THEMIS
THEMIS (Time History of Events and Macro Scale Interactions falling on substorms ) is a NASA program, which consists of five small satellites that explore the substorms in the magnetosphere of the Earth. The two-year mission will first examine in detail the path of the charged particles of the solar wind into the Earth's atmosphere, to find out what the triggers substorms and so the auroras can occur. After launching the satellites received the additional designations Explorer 85 - 89
The THEMIS program is managed by the Goddard Space Flight Center of NASA, the scientific project management at Space Sciences Laboratory, University of California lies. The total costs are estimated by NASA with 200 million U.S. dollars.
THEMIS was decided as part of the Explorer program, NASA on 20 March 2003. Two weeks later received the American air and space company Swales Aerospace, headquartered in Beltsville (Maryland) chosen as the prime contractor for the five small satellites. After a one-year development phase began in June 2004 with the construction of the probes and the instruments.
The support structure of the satellite was built by Swales on the U.S. east coast, while the University of California was responsible for adaptation and installation of the instruments after they have been supplied by the respective institutions. End of November 2005 met the first probe body in a Berkeley - seven months later the last. After the integration of all 25 instruments, the satellites were tested by the Jet Propulsion Laboratory for several weeks on their suitability for space. Then the missiles were packed and brought to the starting point to Florida, where they arrived on 8 December 2006.
Mission Objectives
Although the Swedish physicist Anders Ångström was able to prove in 1867 that it is at the northern lights to self-luminous gas and not to light reflections, whose origin is still not clear. In science, there are two competing models, where exactly caused the geomagnetic storms that trigger the natural spectacle, and as on the physical processes look like in detail.
According to the CDM theory ( Current Disruption model ) performs a local instability in the plasma of the sun opposite side at a distance of about ten Earth radii ( 57,400 km from Earth ) to an interruption in the tail current of the magnetosphere, which is derived on the tagseitige ionosphere. This leads to a " rarefaction wave ", which ( 153,072 km ) to a sudden change in the magnetic field structure in the tail at a distance of 25 Earth radii - called magnetic reconnection - leads.
The NENL model ( Near-Earth Neutral Line ) assumes that in 25 earth radii a Rekonnexionsprozess triggers a substorm that transports large amounts of energy into the inner magnetosphere. These are converted there into thermal energy which is conducted along the plasma boundary layer about the earth. In the loop of Teilchensturms in doing so the currents along the field lines of the geomagnetic field.
With THEMIS basic research is carried out, which does have references to life on Earth. Because the geomagnetic storms in the atmosphere damage satellites, disrupt radio and television broadcasts, skipped grids - in March 1989 caused a violent Teilchensturm within a few seconds in Eastern Canada a breakdown of energy supply, which comprised half the province of Quebec - and lead to high radiation doses in aircraft occupants.
Structure of the satellite
THEMIS consists of five identical satellites that are equipped with five identical instruments. Each probe has a mass of 128 kg of which 26 kg accounting for scientific equipment, and is spin stabilized with 16/min. The energy is supplied via solar cells that are mounted on the satellite.
The satellites are square flat prisms are provided with a total of eight antennas. These are very long, to prevent the satellite itself influences the measurements: are located on all four sides of the THEMIS satellites wire antennas that are streamlined through the centrifugal force and at the ends of sensors are mounted. The two antennas have a length of 20 m, the other pair of 25 m. In addition, there are two three -meter-long telescopic antennas are arranged axially. These six antennas are part of the EFI instrument. In addition to two corners of each satellite, a boom for the FGM ( 2 m) and the SCM (1 m) are attached.
Instruments
- EFI (Electric Field Instrument ): EFI observed the changes in the electric field of the magnetosphere. The instrument records the difference of the electric voltage, which measure the different sensors on the tips of the six antennas and thus determines the spatial distribution and movement of the fields. By comparing the measurements of the five satellites, it is possible to determine the exact extent of the fields. EFI was developed by the University of California in collaboration with the University of Colorado.
- SCM ( search coil magnetometer): SCM is a contribution of the French Centre des environements Terrestre et Planétaires. The instrument has a mass of 2.0 kg and measures the spatial variation of magnetic fields. The low frequency changes ( below 0.1 Hz) in the magnetosphere should play an important role in the formation of substorms.
- FGM ( Flux Gate Magnetometer ): With an accuracy of 0.01 nanotesla the FGM measures ( total mass 1.54 kg ), the magnetic flux density of the magnetosphere, which indicate changes in incipient Teilchenstürme. As sensor serve two toroids, which are periodically magnetized to saturation. Registers the FGM an external magnetic field, this as a " second harmonic " of the excitation signal is measured. This instrument was built by German researchers. The FGM has been developed by the Institute for Geophysics and Extraterrestrial Physics, Technical University of Braunschweig under the direction of Karl -Heinz Glaßmeier and is a development of FGMs, the already successful on Venus Express, Cluster, Cassini -Huygens, or the Rosetta comet mission ( orbiter and the Philae lander ) were used.
- ESA (electrostatic analyzer): ESA 's mission to measure the distribution of electrically charged particles between 3 eV and 30,000 eV. The number of ions and electrons as well as the direction thereof is registered. The scientists obtained by these parameters not only speed and density of the plasma, but also its temperature. By comparisons with the measurements of all five ESA instruments will make it possible to understand the mechanism that triggers the Teilchenstürme. ESA was developed by the Space Physics Research Group at the Space Sciences Laboratory.
