Jeff, Michigan asked:
I have a question about the reversal of the Earthís magnetic field. I hear that the field polarity occasionally switches round periodically so the north turns to be somewhere south. Iíve also heard that between the switch thereís a period of time when there isnít a substantial magnetic field so, my question is, what is the historical estimate of how long it would be before the magnetic field returns?
The Earthís magnetic field does seem to flip every few hundred thousand years and weíre overdue for one at the moment. A few years ago they thought this would take hundreds of thousand of years for it to flip slowly. But recent evidence suggests that itíll maybe flip over faster. In the process of it flipping over it doesnít tend to flip over evenly. What tends to happen is the North Pole gets weaker and weaker and sometimes you get two, three or four other North Poles around the Earth in different places that move around quite fast. Eventually itíll stabilise again the other way aroundÖ maybe after 20-30 years.