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There are some earth phenomena you can count on, but the magnetic field, someday is not of them. It fluctuates in strength, drifts from its axis, and every few 100,000 years undergo, dramatic polarity reversal—a period when north pole beco mes south pole and south pole becomes north pole. But how is the field generated , and why is it so unstable?
Groundbreaking research by two French geophysicists promises to shed some light on the mystery. Using 80 meters of deep sea sediment core, they have obtained me asurements of magneticfield intensity that span 11 polarity reversals and four million years. The analysis reveals that intensity appears to fluctuate with a clear, welldefined rhythm. Although the strength of the magnetic field varies irregularly during the short term, there seems to be an inevitable long term dec line preceding each polarity reversal. When the poles flip—a process that takes several hundred thousand years—the magnetic field rap
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