Black Aurora over Norway

Da "NASA - Picture of the Day" del 29 Marzo 2006:"What causes gaps between aurora curtains? These unusual gaps can make auroral displays appear more detailed and intricate. Research using data from four Cluster Spacecraft orbiting the Earth has likely found the secret: auroral gaps, sometimes knows as "black auroras", are actually anti-auroras. In normal auroras, electrons and/or predominantly negatively charged particles fall toward Earth along surfaces of constant magnetic field. They ionize the Earth's atmosphere on impact, causing the bright glows. In auroral gaps, however, negatively charged particles may be sucked out from the Earth's ionosphere along adjoining magnetic field lines. These dark anti-auroras can climb to over 20.000 Km and last for several minutes. Pictured above, a series of well-defined auroral gaps is seen dividing green aurora curtains high above Harstad, Norway, earlier this month".