't adversely affect the geomagnetic force :)
    Lets hope not! ;)
    But wasn't it you that pointed out that some 30,000 year old clay baked aboriginal fireplaces showed a polar reversal to what we have at present?
    No I didn't - I was showing how we can measure the intensity fluctuations using the fireplaces - and the intensity has indeed fluctuated, but not reversed.
    Doesn't polar reversal seem to occur pretty randomly and therefore the averages are irrelevant,
    They're pretty random, but that doesn't mean that the averages are not irrelevant at all. The fact that the chrons are on average about 250,000 years long rather than say, 25,000 years long, says something fundamental about the nature of the dynamo.
    isn't there also a 50/50 chance as to whether the poles even reverse when they become unstable, a bit like flipping a coin?
    No way! The geomagnetic field regularly destabilises before stabilising itself again; only in extraordinary circumstances does it actually go all the way and flip.



    As for Hearndon's theory, it seems quite plausible, but there's a fundamental flaw, and that's that Uranium and Thorium simply don't like it in the core at all; they are highly concentrated in the crust, less so in the mantle, and are thought to be trace, at best, in the core. This is because fundamentally, U and Th atoms and ions don't fit very well in the dense iron crystal lattices that form at pressure in the Earth - they much prefer the more open lattices of silicates and oxides that you find in the crust and mantle. The theory also relies on the Earth having been formed from an extraordinarily rare type of meteorite, which seems a bit unparsimonious to me.
    Bonny Bonobo alias Brat
    ...there's a fundamental flaw, and that's that Uranium and Thorium simply don't like it in the core at all; they are highly concentrated in the crust, less so in the mantle, and are thought to be trace, at best, in the core. This is because fundamentally, U and Th atoms and ions don't fit very well in the dense iron crystal lattices that form at pressure in the Earth.
    Oliver, how do we know that there are dense iron crystal lattices at the earth's centre?

    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    Oliver Knevitt
    That's what iron does under pressure, and we know its iron because of the way that phases separate under equilibrium in a pressure gradient - not to mention it fits perfectly with our density calculations and our knowledge of the precursors to earth as chondritic meteorites.
    vongehr
    It's clearly a topic of great interest, because there are over 400 comments on the article.
    And almost 35 thousand reads. Lets see whether a very reasonable article without claiming the LHC kills us all can compete on the 'internets'.
    Hank
    Deciphering the Internet is impossible.   Among our top 5 articles of all time, one is because of the picture, one is on blue-eyed people and one is on the Beatles.   That 'LHC will ruin the planet' article may be in the top 100, I didn't look back that far.    Success comes down to either luck or aggressive use of social media and quality rarely matters or Scientific American wouldn't be the most respected name in American consumer science.
    Bonny Bonobo alias Brat
    Well one thing is for sure, you are an expert about all of this unlike me, but I do intend to do better and be more scientific and less speculative in my approach to future blogs.
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    Hank
    You can write anything and include this picture and it will be popular, it seems:



    Seriously, 400K people just because of that picture.
    vongehr
    Now you got me curious. How you know it is just because of the pic? @Helen: No.
    Hank
    You can see a larger snapshot of your traffic trends in your dashboard and then specifics, like IP address for each read, by clicking Track on your article.   After that piece spent a week at the top of the leaderboard I finally clicked on it and saw all of the people were finding the pic in Google and clicking on it.  I doubt they even read the article.
    rholley
    Helen,

    Dinna fesh yesel’!  Sascha’s just being Sascha.

    Anyway, let’s all enjoy a wee bit of German humour, the March of the Fleas by Heinz Erhardt, Germany’s No.1 comedian in the years following WW2.  According to his biography, he served as a pianist in the Marine orchestra during the War.  I wonder, could that be worked into an episode of Dad’s Army?

    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Bonny Bonobo alias Brat
    Helen, Dinna fesh yesel’!  Sascha’s just being Sascha.
    Yes Robert, Sascha is just being Sascha and you are just being Robert. Both of you are moderators and if either of you says '@Helen: No' it makes me nervous as you both have the power to delete me and/or my blogs, just like Bryan Sanctuary was recently deleted. Sascha already has it in for me so I'm walking a fine line and I am trying not to upset him further, but even just explaining this to you publicly might be enough to anger him. Mere mortals are very insignificant to the gods in Science20 Valhalla.







