Definitions

Geomagnetic poles

Magnetometer

[mag-ni-tom-i-ter]
A magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument. Magnetism varies from place to place and differences in Earth's magnetic field (the magnetosphere) can be caused by the differing nature of rocks and the interaction between charged particles from the sun and the magnetosphere of a planet. Magnetometers are often a frequent component instrument on spacecraft that explore planets.

Uses

Magnetometers are used in geophysical surveys to find deposits of iron because they can measure the magnetic field variations caused by the deposits. Magnetometers are also used to detect archaeological sites, shipwrecks and other buried or submerged objects. Magnetic anomaly detectors detect submarines for military purposes.

They are used in directional drilling for oil or gas to detect the azimuth of the drilling tools near the drill bit. They are most often paired up with accelerometers in drilling tools so the both the inclination and azimuth of the drill bit can be found.

Magnetometers are very sensitive, and can give an indication of possible auroral activity before one can see the light from the aurora. A grid of magnetometers around the world constantly measures the effect of the solar wind on the earth's magnetic field, which is published on the K-index.

In space exploration

A three-axis fluxgate magnetometer was part of the Mariner 2 and Mariner 10 missions. A dual technique Magnetometer is part of the Cassini-Huygens mission to explore Saturn. This system is composed of a vector helium and fluxgate magnetometers. Magnetometers are also a component instrument on the Mercury MESSENGER mission. A magnetometer can also be used by satellites like GOES to measure both the magnitude and direction of a planet's or moon's magnetic field.

In paranormal investigation

Magnetometers have recently become popular in paranormal investigation, especially ghost hunting. Some investigators believe that unexplained fluctuations in the local electromagnetic field strength with no discernible cause could be a sign of paranormal activity, such as ghosts. In recent episodes, the Ghost Hunters have used magnetometers (called K-II meters on the show) to allegedly communicate with invisible entities.

Types

Magnetometers can be divided into two basic types:

  • scalar magnetometers measure the total strength of the magnetic field to which they are subjected, and
  • vector magnetometers have the capability to measure the component of the magnetic field in a particular direction.

The use of three orthogonal vector magnetometers allows the magnetic field strength, inclination and declination to be uniquely defined. Examples of vector magnetometers are fluxgates, superconducting quantum interference devices (SQUIDs), and the atomic SERF magnetometer. Some scalar magnetometers are discussed below.

A magnetograph is a special magnetometer that continuously records data.

Rotating coil magnetometer

The magnetic field induces a sine wave in a rotating coil. The amplitude of the signal is proportional to the strength of the field, provided it is uniform, and to the sine of the angle between the rotation axis of the coil and the field lines. This type of magnetometer has been outdated.

Hall effect magnetometer

The most common magnetic sensing devices are solid-state Hall effect sensors. These sensors produce a voltage proportional to the applied magnetic field and also sense polarity.

Proton precession magnetometer

One type of magnetometer is the proton precession magnetometer, also known as the proton magnetometer, which measures the resonance frequency of protons (hydrogen nuclei) in the magnetic field to be measured, due to Nuclear Magnetic Resonance (NMR).

A direct current flowing in an inductor creates a strong magnetic field around a hydrogen-rich fluid, causing the protons to align themselves with that field. The current is then interrupted, and as protons are realigned with Earth's magnetic field they precess at a specific frequency. This produces a weak alternating magnetic field that is picked up by a (sometimes separate) inductor. The relationship between the frequency of the induced current and the strength of Earth's magnetic field is called the proton gyromagnetic ratio, and is equal to 0.042576 hertz per nanotesla (Hz/nT).

Because the precession frequency depends only on atomic constants and the strength of the external magnetic field, the accuracy of this type of magnetometer is very good. Magnetic impurities in the sensor and errors in the measurement of the frequency are the two causes of errors in these magnetometers.

If several tens of watts are available to power the aligning process, these magnetometers can be moderately sensitive. Measuring once per second, standard deviations in the readings in the 0.01 nT to 0.1 nT range can be obtained.

The strength of the Earth's magnetic field varies with time and location, so that the frequency of Earth's field NMR (EFNMR) for protons varies between approximately 1.5 kHz near the equator to 2.5 kHz near the geomagnetic poles.

