Green and Black Aurora Over Norway

What causes gaps between aurora curtains? These unusual gaps can make 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 kilometers 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.