Monday, June 14, 2010

The Earth’s Magnetism

The earth as a strong bipolar magnetic field, as if there was a large bar magnet in its core. In fact, electrical currents in the churning, liquid outer core produce a self sustaining dynamo. The influence of the magnetic field extends beyond the planet. The field meets charged particles streaming from the Sun in a shock wave about 50,000 Km above the Earth and slows hem from 400 to 250 Km per second. The solar wind, as these particles are known sweeps the Earth’s magnetic field into a six million Km tail. Solar flares on the surface of the Sun result in magnetic storms on Earth to days later. There are belts of solar particles trapped in the magnetic field about 3,000 and 25,000Km above the equator, called the Van Allen belts. Aurorae occur where charged particles trapped by the Earth’s magnetic field stream towards the poles.
The earth’s magnetic axis is 11 degree west of the North Pole and is slowly wandering. Eddy currents in the core and magnetism trapped in rocks produce local anomalies. In the geological past, the magnetic field have reversed many times. Between 118 and 83 million years ago it did not reverse at all. For the last few million years it has reversed about once every 220,000 years, revealed by the magnetic alignment in volcanic rocks. We are overdue for another reversal.


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