Astronomy Picture of the Day

Explanation: What causes black aurora? These gaps in normal bright aurora are frequently recorded but rarely questioned. Recent research using data from four Cluster spacecraft orbiting the Earth has now likely found the secret: black aurorae are actually anti-aurorae. In normal aurorae, 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 black anti-aurorae, however, negatively charged particles are sucked out from the Earth's ionosphere along adjoining magnetic field lines. These dark anti-aurorae can climb to over 20,000 kilometres and last for several minutes. Pictured above, a black aurora is seen dividing bright aurorae over Fairbanks, Alaska, USA.