The five small satellites of the THEMIS mission are probing the streams of charged particles in the solar wind that buffet Earth’s magnetosphere and generate polar auroras.

THEMIS (Time History of Events and Macroscale Interactions during Substorms) is a U.S. Explorer mission flying five small satellites carrying instruments conceived by U.S. scientists in collaboration with French research laboratories. The mission is operating inside Earth’s magnetosphere to study the highly explosive phenomena that trigger polar auroras, which scientists call magnetic substorms. The five THEMIS satellites were orbited together by a Delta II launcher from Cape Canaveral, Florida, on 16 February 2007.

Orbiting at different distances from Earth, the THEMIS satellites line up every four days over North America along the Sun-Earth axis, in the region called the magnetotail. This constellation of satellites has already observed first-hand thousands of substorms and the propagation of particles from the solar wind to Earth’s poles. As a result, scientists have been able to trace the point of origin of the phenomenon to a region of the magnetosphere one-third the distance from Earth to the Moon.

Each satellite is carrying five instruments, notably magnetometers, to which the LPP (Laboratoire de Physique des Plasmas, ex-CETP) and the IRAP (Institut de Recherche en Astrophysique et Planétologie, ex-CESR) contributed. IRAP also helped to define the mission and is involved in data analysis and interpretation.