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January 30, 2007
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One out of four Spin Plane Boom (SPB) units of the Electric Field Instrument (EFI), shown connected to the IDPU. Four such boom units are flown per spacecraft. When in orbit, the boom units un-reel a 25m wire boom with a sphere at its end, to measure the voltage locally. Differences in voltage between opposite spheres provide a one-dimensional measurement of the electric field. The four booms provide 2 electric field components on the spin plane, from ambient electric field to its oscillations up to high frequencies (8kHz).

Mass: 13 kg
Average Power: 0.24 W for sensors.
Development Institution: Sensors - Space Sciences Laboratory, University of California, Berkeley. Data Processing - Laboratory for Atmospheric and Space Physics (LASP), University of Colorado, Boulder.
EFI Leads: J. Bonnell and F. Mozer, UCB; Robert Ergun, LASP
Purpose: Measure electric fields in Earth's dynamic magnetosphere to determine plasma flows and electromagnetic waves associated with substorm onset.

The THEMIS Electric Field Instrument (EFI) is designed and built to sense the electric field in Earth's ever-changing magnetosphere. The EFI will provide the observations needed to determine the motion of the electrified gas (plasma) as it travels past each probe. This is the first time a mission will measure these motions with five probes aligned in the magnetotail. This is vital to determining the time and location of the high-speed flows that begin at substorm onset in the Earth's magnetotail.

Both electric and magnetic field variations are seen in association with substorm onset. These "Electromagnetic Fluctuations" cover a range of wave frequencies from several octaves below the lowest key on the piano, up to just beyond the frequencies at the top of the piano (8 kHz) and a range just below the AM band (100-400 kHz). The THEMIS EFI measures this wide range of frequencies, allowing scientists to see the flows of electromagnetic energy associated with the substorm, its onset, and aftermath, and to determine the significance of those flows for substorm onset itself.

Using four, approximately 25-meter long wire cables and two 3-meter long stiff telescopic booms on each probe, THEMIS is able to measure the electric fields in three independent directions. This 3-D measurement helps to determine the direction of plasma flows and wave motions. To make such a 3-D measurement, the four long wire EFI cables are equally spaced around the spinning probe with sensors on the end of each cable. As the probe spins, the force of the spinning pulls the cables and sensors out into space. One pair of sensors is deployed to 20 meters, the other pair to 25 meters. The two stiff telescopic booms extend sensors perpendicular to the four spinning cables and along the probe's spin axis.

The electric field instrument works by measuring the voltage difference between two sensors, similar to a battery tester or voltmeter. Large voltages in Earth's magnetosphere can drive fast flows of the plasma in space. Small voltages drive slower flows of plasma. The two electric field sensors must be far from the body of the probes so that the electrical properties of the probe do not interfere with these voltage measurements. THEMIS meets this criterion for the first time on five identical probes.

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