Dany Waller

Dany Waller

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Voted Most Likely To Be Found Evangelizing About Magnets

The Magnetometer Family Tree

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Introduction

Think of the magnetic field at a given point in space as an arrow: its length represents the field’s strength, and the way it points represents the field’s direction. A vector magnetometer measures the field’s components along three perpendicular axes, usually called x, y, and z. Together, these measurements tell us both how strong the field is and which way it points. A scalar magnetometer measures only the total field strength, giving us the arrow’s length without its direction. We can also calculate that scalar strength from vector measurements by squaring the three components, adding them together, and taking the square root, as shown below.

A magnetic field vector shown as a red arrow on three perpendicular axes labeled Bx, By, and Bz, with its magnitude calculated as the square root of Bx squared plus By squared plus Bz squared
The arrow represents the magnetic field vector; its length is the scalar field strength.

Methods

Vector and scalar describe what a magnetometer measures and the instrument type describes how it makes that measurement. Some instruments sense how a magnetic field affects a material, while others detect an electrical signal or the behavior of tiny magnetic moments inside atoms. Here are a few common techniques.

Vector measurements

  • Fluxgate magnetometers use coils to repeatedly magnetize a small magnetic core in opposite directions. The surrounding field makes the core’s response uneven, and the instrument turns that imbalance into a measurement along one axis. Three perpendicular sensing axes give us the full vector, including steady fields and slow changes.

  • Search-coil magnetometers, also called induction magnetometers, measure the voltage produced when a changing magnetic field passes through a coil of wire. Three perpendicular coils measure the changing vector components. These instruments are useful for detecting magnetic waves, but a stationary coil cannot measure a perfectly steady field.

  • Magnetoresistive magnetometers use materials whose electrical resistance changes in response to a magnetic field. The instrument reads that change to measure a field component. Sensing elements arranged along different axes can provide vector measurements in a compact package.

  • SQUID magnetometers use superconducting loops: materials cooled until they carry electrical current without resistance. A magnetic field passing through a sensing loop changes the instrument’s electrical response, allowing it to detect very weak fields. Loops oriented along different axes can measure vector components, although the instrument needs cooling to work.

Scalar measurements

  • Proton-precession magnetometers contain a liquid rich in hydrogen, such as water. A magnetic pulse temporarily aligns the magnetic moments of the hydrogen nuclei, or protons. After the pulse ends, those moments wobble around the surrounding field like spinning tops. The wobble rate, called the precession frequency, tells us the total field strength.

  • Overhauser magnetometers also measure proton-precession frequency, but use radio waves and special molecules to transfer magnetic alignment from electrons to protons. This strengthens the proton signal without needing the strong magnetic pulse used by a conventional proton-precession instrument. The result is still a scalar measurement.

  • Optically pumped magnetometers use light to prepare atoms in a vapor, such as cesium, potassium, or helium, so their magnetic moments respond together. The instrument detects a magnetic resonance through changes in the light passing through the vapor. The resonance frequency tells us the total field strength. Some designs can also recover vector components by applying known magnetic fields and observing how the measurement changes.

The Lost Art of Making Ring Cores

Some branches of the magnetometer family tree share the same stock of magnetic material. Many fluxgate sensors use a ring core made from thin ferromagnetic foil, whose intrinsic magnetic noise limits the instrument’s sensitivity.1

Many instruments have relied on Infinetics S1000 ring cores, which went out of production in 1996. Their manufacturing process grew out of military research in the 1960s and was insufficiently documented to reproduce their performance. Remarkably, virtually all the permalloy used in North American fluxgates appears to have come from a single batch, likely made by the Hamilton Watch Company around 1969. The alloy contained 6% molybdenum, 81.3% nickel, and the remainder iron.1

The resulting instruments kept working, but the supply of new cores did not keep growing. By 2021, NASA reported that stockpiles were so depleted that some providers were considering dismantling old flight-spare hardware to recover its cores. The manufacturing knowledge had been lost to the civilian community, leaving new missions dependent on a shrinking supply of decades-old components.2

Researchers including David Miles and B. Barry Narod published a replacement process: make a new alloy, cold-roll it into foil, insulate it, wind it around a supporting ring, and heat-treat the assembly. Their 2019 study demonstrated new cores with magnetic noise comparable to many legacy S1000 cores, restoring a manufacturing capability needed by future instruments.1

Spacecraft Missions

Authors Note: Boom lengths are hand-wavey! In some cases, the reported value is the approximate distance between the magnetometer sensor and the spacecraft center rather than the physical boom length.

