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Thursday, 1 June, 2000, 14:50 GMT 15:50 UK
Solar wind has ripping time
Particles streaming from the Sun cause the aurora
Tears or rips in the Earth's magnetic field are responsible for the Northern and Southern Lights, scientists have found.

It has long been known that the aurora borealis and australis are caused by charged particles from the Sun penetrating the Earth's magnetosphere, but no one has seen it happen.

Now an international team of physicists using Nasa's Polar satellite have made the first direct observations of the switch that permits energy to be transferred between the solar wind and the Earth.

The magnetosphere normally acts like a cocoon, protecting the Earth from harmful solar radiation, but every now and then energy finds its way in through a process called "magnetic reconnection".

"Sometimes the solar wind creates tears in the cocoon, and charged particles and energy from the Sun penetrate into the magnetosphere.

"Some of these tears are extremely small - on the order of one kilometre long," said Professor Jack Scudder, principal investigator for the Hot Plasma Analyzer (Hydra) on Nasa's Polar spacecraft.

Poles apart

Scientists involved in the multi-satellite International Solar-Terrestrial Physics (ISTP) program identified two general areas where the rips in the magnetosphere occur.

One region found by Polar is 48,000 to 64,000 kilometres (30,000 to 40,000 miles) on the sunward - or day - side of the cocoon.

The other, observed by the Geotail satellite, is thought to be some 137,000 to 154,000 kilometres (85,000 to 96,000 miles) "downwind" of the Earth, on the night side of the planet and in the tail of the teardrop-shaped magnetosphere.

The discovery answers a half-century-old question surrounding the origin of the energy responsible for aurorae.

Energy from the solar wind enters the magnetosphere through a rip on the day side of the Earth. A slit in the magnetic fields on the night side permits the transfer of energy down to the Earth's atmosphere.

"Everything that happens regarding space weather happens through and because of these slits," Professor Scudder said. "We're excited that the data clearly show the electrons jumping through these slits, doorways really, that open the Earth to these particles from space.

"The $64 question is what makes the tears possible. We'd like to see some more of them to know whether those already detected represent a general or an unusual picture of the switch."

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