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Author Topic: What causes the Earth's magnetic poles to reverse?  (Read 4634 times)

Offline Sprool

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In geological history it is well documented via constructive margins in the sea bed rock, that the magnetic poles swap over every few tens of thousands of years, but what drives this process and when is the next one due?
« Last Edit: 29/03/2012 12:11:48 by chris »


 

Offline Nizzle

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Re: N & S pole time to swap?
« Reply #1 on: 26/03/2012 10:13:05 »
Earth has a magnetic field because it has a liquid metallic core. I believe convection currents in this core is what drives the process.
I also thought that we're already a bit overdue for a reversal
 

Offline syhprum

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Re: N & S pole time to swap?
« Reply #2 on: 26/03/2012 11:22:56 »
In human terms the switch over is very slow it may well be taking place, are our records long enough to tell if it is happening?.
 

Offline Sprool

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Re: N & S pole time to swap?
« Reply #3 on: 26/03/2012 12:26:51 »
it would be interesting to know the timescale needed for the reversal. Presumably at one point there will be a period where N & S are not apparent, in which case chaos may ensue within the scouting, orienteering community and racing pigeon events? What about the earths protection from suns ionised particle stream when the magnetic field lines collapse then invert? Are we all in for a barbequeing?
 

Offline syhprum

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Re: N & S pole time to swap?
« Reply #4 on: 26/03/2012 13:45:25 »
I wonder if does indeed shut down and then restart in the opposite polarity, it could of course just move round via the equator.
I remember an experiment being set up with a massive sphere filled with magnetic fluid to try and emulate it in the lab but I do not know what the results were.

http://www.popularmechanics.com/technology/engineering/4277476

http://gizmodo.com/5865967/inside-this-giant-metal-sphere-lives-a-miniature-version-of-the-earths-spinning-core

http://www.nature.com/news/dynamo-maker-ready-to-roll-1.9582

V
« Last Edit: 26/03/2012 14:02:45 by syhprum »
 

Offline Sprool

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Re: N & S pole time to swap?
« Reply #5 on: 26/03/2012 15:16:48 »

Good ol' Wikipedia:
"Earth's magnetic field (also known as the geomagnetic field) is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun. It is approximately the field of a magnetic dipole tilted at an angle of 11 degrees with respect to the rotational axis—as if there were a bar magnet placed at that angle at the center of the Earth. However, unlike the field of a bar magnet, Earth's field changes over time because it is really generated by the motion of molten iron alloys in the Earth's outer core (the geodynamo). The Magnetic North Pole wanders, fortunately slowly enough that the compass is useful for navigation. At random intervals (averaging several hundred thousand years) the Earth's field reverses (the north and south geomagnetic poles change places with each other). These reversals leave a record in rocks that allow paleomagnetists to calculate past motions of continents and ocean floors as a result of plate tectonics. The region above the ionosphere, and extending several tens of thousands of kilometers into space, is called the magnetosphere. This region protects the Earth from cosmic rays that would strip away the upper atmosphere, including the ozone layer that protects the earth from harmful ultraviolet radiation.

At present, the overall geomagnetic field is becoming weaker; the present strong deterioration corresponds to a 10–15% decline over the last 150 years and has accelerated in the past several years; geomagnetic intensity has declined almost continuously from a maximum 35% above the modern value achieved approximately 2,000 years ago. The rate of decrease and the current strength are within the normal range of variation, as shown by the record of past magnetic fields recorded in rocks.
 
The nature of Earth's magnetic field is one of heteroscedastic fluctuation. An instantaneous measurement of it, or several measurements of it across the span of decades or centuries, are not sufficient to extrapolate an overall trend in the field strength. It has gone up and down in the past for no apparent reason. Also, noting the local intensity of the dipole field (or its fluctuation) is insufficient to characterize Earth's magnetic field as a whole, as it is not strictly a dipole field. The dipole component of Earth's field can diminish even while the total magnetic field remains the same or increases.
 
The Earth's magnetic north pole is drifting from northern Canada towards Siberia with a presently accelerating rate—10 km per year at the beginning of the 20th century, up to 40 km per year in 2003, and since then has only accelerated."
 