MissionTarget / environmentLaunch yearNumber and typeBoom length / location
Luna 13Lunar flyby / heliocentric19591 triaxial fluxgateSpacecraft-mounted
Luna 104Lunar orbit19661 triaxial fluxgateSpacecraft-mounted
Explorer 355Lunar orbit19672 triaxial fluxgatesGSFC sensor: 2.2 m from spin axis
Pioneer 106Jupiter / interplanetary19721 triaxial helium (HVM)\(\sim 6.5\) m from spacecraft center
Pioneer 117,6Jupiter and Saturn19732: 1 triaxial helium (HVM) + 1 triaxial fluxgate (FGM)HVM: \(\sim 6.5\) m from spacecraft center; FGM: experiment platform
Voyager 1/28Outer planets / heliosphere19774 triaxial fluxgates (2 low-field + 2 high-field)13 m
AMPTE-CCE9Geospace19842 triaxial fluxgates2.3 m
AMPTE-IRM10Geospace19841 triaxial fluxgate2 m
AMPTE-UKS11Geospace19841 triaxial fluxgate1 m
Giotto12,13Comet Halley / interplanetary19852: 1 triaxial fluxgate + 1 biaxial fluxgateAntenna tripod: MAG-1 outboard, MAG-4 inboard; no boom
Galileo14Jupiter system19892 triaxial fluxgates11 m
Ulysses15Solar polar orbit19902: 1 triaxial fluxgate + 1 triaxial helium5.6 m
Mars Global Surveyor16Mars orbit19962 triaxial fluxgatesSolar-array tips; \(\sim 2\)–\(2.5\) m
NEAR Shoemaker17Near-Earth asteroid Eros19961 triaxial fluxgate\(\sim 1\) m
ACE18Sun–Earth L119972 triaxial fluxgates4.19 m from spacecraft center
Cassini19Saturn system19972: 1 triaxial fluxgate + 1 vector/scalar helium11 m
Lunar Prospector20Lunar orbit19981 triaxial fluxgate0.8 m; \(\sim 2.6\) m total structure
Cluster21,22Geospace20003: 2 triaxial fluxgates (FGM) + 1 triaxial search-coil (STAFF)5 m radial booms: FGM and STAFF on opposite booms
MESSENGER23Mercury orbit20041 triaxial fluxgate3.6 m
THEMIS24,25Geospace / Lunar orbit20072: 1 triaxial fluxgate (FGM) + 1 triaxial search-coil (SCM)1.2 m FGM; 1 m SCM
SELENE/Kaguya26,27Lunar orbit20071 triaxial fluxgate12 m
Juno28Jupiter orbit20112 triaxial fluxgates\(\sim 10\)–\(12\) m solar-array boom
MAVEN29Mars orbit20132 triaxial fluxgates0.66 m boomlets; sensors \(\sim 5.6\) m from center
Swarm30,31Earth orbit20133: 1 triaxial fluxgate + 2 scalar helium (primary + backup)4.3 m
MMS32,33Geospace20153: 2 triaxial fluxgates (AFG + DFG) + 1 triaxial search-coil (SCM)5 m FGM booms; SCM 4 m along the AFG boom
BepiColombo/MPO-MAG34Mercury orbit20182 triaxial fluxgates2.8–2.9 m
BepiColombo/Mio-MGF35Mercury orbit20182 triaxial fluxgates4.4 m
Parker Solar Probe36Solar corona / heliosphere20183: 2 triaxial fluxgates + 1 triaxial search-coil3.5 m
Solar Orbiter37Solar orbit20202 triaxial fluxgates4.4 m
KPLO/Danuri38Lunar orbit20223 triaxial fluxgates1.2 m
JUICE39,40Jupiter system20233: 2 triaxial fluxgates + 1 scalar rubidium10.6 m
Europa Clipper41Jovian system / Europa20243 triaxial fluxgates7.9–8.5 m

Counts refer to sensor heads per spacecraft within the cited investigations, including backup heads.

Giotto’s dust shield was designed to protect the spacecraft during its fast encounter with Comet Halley, so an exposed magnetometer boom was not included. Both sensors were mounted on the antenna tripod and comparing their readings helped assess interference from the spacecraft’s own magnetic fields. Giotto carried two magnetometer sensor heads: the outboard triaxial MAG-1 and the inboard biaxial MAG-4. MAG-2 was the electronics box. The instrument paper’s hardware inventory does not mention MAG-3, I have not found a satisfying source for why they were named this way.

References

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  2. NASA Science Editorial Team 2021, Rediscovering the Lost Art of Fluxgate Magnetometer Cores, July 6, NASA technology highlight ↩

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