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Plasma whirlpools breach Earth's magnetic shield

Cluster reveals how

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Data returned to Earth from ESA's Cluster quartet of satellites has revealed huge swirling pools of electrified gas that are feeding into Earth's magnetosphere.

The magnetosphere protects Earth from huge amounts of plasma from the sun. But some does still leak through. A zone called the plasma sheet, an inner region of our magnetosphere, sometimes fills up with plasma. This can happen in a matter of hours.

But until now, scientists were not sure exactly how the plasma crossed the magnetopause.

It has been known for some time that weak points in the shield occur where magnetic field lines reconnect. In a reconnection event, ESA explains, magnetic field lines spontaneously break and then join up with other nearby lines. This directs solar plasma along new routes.

But this new data from Cluster reveals that reconnection inside whirlpools of plasma also contribute to the process.

"Wondering how the solar wind could get into the plasma sheet is how I became interested in this problem," says Katariina Nykyri, lead author of the results, from the Imperial College, London.

She began studying an event recorded by Cluster back in 2001, when it was on the dawn side of Earth. In this region of space the solar wind blows over the Earth's magnetopause, much like the wind blows over an ocean.

The solar wind can whip up whirlpools of plasma, known as Kelvin-Helmholtz instabilities. Researchers thought these might conceal reconnection activity, which would show up in the data as a sudden change in the direction of the plasma flow and magnetic field, called a rotational discontinuity.

After what she describes as painstaking analysis, Nykyri and her team have confirmed that this does happen inside the whirlpools. She then developed a computer model of the event which revealed that in these reconnections plasma was being fed down magnetic field lines and into the magnetosphere. ®

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