Ocean Currents Proposed As Cause of Magnetic Field 333
pjt33 notes a recently published paper proposing that ocean currents could account for Earth's magnetic field. The wrteup appears on the Institute of Physics site; the IOP is co-owner, with the German Physical Society, of the open-access journal in which the paper appears. This reader adds, "The currently predominant theory is that the cause of Earth's magnetic field is molten iron flowing in the outer core. There is at present no direct evidence for either theory." "Professor Gregory Ryskin from the School of Engineering and Applied Science at Northwestern University in Illinois, US, has defied the long-standing convention by applying equations from magnetohydrodynamics to our oceans' salt water (which conducts electricity) and found that the long-term changes (the secular variation) in the Earth's main magnetic field are possibly induced by our oceans' circulation."
Summary wrong: Oceans only small variations (Score:5, Informative)
The Slashdot summary is totally wrong.
From the abstract of the paper: "I propose a different mechanism of secular variation: ocean water [...] as it flows through the Earth's main field may [...] manifest itself globally as secular variation."
Meaning: There is a major magnetic field that comes from the molten core. However, certain variations that are as yet unexplained may not result from core phenomena, but from the ocean currents.
I find this much more believable than the swill in the slashdot summary.
Re:Summary wrong: Oceans only small variations (Score:3, Informative)
Its not totally wrong.
FTA: "While Ryskinâ(TM)s research looks only at long-term changes in the Earthâ(TM)s magnetic field, he points out that, âoeIf secular variation is caused by the ocean flow, the entire concept of the dynamo operating in the Earthâ(TM)s core is called into question: there exists no other evidence of hydrodynamic flow in the core.â"
He does go so far to say that there is no examinable proof of a liquid core and that we could have been wrong all these years.
It doesn't seem too far fetched for me, but I'll leave the proof to the geomagnetism community.
Re:I may be wrong, Im not an astrologer (Score:5, Informative)
That's correct. According to their theory, moons like Europa should have a rather strong magnetosphere.
Europa is believed to have a warm, salty ocean under the ice crust. And yet, it shows only slight inducted magnetic field from Jupiter. Contrast that with Ganymede, the only moon with its own magnetosphere and a liquid iron core. Satellite photos dont show very much (or any) water on its surface.
Hmm.
Re:Uh, right. (Score:5, Informative)
Because the other theory hasn't been tested, and might be wrong.
The point is that the chances that each celestial body's magnetic field is due to a unique generator are... Well, let's say that that is not what we typical see in scientific history. Similar effects are generated by similar causes, especially at planetary scales.
(I see that I've been misled by the summary, as usual. Yes, I should RTFA. But the editors should fucking WTFS in a manner resembling responsible journalism. Could currents in the oceans modulate the magnetic field? Worth investigation, I think.)
Re:Summary wrong: Oceans only small variations (Score:1, Informative)
Well, but saying that a dynamo process in the core may not be happening is not the same as claiming that the oceans are doing it. The paper does not actually say what might replace the dynamo model, but obviously the oceans cannot generate a primary field. They can manage the secular variations by moving in the main field, but those are a factor of 1000 weaker than the persistent field.
Re:I may be wrong, Im not an astrologer (Score:3, Informative)
Aren't there planets that do have magnetic fields, but don't have oceans?
IIRC Venus has a weak magnetic field and does not have an ocean.
Re:I may be wrong, Im not an astrologer (Score:5, Informative)
Mercury has a magnetic field, which quite surprised planetary scientists when it was first discovered by MAriner 10, as the prevailing theory at the time was that Mercury's small size would have led to its core solidifying by now and stopping the dynamo that generated the field.
There's obviously a lot we don't know about planetary magentic fields, and I wouldn't want to judge the entire theory just by something I read on Slashdot, but I find it hard to understand how oceanic currents could account for Earth's magnetic field but not for Mercury's.
It doesn't say ocean currents cause the field (Score:5, Informative)
Re:Summary wrong: Oceans only small variations (Score:3, Informative)
http://gsc.nrcan.gc.ca/geomag/field/sec_e.php (about secular variation)
http://en.wikipedia.org/wiki/Magnetic_declination (about magnetic declination, obviously)
Long story short, magnetic declination is the difference between the geographical North Pole and the apparent magnetic North Pole at any one place on earth. The secular variation they're talking about is the gradual change in that magnetic declination, or the apparent movement of the Earth's magnetic North Pole. Secular variation is usually between 0 and 15 arcminutes per year - specific example: a nautical chart of the Thames Estuary from 2008 lists a yearly secular variation of 8' (arcminutes) Eastward.
Re:Polarity switch (Score:4, Informative)
Re:I may be wrong, Im not an astrologer (Score:4, Informative)
As does mars. It has a weak field, but it also is suspected of having a much smaller molten core.
Europa and Ganymede have molten cores due to gravitational churning.
So far, molten cores correlate well with magnetic field strength. Oceans, when present, tend to be on those bodies having molten cores, but their absense does not entirely preclude a magnetic field.
Re:But this would mean?!?!?!?!? (Score:1, Informative)
I wasn't aware water really responded much to solar radiation. Aren't some spaceship plans built with water used as shielding against solar flares? Water is pretty darn stable. There's lots of water ice in comets, and Mars probably does have lots of water, it's just ice.
