July 21, 2014
Humans have known about magnetism for millennia. In the sixth century B.C., the Greek philosopher, Thales, wrote about the attraction of lodestones. Lodestones are magnetized specimens of the mineral, magnetite (Fe3O4), and these stones are attracted to themselves, and to iron. The compass, also known from antiquity, demonstrated the existence of Earth's magnetic field.
As I wrote in a previous article (Magnetic Mars, April 15, 2013), the English scientist, William Gilbert, published the 1600 book, De Magnete, which was the first modern study of magnetism. Gilbert created a sphere of lodestone, which he called a terrella, as a model to provide evidence that the Earth was a giant magnet. A compass placed on its surface acted as a compass does on magnet.
The Earth's magnetic field, which ranges from 0.25 to 0.65 gauss when measured at its surface, is thousands of time less than that of a barium ferrite (BaFe12O19), the magnetic component of refrigerator magnets. The Earth's magnetic field may be small, but it's significant, since it deflects cosmic rays around the Earth.
A magnetic field will deflect a moving charged particles according to the "left-hand rule" taught during introductory physics courses. This rule concerns electrical current, but a moving charge is an electrical current. This deflection of charged particles has been important to the Earth, since it's prevented the solar wind from stripping away our atmosphere. Mars, with no global magnetic field, wasn't as lucky.
So, how stable has Earth's magnetic field been? On average, very good, with one notable feature. Since the Earth's magnetic field is apparently generated by a dynamo action of liquid iron in the core, it's subject to the whims of hydrodynamics. This includes spontaneous reversals of the fluid flow; and, in Earth's case, a spontaneous reversal of the magnetic polarity.
This means that compass needles would point south instead of north. As strange as they might sound, these geomagnetic reversals have happened often in Earth's history. As the figure shows, we've been in an extended period of our present polarity.
Do bad things happen during such field reversals? Scientists have found no extinction events, and they have determined that the Earth's magnetic field decreased by just 5% during the short (440 year) Laschamp reversal event that occurred 41,400 ± 2,000 years ago.
To more closely study Earth's magnetic field, the European Space Agency has launched its Swarm constellation of three identical satellites, two of which are in an initial 460 km orbit, and the third is in a higher, 530 km orbit. The orbits are designed to optimize measurement accuracy. The first observations were presented at the Third Swarm Science Meeting, Copenhagen, Denmark, on June 19, 2014.[3-4]
Six months of observations have shown a weakening of Earth's field over the Western Hemisphere, a field increase at the southern Indian Ocean, and a migration of the North magnetic pole towards Siberia (see figure). These data may indicate an imminent geomagnetic reversal, imminent meaning the usual timescale for such a reversal, hundreds of years.[3-4]
These field changes are an order of magnitude greater than a previous estimate of five percent per century. Now, it's more like 5% per decade. The Sawrm data are expected not only to generally characterize Earth's field, but also as a means to predict earthquakes. I wrote about earthquake prediction in some previous articles (A Century of Earthquake Prediction Possibilities, October 4, 2013 and Earthquake Prediction, January 16, 2013).
|Left, the ESA Swarm satellite constellation. Note that the magnetometers are attached to long booms to distance them from the magnetic influence of the satellite body. The right image shows changes in the Earth's magnetic field from January to June, 2014, with red indicating an increase and blue indicating a decrease. Click for larger map image. (left image, Copyright ESA/AOES Medialab; right image, Copyright ESA/DTU Space.)|
- William Gilbert, "De Magnete," 1600 (original Latin); English translation. The illustration is from Book III, Chapter XII.
- N.R. Nowaczyk, H.W. Arz, U. Frank, J. Kind, and B. Plessen, "Dynamics of the Laschamp geomagnetic excursion from Black Sea sediments," Earth and Planetary Science Letters, Vols. 351-352 (October 15, 2012), pp. 54-69
- Swarm reveals Earth's changing magnetism, ESA Press Release, June 19, 2014.
- Kelly Dickerson, "Earth's Magnetic Field Is Weakening 10 Times Faster Now," Live Science, July 8, 2014.
- ESA's magnetic field mission Swarm, Europeasn Space Agency.
- Swarm fact sheet (PDF File).
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