Mulling the Mystery of Black Aurora

When Jennifer Kimball looked up at the night sky last winter, she noticed something special--a solid green curtain of aurora borealis that seemed to be cleaved in two by a long line of black arcs.

On that February night, Kimball, a graduate student studying space physics at the Geophysical Institute, caught her first live glance of her thesis topic--the black aurora. She later confirmed she wasn't imagining things by watching a video of that night's aurora activity taken at Poker Flat Research Range north of Fairbanks.

Black aurora isn't actually aurora at all; it's the lack of aurora activity where it seems like the aurora should be. Kimball and her advisor, Geophysical Institute Professor Emeritis Tom Hallinan, showed me a black aurora that was captured by a narrow-field camera at the former Ester Dome observatory in Fairbanks in the 1970s. Kimball hit the play button and pointed out three major types of black aurora: black curls, which corkscrew across a sheet of aurora; black rings, which look like dark smoke rings against a pale screen of aurora; and my favorite, black patches, which float like giant black amoebas through a sea of faint aurora.

Not many people have witnessed black aurora because it occurs at what Hallinan refers to as "hot-chocolate time," typically 20 minutes to a half-hour after a spectacular display, a time when most people have retreated to warmer quarters. Black aurora occurs almost directly overhead in Alaska during diffuse auroras.

Diffuse auroras are nondescript displays about as bright as the Milky Way, according to Neil Davis in his book, The Aurora Watcher's Handbook. Davis labeled diffuse auroras as "visually uninteresting" when compared to eye-catching displays such as discrete aurora, which are well-defined arcs, rays and ripples that appear on most aurora postcards, and pulsating aurora, which are patches of the sky that alternately lighten and darken, seeming to turn on and off.

Kimball took advantage of the Geophysical Institute's archives of aurora footage, captured for more than three decades by narrow-field and all-sky cameras at Ester Dome, Poker Flat Research Range, Fort Yukon, and Eagle. For her master's thesis work, Kimball observed dozens of examples of black aurora and came up with some fresh conclusions.

When the aurora draws us outside with impressive displays, we're watching the interaction between particles from the sun and gases in Earth's atmosphere. These gases produce the color in auroras. The familiar whitish-green color of the aurora, for example, results when electron particles from the sun strike nitrogen molecules. Red auroras occur due to the collision between electron particles and oxygen atoms.

Unlike regular aurora emissions, areas of black aurora seem to have a positive, rather than negative, charge. Black curls rotate in the opposite direction of the curls of the aurora we normally see, and black curls sometimes combine with visible aurora curls to produce a series of space-plasma whirlpools called Karman vortex streets.

While Karman vortex streets may not enter most of our minds as we look to the heavens, Kimball's research will help other scientists tackle the question of why and how the aurora gets turned off in patterns that look like black corkscrews, smoke rings and amoebas.