Diffuse auroras are nondescript displays about as bright as the Milky Way, according to Neil Davis in his book, The Aurora Watcher's Handbook. 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. This column is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Ned Rozell is a science writer at the institute.