Spacecraft magnetometers

Magnetometers are one of the most widely used scientific instruments in exploratory and observation satellites. These instruments were instrumental in the discovery of the Van Allen radiation belts around Earth, discovered on the very first rocket mission (now called Explorer 1). They have detailed the magnetic fields of the Earth, Moon, Sun, Mars, Venus and other planets. There are ongoing missions using magnetometers, including attempts to define the shape and activity of Saturn's core.

The first spacecraft borne magnetometer was placed on the Sputnik-3 spacecraft and the most detailed observations of the Earth have been done on Magsat. [ [http://www-ssc.igpp.ucla.edu/personnel/russell/ESS265/History.html History of Vector Magnetometers in Space] ] and Ørsted. Magnetometers were taken to the Moon during the later Apollo missions. Many instruments have been used to measure the strength and direction of magnetic field lines around Earth and the solar system.

Spacecraft magnetometers basically fall into three categories: fluxgate, search-coil and ionized gas magnetometers. The most accurate magnetometer complexes on spacecraft contain two separate instruments, with a helium ionized gas magnetometer used to calibrate the fluxgate instrument for more accurate readings. Many later magnetometers contain small ring-coils oriented at 90° in two dimensions relative to each other forming a triaxial framework for indicating direction of magnetic field.

Magnetometer types

Magnetometers for non-space use evolved from the 19th to mid 20th century, when they were first employed in space flight. A main constraint on magnetometers used in space is the availability of energy and weight. Magnetometers have fallen into 3 major categories: the fluxgate type, search coil and the ionized vapor magnetometers. The newest type is the Obeurhauser type based on nuclear magnetic resonance technology.

Fluxgate magnetometers

Fluxgate magnetometers are used for their electronic simplicity and low weight. There have been several types of fluxgate used in spacecraft, which vary in two regards. Primarily better readings are obtained with three magnetometers, each pointing in a different direction. Some spacecraft have instead achieved this by rotating the craft and taking readings at 120° intervals, but this creates other issues. The other difference is in the configuration, which is simple and circular.

Magnetometers of this type were equipped on the "Pioneer 0"/Able 1, "Pioneer 1"/Able 2, Ye1.1, Ye1.2, and Ye1.3 missions that failed in 1958 due to launch problems. The Pioneer 1 however did collect data on the Van Allen belts. Asif A. Siddiqi [http://history.nasa.gov/monograph24/1958.pdf 1958. Deep space chronicle. A Chronology of Deep Space and Planetary Probes 1958–2000] History. NASA.] In 1959 the Soviet "Luna 1"/Ye1.4 carried a three-component magnetometer that passed the moon en-route to a heliocentric orbit at a distance of convert|6400|mi|km, but the magnetic field could not be accurately assessed. Eventually the USSR managed a lunar impact with "Luna 2", a three component magnetometer, finding no significant magnetic field in close approach to the surface. Explorer 10 had an abbreviated 52 hr mission with two fluxgate magnetometers on board. During 1958 and 1959 failure tended to characterize missions carrying magnetometers: 2 instruments were lost on Able IVB alone. In early 1966 the USSR finally placed Luna 10 in orbit around the moon carrying a magnetometer and was able to confirm the weak nature of the moon's magnetic field. Venera 4, 5, and 6 also carried magnetometers on their trips to Venus, although they were not placed on the landing craft.

Vector sensors

The majority of early fluxgate magnetometers on spacecraft were made as vector sensors. However, the magnetometer electronics created harmonics which interfered with readings. Properly designed sensors had feedback electronics to the detector that effectively neutralized the harmonics. Mariner 1 and Mariner 2 carried fluxgate-vector sensor devices. Only Mariner 2 survived launch and as it passed Venus on December 14th, 1962 it failed to detect a magnetic field around the planet. This was in part due to the distance of the spacecraft from the planet, noise within the magnetometer, and a very weak Venusian magnetic field. Pioneer 6, launched in 1965, is one of 4 Pioneer satellites circling the sun and relaying information to Earth about solar winds. This spacecraft was equipped with a single vector-fluxgate magnetometer.

