‘Black Auroras’ over Alaska

Via Spaceweather.com

Todd Salat is a veteran photographer of auroras in Alaska. For years he has chased the lights and seen most of what Mother Nature has to offer. But even he was puzzled on Nov. 22nd when these strangely-shaped auroras appeared overhead:

“I saw these bizarre auroras drift over southcentral Alaska around 4 am last Friday morning,” says Salat. “It came up from the northwest and I was like, whoa! It looked like the letter E to me.”

Salat may have witnessed an episode of ‘black auroras.’ They are dark rings or black blobs that sometimes appear in an otherwise ordinary expanse of auroral light. For example, look at Figure 1 in this research paper on the topic. Some researchers call them “anti-auroras.” The black auroras in Salat’s photo are circled here.

Ordinary auroras are caused by electrons raining down from space. Black auroras are the opposite. Instead of electrons raining down, electrons are propelled upwards back into space. Europe’s fleet of Cluster spacecraft flew over a black aurora on Jan. 14, 2001, and saw the process in action:

Sensors onboard the spaceraft detected strong positive electric fields in the black aurora zone. These fields reversed 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 alert for black. Aurora alerts: SMS Text.