A 55-year-old mystery about the temperature of the Sun's corona - the outermost layer, visible during a solar eclipse - has been solved by a European Space Agency (ESA) spacecraft. The answer, it turns out, is that the Sun is covered in carpets.

They are not physical carpets. Instead, they consist of moving waves of magnetism which carry more energy than a hydroelectric plant could generate in a million years.

For years, scientists have known that the corona has a temperature of millions of degrees. Yet the visible surface of the Sun is relatively cold, at about 6,000 degrees centigrade.

Since it is physically impossible in a closed system for thermal energy to be transferred from the cool surface to the hotter one, scientists have long theorised that some form of electromagnetic activity was causing the necessary energy movement.

The Solar and Heliospheric Observatory (Soho) spacecraft, run by ESA and the US space agency Nasa, has discovered that there are huge "carpets" consisting of loops of charged (and hence magnetic) particles flowing across the Sun's surface.

Where the carpets meet they produce "short circuits" which flow upwards into the corona and heat it to its multi-million degree temperature. Images collected by Soho show the hot gases of the corona reacting as the shifting magnetic fields on the surface move and develop.

"We found that after a typical small magnetic loop emerged, it fragments and drifts around and then disappears in only 40 hours," said Alan Title of the Stanford-Lockheed Institute for Space Research in Palo Alto, California.