Ring core and spherical

Ring core sensor fluxgate magnetometers began replacing vector sensor magnetometers with the Apollo 16 mission in 1972, were a three axis magnetometer was placed on the moon. These sensors were used on a number of satellites including Magsat, Voyager, Ulysses, Giotto, AMPTE. The Lunar Prospector-1 uses ring-coil made of these alloys extended away from each other and its spacecraft to look for remnant magnetism in the moons 'non-magnetic' surface. [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1998-001A-05 Lunar Prospector Magnetometer (MAG)] National Space Science Data Center, NASA] cite journal |author=Konopliv AS, Binder AB, Hood LL, Kucinskas AB, Sjogren WL, Williams JG |title=Improved gravity field of the moon from lunar prospector |journal=Science (journal) |volume=281 |issue=5382 |pages=1476–80 |year=1998 |month=September |pmid=9727968 |doi= |url=http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=9727968] . The metal alloys that form the core of these magnetometers has also improved since Apollo-16 mission with latest using advanced molybdenum-pemalloy alloys, producing lower noise with more stable output. [ [http://mgs-mager.gsfc.nasa.gov/instrument.html The MGS Magnetometer and Electron Reflectometer] Mars global surveyor, NASA]

earch-coil magnetometer

Search-coil magnetometers are wound coils around a core of high magnetic permeability. Search coils concentrate magnetic field lines inside the core along with fluctuations. [ [http://www.nasa.gov/mission_pages/themis/spacecraft/SCM.html Search Coil Magnetometers (SCM)] THEMIS mission. NASA] . The benefit of these magnetometers is that they operate on alternating current and so can resolve changes in magnetic fields quickly, many times per second. The Pioneer 5 mission finally managed to get a working magnetometer of this type in orbit around the sun showing that magnetic fields existed between Earth and Venus orbits. [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1960-001A-02 Magnetometer - Pioneer 5 mission] ] A single magnetometer was oriented along the plane perpendicular to the spin axis of the space craft. Search coil magnetometers have become increasingly more common in Earth observation satellites. A commonly used instrument is the triaxial search-coil magnetometer. Orbiting Geophysical Observatories (OGO missions - OGO-1 to OGO-6) [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1964-054A-01 Search coil magnetometer - OGO1 mission] , National Space Science Data Center, NASA] [Frandsen, A. M. A., Holzer, R. E., and Smith, E. J. "OGO Search Coil Magnetometer Experiments". (1969) IEEE Trans. Geosci. Electron. GE-7, 61-74.] The Vela (satellite) mission used this type as part of a package to determine if nuclear weapons evaluation was being conducted outside earth's atmosphere. [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1964-040A-08 Search coil magnetometers - Vela2A mission] National Space Science Data Center, NASA] In September 1979 a Vela satellite collected evidence of a potential nuclear burst over the South Western Indian Ocean. In 1997 the US created the FAST that was designed to investigate aurora phenomena over the poles. [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1996-049A-04 Tri-Axial Fluxgate and Search-coil Magnetometers - FAST Mission] National Space Science Data Center, NASA] And currently it is investigating magnetic fields at 10 to 30 Earth radia with the THEMIS satellites [ [http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=2007-004A Search coil magnetometer - Themis-A] National Space Science Data Center, NASA] THEMIS, which stands for "Time History of Events and Macroscale Interactions during Substorms" is an array of five satellites which hope to gather more precise history of how magnetic storms arise and dissipate. [ [http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=2007-004A Themis-A] National Space Science Data Center, NASA]

Ionized gas magnetometers

Heavy metal — scalar

Certain spacecraft, like Magsat are equipped with scalar magnetometer. The output of these device, often in out frequency, is proportional to the magnetic field. The Magsat and Grm-A1 had cesium-vapor (Cesium-133) sensor heads of dual-cell design, this design left two small dead zones. Explorer 10 (P14) was equipped with a rubidium vapor magnetometer, presumably a scalar magnetometer since the spacecraft also had a fluxgate. The magnetometer was fouled accidentally which caused it to overheat, it worked for a period of time but 52hr into the mission transmission went dead and was not regained. [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1961-010A-01 RB-Vapor and Fluxgate Magnetometers] National Space Science Data Center, NASA] Ranger 1 and 2 carried a rubidium vapor magnetometer, failed to reach lunar orbit.