    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    Hank
    Prof. Sanctuary had his article unpublished and his posting access disabled after writing three pieces in the same theme; he only signed up to launch attacks at a contributor in a field he knows nothing about - he is a chemist pontificating on quantum mechanics and defending a guy even the Perimeter Institute thinks is too kooky for them.  

    Alleging that there is some arbitrary elimination if anyone simply disagrees when there have been maybe 5 removals in 5 years, out of 8,000 people signing up, is unfair to the moderators.    

    Your geomagnetic piece not being pulled should be an affirmation that the moderators don't unpublish things very often - instead you seek to once again exaggerate for effect.
    vongehr
    a guy even the Perimeter Institute thinks is too kooky for them
    What do you mean? I have seen no distancing from PI and his garbage stays on their webpage as if it is one of their best guys.
    @ Helen: Off your meds again while in Bali? You can import a weeks personal supply with your doc's prescription.
    Bonny Bonobo alias Brat
    I don't need medication Sascha, unlike you I am naturally happy and focused :)
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    rholley
    Helen,

    I have hardly ever exercised my moderatorship, only once or twice to delete some advertising replies which have nothing to do with the topic.

    You have never put anything on this site which even made me think of moderation (though talking of moderation, that makes me think of whisky!)

    As for gods and Valhalla, do you have something like this in mind?  (from Earthworm Jim – Assault and Battery)


    (centre: Japeus, god of Puns; left: Phlebephus, god of Nasal Discharge; right: Terpsichore, goddess of Disco.  They took the battery from her Walkgod to bring power back to Earthworm Jim’s suit.)

    Neither was my posting of Heinz Erhardt anything like an Anglo-German war front, for two reasons:

    (1) I’m not exactly Anglo;

    (2) I post Heinz Erhardt because he’s fun.  Sometimes when I’m stressed I relieve the tension by reciting to myself one or two of his poems.

    However, to be fair to Scotland, here’s a YouTube to balance things out.  The first two minutes are Will Fyffe making up; you can pull the bar forwards to get to the stage act.

    Robert H. Olley / Quondam Physics Department / University of Reading / England
    calm mongering and understandable explaination of complex ideas at it's finest.

    thank you.

    "..so our best hypothesis is that it hasn't cooled yet. (hardly) Or that we don't know enough about the generating geomagnetic fields - which is certainly possible. (indeed)
    So, I hope that's cleared up a few things about geomagnetism."

    Err! You have got to love scientists.

    Bonny Bonobo alias Brat
    Ha ha, very funny Mickxx!
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    Oliver Knevitt
    All I can say is, we're working on it. This is how science works; we make hypotheses and test them with experiments. It just so happens that we don't have enough data to constrain this one, so the best we have are conjectures; hopefully with more research we'll know. Maybe you'd rather I made up something and said that that was how it worked, end of?

    And what exactly are you trying to imply by "hardly"? If you have a better hypothesis, then go ahead; write a paper.

    (Thanks for spotting that phantom "the" btw; I've now removed it)
    By 'hardly' I mean it has had millions and millions of years to cool down and probably has (like Mars)

    A new hypothesis? OK How about induced currents. Currents induced in Ganymede by being in Jupiters mag field and Earth having currents induced by the Sun's mag field.

    Oliver Knevitt
    It's way too strong to be produced just by induction. That's the whole interesting thing; Europa and Callisto have fields that can be accounted for by induction, but Ganymede has an unusually strong magnetic field.
    Fair enough. Higher current induction possibly determined by higher metallic content?
    But the Sun's mag field therefore effects the Earth's mag field which is at least 'interesting' if that's the right word.
    http://en.wikipedia.org/wiki/Telluric_current

    Oliver Knevitt
    Agreed; it's a very interesting effect, and I believe this effect has been detected on some satelites too.

    BTW, in case you were interested, there's a great recent review of the generation of geomagnetic fields here and here, and there's papers focusing on just ganymede here and here.
    If you have any problems with paywalls, I'm very happy to email you pdfs of them.
    Thanks for refs I will read them. You will probably stop replying after this next comment but TRY IT OUT never the less (it's a bit outside the box). With ref to telluric currents, get a globe (please tell me you have one) and join Gizza to Los Angels via the artic circle and see where the extended great circle crosses the equator. Then try joining Seattle, Mexico city and Santiago (South america) and see where that great circle crosses the equator. Someone is messing around with telluric currents. Told you you wouldn'y rely (wink) Don't think outsdie the box...just ignore it completely.