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Description of the mission
Time History of Events and Macroscale Interactions during Substorms (THEMIS) aims to resolve one of the oldest mysteries in space physics, namely the origin of magnetospheric substorms. These violent eruptions are at the origin of the aurora that are observed in the polar regions.
To understand the mystery of substorms the following points are sudied:
- The outbreak of the substorms
- The interaction of the different structures involved during substorms
- The activation of aurora by substorms
- The relationship between local disturbances of currents in the tail and substorms.
THEMIS is the first space mission with five spacecraft. They were launched by a single rocket in February 2007. The mission was originally scheduled to last two years. It is currently extended by NASA until 2015.
An Earth-space collaboration
The originality of THEMIS is the fact that it is made up of five satellites co-ordinated with a network of ground-based instruments (magnetometers and all-sky cameras). This structure offers the possibility to characterize the start of a magnetospheric substorm by observing its triggering watching both with in situ measurements in the magnetotail and on the ground in the auroral regions.
The probes were placed on three different equatorial orbits. They have different apogees. Three satellites have an apogee at 10 Earth radii (RT), one at 20RT, and the last around 30RT. When satellites simultaneously reach their apogee, they are located in the tail of magnetosphere. We can then determine the time of onset of substorms. It becomes possible to characterize the acceleration region. Conjunctions between the five probes and the network of ground-based instruments are held every four days.
During these conjunctions, in situ measurements can be related to measurements made by the ground networks (all sky cameras and magnetometers). Therefore scientists have access simultaneously to in situ measurements in the acceleration region and to the image of this region. The image is formed in the upper atmosphere by precipitation of accelerated particles. It is the image of the aurorae borealis. The ground networks are located in Canada and the northern United States.
The other lives of THEMIS
The strategy of the mission was to have five satellites in orbit around the Earth. In 2010, plans were changed. Two satellites have been repositioned in orbit around the moon. They are the ARTEMIS mission. The first objective is to explore the wake of the moon in the solar wind. The second is to study the dynamics of the distant magnetotail (at 60RT).
In August 2012, the Mission RBSP (Radiation Belt Storm Probes) renamed Van Allen probes was launched by NASA. It aims to analyze the radiation belts of our magnetosphere. The radiation belts are characterized by a high energy dense plasma, so they are potentially very dangerous regions for the onboard electronics of spacecraft or manned flights. They are located at the geomagnetic equator. Coordinated studies are planned between RBSP and the three remaining THEMIS probes.
For more information about the French participation : THEMIS (French participation)
Image : Representation the 5 spacecraft of THEMIS in the tail of the Earth magnetosphere Source : http://www.nasa.gov/mission_pages/themis/news/speedy-particles.html
