Solar wind to shield Earth during pole flip
By Marcus Chown
Hollywood now has one less disaster scenario to worry about. The Earth, it seems, will be safe when its magnetic field falters during the next reversal of its magnetic poles.
A new model of the way the Earth interacts with the solar wind indicates that a replacement field will form in the upper atmosphere during the switch.
Scientists had previously thought that the planet would be left without a protective shield to stop lethal radiation from space reaching the surface.
The strength of the Earth's magnetic field is known to drop during "magnetic reversals", when the north and south poles swap places. Records of the field direction, frozen into sediments laid down on the seabed, show that the magnetic field has reversed hundreds of times in the past 400 million years.
In normal circumstances, the magnetic field protects the Earth's surface from dangerous high-energy particles, including particles from the sun and cosmic rays from deep space.
But as the field switches polarity, it can drop to below 10 per cent of its normal strength for thousands of years. Such a weakened field would allow lethal radiation to reach the Earth's surface, with potentially disastrous consequences for the atmosphere, the climate and particularly for life.
Opportune moment
In a paper to be published in the journal Astronomy and Astrophysics, Guido Birk and Harald Lesch of the University of Munich, Germany, and Christian Konz of the Max Planck Institute for Plasma Physics in Garching report an investigation of exactly what happens when the field is drastically reduced or vanishes altogether.
Their simulations show that the solar wind – the million-kilometre-an-hour stream of hydrogen and helium nuclei from the sun – wraps itself around the Earth in a way that induces a magnetic field in the ionosphere as strong as the original field.
"We were quite surprised about its effectiveness," Lesch says.
The news comes at an opportune moment. The Earth's magnetic field is showing worrying signs that it is about to reverse again. Not only has the magnetic north pole wandered by 1100 kilometres in the past 200 years, but its strength is dropping at a rate of 5 per cent a century.
"This is the fastest decrease since the last reversal 730,000 years ago," Lesch says.