Flip-flop of Earth's magnetic field probed The polarity reversal was a global event. Credit: Norbert R. Nowaczyk/GFZ POTSDAM, Germany, Oct. 16 (UPI) -- Some 41,000 years ago a complete reversal of the Earth's magnetic field occurred and compass needles would have pointed south not north, German scientists say. Researchers at the Helmholtz Center Potsdam -- GFZ German Research Center for Geosciences report the most remarkable feature of the phenomenon was the speed of the reversal. "The field geometry of reversed polarity, with field lines pointing into the opposite direction when compared to today's configuration, lasted for only about 440 years, and it was associated with a field strength that was only one quarter of today's field," GFZ researchers Norbert Nowaczyk said. The actual polarity changes lasted only 250 years. In terms of geological time scales, that is very fast. Most reversals, which the Earth has experienced periodically, are estimated to take between 1,000 and 10,000 years. During the reversal 41,000 years ago the field was very weak with only 5 percent of today's field strength, researchers said, and as a consequence Earth nearly completely lost its protection shield against hard cosmic rays, leading to a significantly increased radiation exposure.