Re:What happens when Ocean current patterns change (Score:2, Informative)
While I have no expertise in this area ... Are ocean current patterns really as static as the Earth's magnetic field?
Yes. Neither are static, both change continuously, but both are relatively slowly changing phenomena.
I'd think that there would be more fluctuations/variations in the Earth's magnetic field if it depended on the waterbodies.
On geological timescales, it would change dramatically. Which, we know, it does.
Wouldn't this also require compasses / magentic fields being disrupted when there are earthquakes/tsunamis or major storms?
No, since large-scale ocean currents are not noticeably affected by these things.
Re:But this would mean?!?!?!?!? (Score:4, Informative)
Yes exactly water ice in comets. Do you know what a comet's tail is made out of? Water evaporated and removed from the comet by solar radiation.
Re:Polarity switch (Score:4, Informative)
No, the poles already reversed once in theory http://en.wikipedia.org/wiki/Earth's_magnetic_field [wikipedia.org], and are likely to keep reversing, though none of us will be around to find out.
If you do some non-wiki research, you will find out that Earth's magnetic field has reversed many times over the eons. We're overdue now by several thousand years. This Global Warming may be just another indicator that such a change is imminent.
Re:I may be wrong, Im not an astrologer (Score:2, Informative)
Re:I may be wrong, Im not an astrologer (Score:3, Informative)
I don't know if you are joking, but we certainly knew Mars gravity to within some tiny fraction of a percent. The period and distance of it's orbiting moons can be used to figure it out pretty accurately. You may be thinking of the spaceship that was lost because of metric/english unit confusion?
Re:Summary wrong: Oceans only small variations (Score:2, Informative)
Re:Polarity switch (Score:5, Informative)
Hundreds if not thousands of direction changes are documented, back into the Triassic at least and possibly into the Late Palaeozoic. There are sufficient that, in more recent times (Cretaceous onwards) the reversal record has been used as a tool for correlation. (Such work may go back further into the geological record ; I've certainly seen it used in Cretaceous mudrock sequences as a petrophysical indicator that can be measured faster and with less skilful operators than other techniques like palynology.)
For certain values of "overdue" ; the distribution of durations between reversal events seems to be essentially random, and since we're over the average duration between reversals, then one could meaningfully "expect" a reversal sooner rather than later. But once you start looking at the statistics, you have to accept that, if the model is accurate, then the probability of a reversal in the next thousand years (say), is the same as the probability of a reversal in the first thousand years after the last reversal. It's the same logic as tossing coins - if you get ten heads in a row, the probability of your next toss being a head is still 1/2, even if the probability of getting 11 heads in a row is 1/2048. Random variables - love 'em or hate 'em, but you can't predict 'em.
That said, outside the statistical description of the record, the physical models suggest that some events seen at the moment (decreasing field strength ; regional anomalies) may be precursors to a reversal.
Has someone been claiming global warming to be related to magnetic field strength? Whooo, can I get a smoke of that? Sounds like good gear.
But there is a molten outer core.. (Score:3, Informative)
We have proven the existence of a molten outer core inside the Earth, and the proof doesn't depend on the magnetic field, but rather, seismology. Sound and vibration can travel in any substance as a pressure wave - material compressing and decompressing (P-waves). In solids, vibration can also be orthogonal to the direction of propagation (S-waves). Think of vibration in a string, or in a tuning fork. It is known empirically that S-waves travel through the Earth only to certain depth. Because they can't propagate deeper than that, the material must be unsuitable for S-waves, which means liquid.
Now, if there's a liquid, a gravitational field, and a temperature difference, convective flow must be present too. In addition, this liquid outer core is circulating around the Earth's axis. So the "geodynamo" still seems like the best explanation to me (I recommend Fowler's The Solid Earth if anyone's actually interested in the science and reasoning behind all this).
the long-term changes (the secular variation) in the Earth's main magnetic field are possibly induced by our oceans' circulation.
This here is what the article actually states. I'm not surprised that oceanic currents can correlate with the details of the magnetic field, as the field is known to be the result of several phenomena. Actually this finding can turn out to be supporting the geodynamo idea, as one problem with the geodynamo is why the magnetic field is such a mess ;). Maybe core currents generate most of the magnetic field and oceans add variation to it.
Re:I may be wrong, Im not an astrologer (Score:3, Informative)
>Nobody is saying that the iron itself is magnetic (because then it would be magnetite and not iron anyway).
I'm a little confused by this.
There's lots and lots of iron that's magnetic but isn't magnetite. Magnetite and hematite are both iron oxides; magnetite happens to be magnetic, while hematite isn't. (Well, technically, hematite is antiferromagnetic until it's reasonably hot, at which point it's a type of antiferromagnetic that makes it act magnetic, but that's not something you're ever going to notice unless you're a geek with good equipment.)
The more critical point here is the Curie point [wikipedia.org], the temperature above which iron is non-ferromagnetic. That's way below the temperature of molten iron, and we assume the core of the planet is molten. As such (if the Curie point isn't pressure-sensitive, which I don't know anything about) we can feel pretty confident that the iron core isn't ferromagnetic, but that it has a magnetic field induced by motion.