NASA launched a constellation of five small satellites on 17 February 2007, all carrying identical suites of electric, magnetic, and particle detectors to study aurora. These satellites fly in carefully coordinated orbits. Every four days, they line up along the Earth's magnetic tail to track disturbances in the magnetosphere.
Satellite data from the THEMIS mission (for "time history of events and macroscale interactions during substorms") are compared to observations from ground stations across the Arctic Circle. In North America, 20 observatories equipped with automated, all-sky cameras take pictures every three seconds over the two-year mission, for a total of 140 million pictures.