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THEMIS (satellite)
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The THEMIS mission uses a constellation of five NASA satellites to study energy releases from Earth's magnetosphere known as substorms, magnetic phenomena that intensify auroras near Earth's poles. The name of the mission is an acronym for Time History of Events and Macroscale Interactions during Substorms, alluding to the Titan, Themis.
The THEMIS satellites were launched February 17, 2007 from Cape Canaveral Air Force Station Launch Complex 17 aboard a Delta II rocket.
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Encyclopedia
The THEMIS mission uses a constellation of five NASA satellites to study energy releases from Earth's magnetosphere known as substorms, magnetic phenomena that intensify auroras near Earth's poles. The name of the mission is an acronym for Time History of Events and Macroscale Interactions during Substorms, alluding to the Titan, Themis.
The THEMIS satellites were launched February 17, 2007 from Cape Canaveral Air Force Station Launch Complex 17 aboard a Delta II rocket. Each satellite carries identical instrumentation, including a fluxgate magnetometer (FGM), an electrostatic analyzer (ESA), a solid state telescope (SST), a search coil magnetometer (SCM) and an electric field instrument (EFI). Each has a mass of 126 kg, including 49 kg of fuel.
Launch
THEMIS was originally scheduled to launch on 19 October 2006. Owing to delays caused by workmanship problems with Delta II second stages -- an issue that also affected the previous mission, STEREO -- the THEMIS launch was delayed to Thursday, February 15, 2007. Due to weather conditions occurring on Tuesday, February 13, fueling of the second stage was delayed, and the launch pushed back 24 hours. On February 16, the launch was scrubbed in a hold at the T-4 minute point in the countdown due to the final weather balloon reporting a red, or nogo condition for upper level winds. A 24-hour turnaround procedure was initiated, targeting a new launch window between 23:01 and 23:17 GMT on 17 February.
Favourable weather conditions were observed on 17 February, and the countdown proceeded smoothly. THEMIS successfully launched at 6:01 p.m. EST. The spacecraft separated from the launch vehicle approximately 73 minutes after liftoff. By 8:07 p.m. EST, mission operators at the Space Sciences Laboratory (SSL) of the University of California, Berkeley, commanded and received signals from all five spacecraft, confirming nominal separation status.
The launch service was provided by the United Launch Alliance through NASA Launch Services Program (LSP).
Mission Status
From February 15,2007 until September 15,2007 the five Themis satellites coasted in a string of pearls orbital configuration. From September 15,2007 until December 4,2007 the satellites were moved to more distant orbits in preparation for data collection in the magnetotail. This phase of the mission was called the "Dawn Phase" because the satellites' orbits were in apogee on the dawn side of the magnetosphere. On December 4,2007 the first tail science phase of the mission began. In this segment of the mission scientists will collect data from the magnetotail of the earth's magnetosphere. During this phase the satellites' orbits are in apogee inside the magnetotail. The scientists hope to observe substorms and magnetic reconnection events. During these events charged particles stored in the Earth's magnetosphere are discharged to form the aurora borealis. Tail science is performed in the winter of the northern hemisphere because the ground magnetometers that Themis scientists correlate the satellite data with have relatively longer periods of night. During the night, observations are not interrupted by charged particles from the Sun.
In 2007, THEMIS "found evidence of magnetic ropes connecting Earth's upper atmosphere directly to the sun," reconfirming the theory of solar-terrestrial electrical interaction (via "Birkeland currents" or "field-aligned currents") proposed by Kristian Birkeland circa 1908. NASA also likened the interaction to a "30 kiloVolt battery in space," noting the "flux rope pumps 650,000 Amp current into the Arctic!"
On 26 February 2008, THEMIS probes were able to determine, for the first time, the triggering event for the onset of magnetospheric substorms . Two of the five probes, positioned approximately one third the distance to the moon, measured events suggesting a magnetic reconnection event 96 seconds prior to Auroral intensification. Dr. Vassilis Angelopoulos of the University of California, Los Angeles, who is the principal investigator for the THEMIS mission, claimed, "Our data show clearly and for the first time that magnetic reconnection is the trigger.".
Extended mission
On May 19, 2008 the Space Sciences Laboratory (SSL) at Berkeley announced NASA had extended the THEMIS mission to the year 2012. In addition a new mission that would send two THEMIS probes into lunar orbits was provisionally approved by NASA, pending a technical review before February 2009.
Instruments
Aboard the spacecraft:
- Instrument Data Processing Unit (IDPU): The IDPU houses most of the electronics for the instruments on the THEMIS spacacraft.
- Electric Field Instruments (EFI): The EFI is designed and built to sense the electric field in Earth's ever-changing magnetosphere
- Flux Gate Magnetometer (FGM): The FGM measures the background magnetic field to identify and time the abrupt reconfigurations of the magnetosphere during substorm onset.
- Search Coil Magnetometers (SCM): The SCM measures low frequency magnetic field fluctuations and waves in three directions in Earth's magnetosphere.
- Electrostatic Analyzer (ESA): The ESA measures thermal electrons and ions to identify and track high-speed flows through the magnetotail and identify pressure pulses.
- Solid State Telescope (SST): The SST measures energetic particle distribution functions.
- Digital Fields Board (DFB): The DFB uses an FPGA to perform configurable on board band pass processing and fast Fourier transformations(FFT) on instrument data.
Ground based:
As the satellites monitor the magnetosphere from orbit, twenty ground stations in North America simultaneously monitor auroras. Ground station mission and science operations are being managed by the University of California Space Sciences Laboratory.
- Ground-Based All-Sky Imager (ASI) Array: The ground-based All-Sky Imager (ASI) array observes the aurora over the Northern American continent from Canada to Alaska in order to determine where and when the auroral substorm onset occurs.
- Ground-Based Magnetometer (GMAG) Array: The GMAG Measure changes in Earth’s magnetic field near Earth’s surface due to substorm onset to help determine the timing of substorm events.
Swales Aerospace
Swales Aerospace, now Alliant Techsystems Inc., (Beltsville, Maryland) manufactured all five probes for this mission. Each was built-up and tested at the Beltsville facility, before being delivered to the University of California, Berkeley for instrument integration. Swales was responsible for integrating the BAU, IRU, Solar Arrays, Antenna, Battery, and other components necessary for functionality. This was the second major satellite built by Swales, the first being the EO-1 spacecraft, which continues to orbit the Earth. Swales was also responsible for designing and building the Electrical Ground Support Equipment (EGSE) used for monitoring the probes during all phases of pre-launch activities, including use at the launch site.
JPL
Testing after the installation of instruments at SSL, Berkeley, including vacuum testing, was done at NASA's Jet Propulsion Laboratory in Pasadena, California.
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