Fluxgate Magnetometer

A sliver of permalloy driven in and out of magnetic saturation sixty thousand times a second — measuring fields ten billion times weaker than a fridge magnet.

A fluxgate magnetometer measures the vector components of a static or slowly-varying magnetic field — amplitude, sign, and direction — without any moving parts and down to frequencies as low as DC. It can resolve fields of one nanotesla or less against a background of up to tens of microteslas, making it the instrument of choice for geomagnetic surveys, submarine detection, spacecraft attitude sensing, and every interplanetary mission that needs to characterise a planetary magnetic field. Three sensors aligned orthogonally give the full three-dimensional field vector. The technique works equally well in a rugged field-portable probe, a deep-sea ocean floor sensor, or a space-qualified instrument weighing a few hundred grams on a boom extending from a spacecraft.

At the heart of each sensor axis is a core of high-permeability magnetic alloy (typically 6-81 permalloy: 81% nickel, 6% molybdenum, remainder iron) wound with two coils. A drive coil periodically drives the core into magnetic saturation with an alternating current, switching it back and forth at a carrier frequency, typically 10 kHz. When the ambient magnetic field is zero, the core saturates symmetrically in each half-cycle and the sense coil sees only even harmonics of the drive frequency. When an ambient field is present it biases the saturation, breaking the symmetry: the core saturates earlier in one half-cycle than the other. This asymmetry generates a second-harmonic signal in the sense coil whose amplitude is proportional to the ambient field component along the core's axis. A feedback winding that cancels the field at the core converts this to a ratiometric measurement, giving excellent linearity. The entire chain from core to demodulated output is a DC-coupled, phase-sensitive detection scheme whose noise floor is set by the intrinsic magnetic noise of the permalloy core material, typically 6–10 pT/√Hz at 1 Hz for high-quality commercial sensors.

In plain terms

Imagine you have a sponge that can only soak up a fixed amount of water. In a dry room it fills up slowly; in a humid room it fills up faster. A fluxgate magnetometer works like that sponge: it fills up with magnetism faster or slower depending on the surrounding magnetic field, and by timing exactly when it saturates you can tell how strong the field is — to within a fraction of the Earth's own field.

Also called: fluxgate sensor, saturable-core magnetometer, MAG instrument, vector magnetometer.

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A short history

The fluxgate principle was first applied in a practical instrument by Victor Vacquier and colleagues at Gulf Oil Company around 1939–1941, motivated by oil exploration and, during World War II, by the US Navy's need to detect submerged submarines from the air. The Navy flew Vacquier's Magnetic Airborne Detector (MAD) in patrol aircraft from 1942 onwards. In the 1950s and 1960s, oceanographers fitted fluxgate sensors to research ships to map the seafloor magnetic field, producing the striped anomaly patterns that provided crucial evidence for plate tectonics and seafloor spreading (Vine–Matthews–Morley hypothesis, 1963). Space applications followed almost immediately: Mario Acuña at NASA Goddard Space Flight Center developed a fluxgate heritage that placed instruments on more than fifty planetary missions, including Mariner, Pioneer, Voyager, Cassini, MAVEN, and MMS. The Voyager 1 and 2 fluxgate magnetometers, launched in 1977, remain the longest continuously-operating scientific instruments in the history of space exploration, still measuring magnetic fields in interstellar space.

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