Helium

This type of magnetometer depends on the variation in helium absorptivity, when excited, polarized infrared light with an applied magnetic field. [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1967-060A-05 Triaxial Low Field Helium Magnetometer - Mariner 5 mission] National Space Science Data Center, NASA] A low field vector-helium magnetometer was equipped on the Mariner 4 spacecraft to Mars like the Venus probe a year earlier, no magnetic field was detected. [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1964-077A-02 Helium Magnetometer-Mariner 4 mission] National Space Science Data Center, NASA] Mariner 5 used a similar device For this experiment a low-field helium magnetometer was used to obtain triaxial measurements of interplanetary and Venusian magnetic fields. Similar in accuracy to the triaxial flux-gated magnetometers this device produced more reliable data.

Other types

Overhauser magnetometer provides extremely accurate measurements of the strength of the magnetic field. The Orsted (satellite) uses this type of magnetometer to map the magnetic fields over the surface of the earth.

On the Vanguard 3 mission (1959) a proton processional magnetometer was used to measure geomagnetic fields. The proton source was hexane. [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1959-007A-01 Proton Processional Magnetometer] National Space Science Data Center, NASA]

Configurations of magnetometers

Unlike ground based magnetometers that can be oriented by the user to determine the direction of magnetic field, in space the user is linked by telecommunications to a satellite traveling at 25,000 km per hour. The magnetometers used need to give an accurate reading quickly to be able to deduce magnetic fields. Several strategies can be employed, it is easier to rotate a space craft about its axis than to carry the weight of an additional magnetometer. Other strategy is to increase the size of the rocket, or make the magnetometer lighter and more effective. One of the problems, for example in studying planets with low magnetic fields like Venus, does require more sensitive equipment. The equipment has necessarily needed to evolve for today's modern task. Ironically satellites launched more the 20 years ago still have working magnetometers in places where it would take decades to reach today, at the same time the latest equipment is being used to analyze changes in the Earth here at home.

Uniaxial

These simple fluxgate magnetometers were used on many missions. On Pioneer 6 and Injun 1 the magnetometers were mounted to a bracket external to the space craft and readings were taken as the spacecraft rotated every 120°. [ [http://nssdc.gsfc.nasa.gov/nmc/experimentSearch.do?spacecraft=Pioneer%20%206 Uniaxial Fluxgate Magnetometer - Pioneer 6] National Space Science Data Center, NASA] Pioneer 7 and Pioneer 8 are configured similarly. [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1967-123A-01 Single-Axis Magnetometer-Pioneer 9] National Space Science Data Center, NASA] The fluxgate on Explorer 6 was mounted along the spin axis to verify spacecraft tracking magnetic feild lines. Search coil magnetometers were used on Pioneer I, Explorer 6, Pioneer V, and Deep Space 1.

Diaxial

A two axis magnetometer was mounted to the ATS-1 (Applications Technology Satellite). [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1966-110A-02 Biaxial Fluxgate Magnetometer - Application Technology Satellite -1 (ATS-1)] National Space Science Data Center, NASA] One sensor was on a 15 cm boom and the other on the spacecrafts spin axis (Spin stabilized satellite). The sun was used to sense the position of the boom mounted device, and triaxial vector measurements could be calculated. Compared to other boom mounted magnetometers, this configuration had considerable interference. Interestingly with this spacecraft, the sun induce magnetic oscillations and this allowed the continued use of the magnetometer after the sun sensor failed. Explorer 10 had two fluxgate magnetometers but is technically classified as a Dual technique since it also had a rubidium vapor magnetometer.

Triaxial

The Sputnik-3 had a vector fluxgate magnetometer, however because the orientation of the spacecraft could not be determined the direction vector for the magnetic field could not be determined. Three axis magnetometers were used on Luna 1, Luna 2, Pioneer Venus, Mariner 2, Venera 1, Explorer 12, Explorer 14, and Explorer 15. Explorer 33 was 'to be' the first US spacecraft to enter stable orbit around the moon was equipped with the most advanced magnetometer, a boom-mounted triaxial fluxgate (GFSC) magnetometer of the early-vector type. It had a small range but was accurate to a resolution of 0.25 nT. [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1966-058A-01 GFSC Magnetometer - Explorer 33] National Space Science Data Center, NASA] However after a rocket failure it was left in an highly elliptical orbit around Earth that orbited through the electro/magnetic tail. [ Behannon KW. "Mapping of the Earth's Bow Shock and Magnetic Tail by Explorer 33. 1968. J. Geophys. Res. 73: 907-930] (ALSEP). [ [http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1969-099C Lunar Surface Magnetometer - Apollo-12 Lunar module] National Space Science Data Center, NASA] [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1969-099C-04 Lunar Surface Magnetometer] National Space Science Data Center, NASA] The magnetometer continued to work several months after that return module departed. As part of the Apollo 14 ALSEP, there was a portable magnetometer.

