Aurora Borealis on April 18, 2021 @ Hopeman, Moray, Scotland
© Alan C Tough
Aurora Borealis on April 18, 2021 @ Hopeman, Moray, Scotland
Red. Green. Purple. These are the colors we usually see during any display of auroras. On April 18th, Alan C. Tough of Hopeman, Moray, Scotland saw something else. "Black," he says. In the photo below, "note the dark vertical strip above the green band, which is devoid of any normal auroral colour."

Black auroras have been seen before. They are dark rings or black blobs that sometimes appear in an otherwise ordinary expanse of auroral light. Some researchers call them "anti-auroras." The black auroras in Tough's photo are circled here.

Ordinary auroras are caused by electrons raining down from space, hitting Earth's upper atmosphere and making the air glow. Black auroras are the opposite. Instead of electrons raining down, electrons are propelled upward, back into space. This diagram shows what happens:

black aurora
Europe's fleet of Cluster spacecraft flew in formation over a black aurora on Jan. 14, 2001, and saw the process in action. Sensors onboard the spacecraft detected strong positive electric fields in the black aurora zone. These idiosyncratic fields were stopping, and even reversing, the normal downward rain of aurora-causing electrons.

The study of black auroras is still in its infancy, and forecasters cannot yet predict when or where they might appear. Aurora watchers, the next time a geomagnetic storm erupts, be on the lookout for black.