Orbiting ‘Magnetism to Light Converter’ Maps Earth’s Magnetic Field

Three identical satellites of ESA Swarm mission will move into separate polar orbits to blanket the planet with high-precision magnetic measurements.

The European Space Agency’s Swarm expedition was launched from the Plesetsk Cosmodrome on 22 November, on a mission to study how the Earth’s magnetic field and ionosphere vary in time and space. It started sending back data four days later.

The mission consists of three identical satellites launched into separate polar orbits that will let them sheathe the Earth in a web of magnetic measurement. Swarm will gauge the direction and strength of the planet’s magnetic field more precisely than ever before, using instruments up to five times as sensitive as those deployed on the Danish Øersted (launched 1999) and German CHAMP (2000) satellites. These measurements will return data on every part of the Earth, from the dynamo at the planet's core to the workings of the ionosphere and magnetosphere. It may even, perhaps, explain more about the magnetic “soft spot” that hovers over the South Atlantic (and might presage one of the periodic reversals in the Earth’s magnetic polarity).

Each Swarm spacecraft looks like an elongated horseshoe crab, with a solar-cell-covered carapace and a rapier of a tail. Once unfolded, the tail is a tubular, 4.3-meter-long, carbon-fiber-reinforced polymer boom. It’s manufactured without any magnetic components, because it is carries some of the most advanced magnetic-field-measurement instruments yet built.

About halfway down the boom is an optical bench that couples a three-axis star-tracking telescope with a Vector Field Magnetometer (VFM)—a highly sensitive device for measuring the intensity and orientation of magnetic lines of flux. Overall, the assembly (devised by researchers at the Danish Technical University) is accurate to within about 0.5 nanotesla (0.5 x 10-9 T) in field strength and 0.1 degrees in satellite attitude. The VFM, which is the satellite’s primary instrument, will measure not only the direction and strength of the surrounding magnetic field, but also plot it against positions confirmed by the three-way star-sight.

At the end of the boom is a new design—the Absolute Scalar Magnetometer, (ASM) built by CEA-Leti (Grenoble, France), with scientific support from the Institut de Physique du Globe de Paris and financing and logistics from the Centre National d’Etudes Spatiales (CNES), the French national space agency.

The ASM’s nominal role is to understudy the VFM, helping to keep the vector instrument calibrated. What it actually offers, say the designers, goes a lot farther. (For a collection of papers on ASM’s design, see this CNES library.)

The device uses low-density helium as its sensor, and exploits the Zeeman effect—the splitting of the element’s emission-spectrum lines in a magnetic field.

To measure this spread, the ASM first applies radio frequency energy to lift electrons from their ground state to a metastable intermediate energy level (actually, one of three levels, since the Earth’s magnetic field splits this level into three levels, depending on the combined spins of the atom and its electrons).

Then a linearly polarized laser beam further pumps electrons to an even higher, though very short-lived, excited state. In one-tenth of a microsecond, the electrons drop back into one of the metastable levels, giving off photons and creating three closely grouped spectral lines (clustering at around 1083 nanometers wavelength in the infrared). The gap between the lines is proportional to the ambient magnetic field.

The instrument incorporates a number of refinements. To maintain accuracy, the designers had to maintain a constant relationship between the stimulating laser beam and the applied magnetic fields. By using a beam that’s polarized linearly rather than (as in previous designs) circularly, the designers were ab keep the system aligned by adjusting the direction of polarization rather than the direction of beam propagation—and it’s much easier to change polarization than to move the laser. In the ASM, non-magnetic piezoelectric motors control the orientations of the RF coils.

The net result is that the Absolute Scalar Magnetometer is about ten times as sensitive as the Vector Field Magnetometer, with a maximum error of less than 65 picotesla (65 x 10-12 T). By comparison, that’s about one millionth of the Earth’s surface magnetic field, which ranges from about 30 to 60 microtesla (30-60 x 10-6 T), and about one one-hundred-millionth the magnetic field strength of a common household refrigerator magnet (5 x 10-3 T). The precision should be better than 1 picotesla, and the noise levels are low, better than 1 pT / (Hz)1/2.

Also, the ASM uses three orthogonal sets of RF coils. The device is able to report how much of the scalar field is projected along each of these axes…so, voila, the scalar magnetometer can also function as a vector magnetometer, although at a lower sampling rate and with reduced accuracy. Among other experiments, the Swarm mission will test whether the understudy ASM might be ready to step out from backstage and take on the starring role of the VFM.

Illustrations: ESA 

Modified 18 Dec. 2013 to include correct launch site at Plesetsk.

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