The first use of the three axis ring-coil magnetometer was on the Apollo 16 moon mission. Subsequently is was used on the Magsat. The MESSENGER mission has triaxial ring-coil magnetometer with a range of +/- 1000 mT and a sensitivity of 0.02 mT, still in progress, the mission is designed to get detailed information about Mercurian magnetosphere. [ [http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=2004-030A MESSENGER] Space Science Data Center, NASA] ] The first use of spherical magnetometer in three axis configuration was on the Orsted (satellite).

Dual technique

Each type of magnetometer has its own built in 'weakness'. This can result from the design of the magnetometer to the way the magnetometer interacts with the spacecraft, radiation from the sun, resonances, etc. Using completely different design is a way to measure which readings are the result of natural magnetic fields and the sum of magnetic fields altered by spacecraft systems. In addition each type has its strengths. The fluxgate type is relatively good at providing data that finds magnetic sources. One of the first Dual technique systems was the abbreviated Explorer 10 mission which used a rubidium vapor and biaxial fluxgate magnetometers. Vector helium is better at tracking magnetic field lines and as a scalar magnetometer. Cassini spacecraft used a Dual Technique Magnetometer. One of these devices is the ring-coil vector fluxgate magnetometer (RCFGM). The other device is a vector/scalar helium magnetometer. [ [http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-mag.cfm SPACECRAFT - Cassini Orbiter Instruments - MAG] ] The RCFGM is mounted 5.5 m out on an 11 m boom with the helium device at the end.

Explorer 6 (1959) used a search coil magnetometer to measure the gross magnetic field of the Earth and vector fluxgate. [ [http://nssdc.gsfc.nasa.gov/nmc/experimentSearch.do?spacecraft=Explorer%20%206 Experiments Explorer 6] National Space Science Data Center, NASA] , however because of induced magnetism is the space craft the fluxgate sensor became saturated and did not send data. Future missions would attempt to place magnetometers further away from the space craft.

Magsat Earth geological satellite was also Dual Technique. This satellite and Grm-A1 carried a scalar cesium vapor magnetometer and vector fluxgate magnetometers. [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1979-094A-01 Scalar Magnetometer Magsat mission] National Space Science Data Center, NASA] [ [http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1979-094A-02 Vector Magnetometer Magsat mission] National Space Science Data Center, NASA] The Grm-A1 satellite carrier the magnetometer on 4 meter boom. This particular spacecraft was designed to hold in a precised equi-gravitational orbit, while taking measurements. [ [http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=GRM-A1 GRM-A1] National Space Science Data Center, NASA] For purposes similar to Magsat, theØrsted satellite, also used a dual technique system. The Overhauser magnetometer is situated at the end of an 8 meter long boom, in order to minimize disturbances from the satellite's electrical systems. The CSC fluxgate magnetometer is located inside the body and associated with a star tracking device. One of the greater accomplishments of the two missions, the Magsat and Orsted missions happen to capture a period of great magnetic field change, with the potential of a loss of dipole, or pole reversal.cite journal |author=Hulot G, Eymin C, Langlais B, Mandea M, Olsen N |title=Small-scale structure of the geodynamo inferred from Oersted and Magsat satellite data |journal=Nature |volume=416 |issue=6881 |pages=620–3 |year=2002 |month=April |pmid=11948347 |doi=10.1038/416620a |url=] [ [http://www.nasa.gov/centers/goddard/news/topstory/2004/0517magnet.html NASA AND USGS MAGNETIC DATABASE "ROCKS" THE WORLD] NASA Web Feature, NASA]

References