Wednesday, June 13, 2012

The Earth’s internal magnetic field has not always been oriented as it is today

The Earth’s internal magnetic field has not always been oriented as it is today. The direction of the dipole component reverses, on an average, about every 300,000 to 1,000,000 years. This reversal is very sudden on a geologic timescale, apparently taking about 5,000 years. The time between reversals is highly variable, sometimes occurring in less than 40,000 years and at other times remaining steady for as long as 35,000,000 years. No regularities or periodicities have yet been discovered in the pattern of reversals. A long interval of one polarity may be followed by a short interval of opposite polarity.

Available data suggest that during a reversal the strength of the dipole component shrinks to zero while maintaining its orientation. It then grows again to its former strength but with opposite orientation. During the interval in which there is no dipole component, the non-dipole part of the field appears to persist.

During field reversals the outer portion of the Earth’s magnetic field is greatly altered. The absence of a dipole component would mean that the solar wind would approach much closer to the Earth. Cosmic-ray particles that are normally deflected by the Earth’s field or are trapped in its outer portions would reach the surface of the planet. These particles might cause genetic damage in plant or animal communities, leading to the disappearance of one species and the appearance of another.