ESA Science & Technology11-Jul-2005 15:00:44

Science Results

The secret of the black aurora

The four Cluster spacecraft showed that black aurorae are a kind of 'anti-aurora'

Anyone living near the Arctic Circle will be familiar with aurorae, the legendary red and green curtains that illuminate the long winter nights. Much less familiar is the mysterious 'black aurora', a strange electrical phenomenon that produces dark, empty regions within the visible Northern and Southern Lights.

In an effort to solve the puzzle of the black aurora, Swedish and British researchers have used the four Cluster spacecraft to make the most detailed observations yet of the regions where they are generated.

The black aurora takes on various guises - dark rings, curls or black blobs in a sea of faint, glowing aurora. The Cluster data show that the black aurora occurs where there are 'holes' in the ionosphere, the part of the upper atmosphere where aurorae are created. Here, the particles that make up the ionosphere are shooting upwards into space inside regions known as 'positively charged electric potential structures'.

This is the opposite process to that which creates visible aurorae, where electrons spiral down from space into the atmosphere within similar, but negatively charged, structures. Cluster has allowed to discover how the huge vertical structures associated with the black aurora form, how long they last and how they vary with altitude. These results were published in Nature.

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