Offline CliffordK

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Re: N & S pole time to swap?
« Reply #6 on: 26/03/2012 17:53:41 »
It is my belief that Earth's magnetic field strength is related to the solar magnetic field strength.

The sun's magnetic field polarity is flipping approximately every decade.  This flipping of the sun's magnetic field then impacts Earth.  This, however, is not necessarily the "normal" state of affairs.  It is quite possible that the solar magnetic field ceased its flipping during the Maunder Minimum/Little Ice Age, and I believe some data indicates that Earth's magnetic field strength increased during this period. 

The sun's magnetic field may also stabilize during the glacial periods, and become more erratic during the interglacial periods.  Thus, a weakening magnetic field on Earth may in fact be a very good sign.  A strengthening field could indicate a significant decrease in solar output and temperatures on Earth.

I'm curious about what the Voyager probes will determine about the Milky Way magnetic field.  Currently they are going through an area being described as "foam" or "froth".  I am just hoping that they will get far enough away from the sun to get some data independent of the solar system before running out of power.
 

Offline Sprool

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Re: N & S pole time to swap?
« Reply #7 on: 27/03/2012 12:26:50 »
Does the magnetic field strength of the sun actually reach as far as inflencing the earth, 93 million miles away? I thought that was a relatively weak force but if you have info on it affecting the earths field it would be interesting to read up further.
 

Offline CliffordK

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Re: N & S pole time to swap?
« Reply #8 on: 28/03/2012 05:43:12 »
Here is a chart I made up a while ago, cutting and pasting Glacial temperature records and the inverse of the magnetic field strength (from Wikipedia).



I must admit that when I first made the chart, I found it difficult to match two sets of squiggly lines.  The period around 700,000 years ago (red) seemed to give me a good match.  Other periods seemed to be representative, but perhaps not quite as good of a match.

Earlier I had found several articles discussing this, but I don't have access to those files at this time.  Here is one article with similar charts covering the last thousand years (Fig 4), note they show cold on top, warm on bottom.

http://sciences.blogs.liberation.fr/home/files/Courtillot07EPSL.pdf

As far as solar magnetic field strength vs the Earth's...  Obviously the sun is much larger than the Earth, but of course, quite distant.  I believe one can also match these magnetic field strength charts to solar activity, for example stronger magnetic field strength during the Maunder Minimum shortly after 1600.  Anyway, I can't say for certain that the solar field is actually influencing Earth's field, but it seems to be a possibility. 
« Last Edit: 28/03/2012 05:59:24 by CliffordK »
 

Offline Ophiolite

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Re: N & S pole time to swap?
« Reply #9 on: 28/03/2012 12:53:02 »
Clifford, your investigations in this are intersting, but I think they are something of a red herring as far as the OP is concerned: while I fully accept that solar activity does influence superficial expression of Earth's field I see no conceivable mechanism by which the field could influence the generation and hence fundamental properties of that field, specifically its polarity. You may be interested in this paper: "TIME VARIATIONS OF THE EARTH'S MAGNETIC FIELD: From Daily to Secular" It is a comprehensive review of time variations in the geomagnetic field.

Turning to the matter of field reversal here are some pertinent facts:

1. The consensus view is that reversal would take of the order of a few thousand years to complete.
2. However, a recent study appears to reveal a very rapid change within less than a century. (Frantically trying to find this paper.)
3. During reversal the field does not vanish, but falls to a low percentage of its 'normal' level.
4. Simulations suggest that the reversal involves the appearance of multiple north and south poles around the planet.
5. The appearance of the present South Atlantic Magnetic Anomaly could be such an instance.
6. The timing of reversals is highly variable, so that it is speculative to say that we are overdue for a reversal.


Edit: This is interesting. I have located the paper referred to in 2. above. I had clearly misrecalled what was being described. The authors still accept that reversals take several thousand years, but during this period there may be very rapid changes in the field of six degrees per day.

You may also be intrigued by their comments on the work of Russian researchers that when the field is weak during a transition it is much more easily influenced by the sun (page 692 - left hand column). They lean towards discarding this hypothesis, but it clearly shows others are thinking along the same lines as yourself.
« Last Edit: 28/03/2012 13:35:56 by Ophiolite »
 

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Re: N & S pole time to swap?
« Reply #9 on: 28/03/2012 12:53:02 »

 

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