- SST (Solid State Telescope ): This third unit of the University of California, Berkeley, measures the exact density of the electrically charged particles and the direction from which they strike the probe. The SST measures the ion and electron distribution in the vicinity of the probe in a range between 25 keV and 6 MeV. These particles have a higher energy potential than the rest of the dipolar magnetosphere and play an important role in the heating and motion of the plasma. The instrument consists of two side- mounted SST devices, each of which three particle detectors ( built by the Lawrence Berkeley National Laboratory ) equipped. One side of the SST can be a magnet, only the positively charged ions to pass, while a metal film on the opposite side only transmits electrons.
Ground-based observation
The THEMIS mission will be complemented by observations from Earth. A total of 20 ground stations in Canada ( 16) and Alaska ( 4) were equipped with automatic cameras ASI ( All- Sky Imager ). These are housed in glass domes and have highly sensitive CCD photo sensors and fisheye lenses, so they have a field of view from horizon to horizon. Every three seconds, is made in visible light a black and white picture of the sky.
In addition, were set up at 21 locations in Canada and the northern United States Flux Gate Magnetometer: ten units in the ASI cameras and eleven of these so-called GMAG instruments in schools. The magnetometer, the operation of which complies with the FGMs on the satellite, are a development of University of California at Los Angeles and measure the interference of the ground-level geomagnetic field. So auroras can be detected, even if the ASI cameras due to heavy cloud cover record anything. In each case, a specially trained teacher who participates in this annual seminars, is responsible at his school for the GMAG. This would oversee operations, plans project-related activities and analyzes the data, together with pupils. All participating schools, teachers and students are members of the geons network ( Geomagnetic Event Observation Network by Students) and exchange their experiences and results with each other via the Internet.
Mission History
The launch took place on 17 February 2007, at 23:01 UTC (two days before the proclaimed by the United Nations International Heliophysical year began ). A Delta II 7925 -10C rocket took off with all five THEMIS satellites on board from launch pad 17B of the Cape Canaveral Air Force Station. Never before have so many research satellites were brought into orbit with a single rocket.
The initial orbit of all THEMIS probes was initially 1.073 to 14.697 Earth radii, corresponding to 470 km and 87,630 km above the earth's surface, at an inclination of 15.7 °. Thus, the railway was to 4200 km lower than calculated in the apogee. The individual satellites are not proper names, but are spelled from A to E.
In order to prevent that the four belonging to the EFI instrument wire antennas fall into each other while being rolled, the ground control reduced the rate of revolution of the satellites. For this, the frequency of 16/min was the end of February 2007 Reduced for two days at 5 rpm. After completion of the maneuver, the rate was temporarily increased to 20 rpm, so that the wire antennas could fully unfold.
The first scientific test of the entire THEMIS system took just five weeks after the start: On 23 March 2007, on the North American continent Teilchensturm occurred, its dynamics surprised the researchers. The Northern Lights, have been observed for two hours both of the satellite and from the ground cameras, moving twice as fast as expected - the luminous phenomena withdrawn at a rate of 15 degrees of longitude per minute westward. Vassilis Angelopoulos, the scientific director of the THEMIS program at the University of California, explained that the Teilchensturm had acted totally unexpected. The released energy equivalent to estimates Angelopoulos ' an earthquake of magnitude 5.5.
Using the satellite THEMIS the existence of magnetic tapes could be detected. On May 20, the THEMIS satellites observed a magnetic tape that about 65,500 km above the earth's surface was, and had an area of approximately 13,000 km. The bands are formed in this magnetopause mentioned area where the solar wind meets the Earth's magnetic field. Other satellites had already provided evidence of this twisted magnetic fields into each other, but could not collect reliable data on its structure.
The THEMIS satellites discovered in June 2007, two temporary holes in the Earth's magnetosphere.
Phases of flight
The two-year mission is divided into several sections: First, the satellites were tested, the experiments enabled (so that was begun five days after the start) and calibrated. The official science operations began with the " Coast " phase on 1 July 2007.
From September 2007, the orbits of the five probes were changed. During the erdnächste point remains unchanged, the final orbits differ in apogee ( apogee ) differ considerably: a satellite has its apogee at 10 Earth radii ( 57,400 km ), two at 12 radii ( 70,160 km ), the fourth has a position of erdfernste 20 Earth radii ( 121,180 km ) and the last one of 30 radii ( 184,960 kilometers ). Since all five satellites are identical, positioning phase was only decided during this, which probe flies on which track.
On 4 December 2007 which began " Tail Science Phase". For four months, data are collected in the tail of the magnetosphere. To this end, the THEMIS satellites are run through their apogee points in the Earth's shadow while.
In mid-April 2008, the paths of the satellites quintet changed so that they reach their Apogäumspunkt at a 90 ° angle of the Sun-Earth system. Your highly elliptical orbits are created for eight weeks in their longitudinal axis along the direction of rotation of the earth around the sun.
The conclusion is the " Dayside Science Phase," which extends from June through October 2008. As with a solar eclipse, the longitudinal axes of the satellite orbits are aligned so that all probes are at apogee between Earth and the Sun.
The trajectories are chosen so that all five satellites are every fourth day for around 15 hours in the extended Sun-Earth line. This makes it possible to perform coordinated measurements at the two points that are supposed to be responsible for the formation of part of the geomagnetic storms.
Advanced Mission
In May 2008, the mission was extended to 2012. THEMIS B and THEMIS C was maneuvered under the name ARTEMIS P1 and P2 to the Earth-Moon libration points. Objective of the mission is the study of space weather. 2010 a hitherto unknown natural phenomenon, a space quake was discovered. On 27 June 2011 ARTEMIS P1 entered an eccentric lunar orbit, ARTEMIS P2 followed on 17 July 2011.