Flight unit of the FluxGate Magnetometer (FGM) instrument (silver-colored cylinder at the foreground) and of the Search Coil Magnetometer (SCM) instrument (3 black colored, orthogonal rods at the background) mounted on their carbon-composite booms. The combination of these instruments measures the ambient magnetic field and its oscillations up to high frequencies (4kHz).
Mass: 1.54 kg (including boom, cable and blankets)
Average Power: 0.85W
Development Institutions: Technical University of Braunschweig, (TU-BS), Germany
FGM Leads: K. H. Glassmeier, TU-BS
Purpose: Measures the background magnetic field to identify and time the abrupt reconfigurations of the magnetosphere during substorm onset.
The Flux Gate Magnetometer (FGM) measures the background magnetic field and any low frequency (to 64 Hz) fluctuations superimposed upon it. The instrument will be used to identify and time the abrupt reconfigurations of the magnetospheric magnetic field that occur at substorm onset. For the first time, this measurement will be made on five satellites, all five THEMIS probes, aligned during substorm onset.
At the locations in the magnetotail where substorm onsets occur, magnetic field strengths are typically on the order of several tens of nanoTesla (nT). The amplitude of variations in the field range from about 0.1 to 30 nT. For comparison, the strength of the Earth's magnetic field at its surface is up to 60,000 nT and the strength of a standard bar magnet is more than ten times higher than the Earth's field. The FGM is highly sensitive: it can detect variations in the magnetic field within the accuracy of 0.01 nT. This type of instrument has been flown on many space missions before, but the THEMIS FGM uses an updated technology developed in Germany that digitizes the sensor signals directly and replaces the analog hardware by software. Using the digital fluxgate technology results in lower mass of the instruments (sensor 80 gr.: electronics 150 gr.) and improved robustness which both are important when launching five probes with identical instrumentation on the same spacecraft.
The FGM sensor is about half the size of a soda can. Inside the metal casing there are two small rings, 'ring cores' made from a material that can be magnetized easily. Copper wire is wound around these cores and an alternating electrical current is driven through the copper wire. This alternating current causes an alternating magnetic field around the cores. The external magnetic field, the one in space that is being measured, affects the symmetry of the magnetic field that is generated in the ring cores. Second sets of wires are wound around the cores and these are used to sense this magnetic field. From this it is possible to calculate the magnetic field in space.
The development of the instrument was done with close collaboration with UC Berkeley and Institut fuer Weltraumforschung, (IWF) in Graz, Austria. UCB provided the electronic parts and the boom for each spacecraft and IWF contributed to the instrument design and calibration.