Weather Notebook
Bryan Yeaton

Black Aurora
Thu Feb 05, 2004

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You may have been fortunate to witness the legendary red and green curtains of the aurora borealis. But did you notice dark empty regions within the aurora? Those dark spaces are the mysterious black aurora. Hi, I'm Bryan Yeaton for The Weather Notebook.

Graduate students and researchers at Alaska's Geophysical Institute coined the term "black aurora" in jest during the 1960s. When making visual observations of the Northern Lights, they occasionally noted unusual auroral forms with dark spaces between pale, diffuse aurorae which they first thought to be optical illusions.

Further research determined black aurorae were indeed real and represented holes in the ionosphere, the upper atmosphere region where aurorae form. The black aurorae can take on many faces: dark rings, curls, or black blobs floating upon a sea of faint, glowing aurora. These anti-auroras can ascend to over 20,000 kilometres and last for several minutes.

Only recently have Swedish and British researchers begun to unravel the puzzle of black aurora, using the Cluster satellites -- Rumba, Salsa, Samba and Tango -- to make detailed observations of the ionosphere.

It appears that black aurorae form in patches of the ionosphere where conditions are the exact opposite of those forming normal aurora, where, in regions known as positively charged electric-potential structures, negatively charged electrons are accelerated upwards into space. In contrast, aurorae arise within negatively charged electric-potential structures where electrons spiral down from outer space into the atmosphere.

When black aurora mixes with diffuse aurora displays, its black curls can combine with visible auroral curls to form a series of space-plasma whirlpools wafting across the heavens. Now that's cool.

Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook is supported by Subaru of America and the National Science Foundation. And thanks to Davis Weather Instruments for making our 2004 tour possible!

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