The Case of the Missing Aurora

Sometimes, near midnight, auroras suddenly stop. Nobody knows why. This nightside gap in aurora was confirmed recently by D. Chua ( U. Washington ) and colleagues in data from the Ultraviolet Imager onboard the Polar spacecraft. The gap appears from space as a slight break in a more full auroral arc surrounding a magnetic pole of the Earth. Pictured above are clouds and auroras occurring last August near Wildcat Mountain in Wisconsin.