THEMIS (Time History of Events and Macroscale Interactions during Substorms) is a NASA mission designed to study the causes of auroral substorms, which are sudden brightenings of the aurora borealis (northern lights) and aurora australis (southern lights) that occur in the Earth's upper atmosphere.
The mission consists of five identical spacecraft that were launched into orbit in 2007. These spacecraft are equipped with a variety of instruments to measure the electric and magnetic fields, particles, and waves in the Earth's magnetosphere, as well as the energy inputs that drive auroral substorms. The THEMIS mission has made a number of important discoveries about the physics of auroral substorms, including the role of magnetic reconnection in driving these events, and the existence of a new type of wave called an electrostatic solitary wave.