Monday, November 9, 2009

Magnetic Reversal Round-up

The sun reverses its magnetic field like clockwork every eleven years at the peak of the sunspot cycle. The next solar flip is due in 2012. South-pointing magnetic flux moves from sunspots, which are intense magnetic loops near the equator of the sun, along “meridional flows” to the north magnetic pole, and vice versa. As the oppositely-directed charge accumulates at the poles the field declines, until eventually the reverse charge predominates.

The mechanism that controls earth's field reversals may not be based on similar principles. For one thing, a planet does not seem to have any equivalent to the powerful sunspots. McFarlane refers to there being more than one north-south pole system and about 10% of the total field being involved in smaller extra fields. If these subordinate minor magnetic fields take up more of the magnetic activity during the main field’s decline, they might become active enough to sustain a minimal protective layer shielding the biosphere, even if the main dipole field declines to zero gauss. This could be important for our survival, as the Steen’s mountain lava flows indicate that the reversal took 4,500 years to be completed!'s_magnetic_field

The Earth's field changes in strength and position. The two poles wander independently of each other and are not at directly opposite positions on the globe. Currently the magnetic south pole is farther from the geographic south pole than the magnetic north pole is from the geographic north pole.

Based upon the study of lava flows of
basalt throughout the world, it has been proposed that the Earth's magnetic field reverses at intervals, ranging from tens of thousands to many millions of years, with an average interval of approximately 250,000 years. The last such event, called the Brunhes-Matuyama reversal, is theorized to have occurred some 780,000 years ago.

In each case, the reversal is preceded by at least 20,000-40,000 years of fairly continuous decay in field strength to about the same (very low) value, with a much more rapid recovery in field strength following the transition. In this context, a couple of centuries' worth of field decay is not particularly significant, especially since the present field strength (about 8 on the scale in this figure) is still a lot higher than the value reached during all of these reversals (<2).

If you compare the duration and magnitude of the recent decrease (the red box in the figure below) to the field behaviour for past reversals, it doesn't look like we have to worry about the field reversing for a while - it looks like it needs to continue weakening for a few thousand more years, and reach a much lower strength, before a reversal is going to happen.

Of course, another thing you should get from the Valet et al. paper is that a full reversal sequence is not an instantaneous event; our compasses will not point north today and south tomorrow. Instead, the geomagnetic field will weaken, and the magnetic poles will start to wander to lower latitudes, and possibly multiply, over a period of hundreds and thousands of years. It looks dramatic from the perspective of Deep Time, but during a reversal the changes over a human lifetime will probably be little different from the secular variation we see today. No extinction event has ever been linked to a magnetic field reversal, and I think that we might just cope with the next one too - whenever it might occur.

The last linked article is actually quite good and goes into a lot of detail and has some good charts and graphs in it.

So, it looks like a 2012 pole reversal is not likely and even if it were, it is not expected to be an extinction event. This all comes up because I made the mistake of watching a show last night on the Syfy channel that was all about 2012 and one of the things they talked about was the pole reversal and of course more fact checking is done for a second grade book report than was done for this show.

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