The measurement of the precession frequency of proton spins in a magnetic field can give the value of the field with high accuracy and is widely used for that purpose. In low fields, such as the Earth's magnetic field, the NMR signal is expected to be weak because the nuclear magnetization is small, but special devices can enhance the signal 100 or 1000 times. Incorporated in existing portable magnetometers, these devices make them capable of measuring fields to an absolute accuracy of about one part in 1,000,000 and detecting field variations of about 0.1nT. Apart from the direct measurement of the magnetic field on Earth or in space, these magnetometers prove to be useful whenever a phenomenon is linked with variations of magnetic field in space or in time, such as anomalies arising from submarines, skiers buried under snow, archaeological remains, and mineral deposits

Fluxgate magnetometer

A fluxgate magnetometer consists of a small, magnetically susceptible, core wrapped by two coils of wire. An alternating electrical current is passed through one coil, driving the core through an alternating cycle of magnetic saturation (ie magnetised - unmagnetised - inversely magnetised - unmagnetised - magnetised). This constantly changing field induces an electrical current in the second coil, and this output current is measured by a detector. In a magnetically neutral background, the input and output currents will match. However, when the core is exposed to a background field, it will be more easily saturated in alignment with that field and less easily saturated in opposition to it. Hence the alternating magnetic field, and the induced output current, will be out of step with the input current. The extent to which this is the case will depend on the strength of the background magnetic field. Often, the current in the output coil is integrated, yielding an output analog voltage, proportional to the magnetic field.

Fluxgate magnetometers, paired in a gradiometer configuration, are commonly used for archaeological prospection. In Britain the most common such instruments to be used are the Geoscan FM series of instruments and the Bartington GRAD601. Both are capable of resolving magnetic variations as weak as 0.1nT (roughly equivalent to one half-millionth of the earth's magnetic field strength).

Overhauser magnetometer

The Overhauser effect takes advantage of a quantum physics effect that applies to the hydrogen atom. This NMR effect occurs when a special liquid (containing free, unpaired electrons) is combined with hydrogen atoms and then exposed to secondary polarization from a radio frequency (RF) magnetic field (i.e. generated from an RF source).

RF magnetic fields are ideal for use in magnetic devices because they are transparent to the Earth's DC magnetic field and the RF frequency is well out of the bandwidth of the precession signal (i.e. they do not contribute noise to the measuring system).

The unbound electrons in the special liquid transfer their excited state (i.e. energy) to the hydrogen nuclei (i.e. protons). This transfer of energy alters the spin state populations of the protons and polarizes the liquid – just like a proton precession magnetometer – but with much less power and to much greater extent.

The proportionality of the precession frequency and magnetic flux density is perfectly linear, independent of temperature and only slightly affected by shielding effects of hydrogen orbital electrons. The constant of proportionality is known to a high degree of accuracy and is identical to the proton precession gyromagnetic constant. As with the proton magnetometer, magnetic impurities and inaccuracies in frequency measurement are two causes of error in the measurement. The Overhauser magnetometer may have an additional error because the frequency produced can be changed slightly by an interaction between the protons and the coil used to detect the magnetic field.

Overhauser magnetometers achieve some 0.01 nT/√Hz noise levels, depending on particulars of design, and they can operate in either pulsed or continuous mode.

Caesium vapor magnetometer

A basic example of the workings of a magnetometer may be given by discussing the common "optically pumped caesium vapour magnetometer" which is a highly sensitive (0.004 nT/√Hz) and accurate device used in a wide range of applications. Although it relies on some interesting quantum mechanics to operate, its basic principles are easily explained.

The device broadly consists of a photon emitter containing a caesium light emitter or lamp, an absorption chamber containing caesium vapour and a "buffer gas" through which the emitted photons pass, and a photon detector, arranged in that order.

Polarization

The basic principle that allows the device to operate is the fact that a caesium atom can exist in any of nine energy levels, which is the placement of electron atomic orbitals around the atomic nucleus. When a caesium atom within the chamber encounters a photon from the lamp, it jumps to a higher energy state and then re-emits a photon and falls to an indeterminate lower energy state. The caesium atom is 'sensitive' to the photons from the lamp in three of its nine energy states, and therefore eventually, assuming a closed system, all the atoms will fall into a state in which all the photons from the lamp will pass through unhindered and be measured by the photon detector. At this point the sample (or population) is said to be polarized and ready for measurement to take place. This process is done continuously during operation.

Detection

Given that this theoretically perfect magnetometer is now functional, it can now begin to make measurements.

In the most common type of Caesium magnetometer, a very small AC magnetic field is applied to the cell. Since the difference in the energy levels of the electrons is determined by the external magnetic field, there is a frequency at which this small AC field will cause the electrons to change states. In this new state, the electron will once again be able to absorb a photon of light. This causes a signal on a photodetector that measures the light passing through the cell. The associated electronics uses this fact to create a signal exactly at the frequency which corresponds to the external field.

Another type of Caesium magnetometer modulates the light applied to the cell. This is referred a Bell-Bloom magnetometer after the two scientists who first investigated the effect. If the light is turned on and off at the frequency corresponding to the earth's field, there is a change in the signal seen at the photodetector. Again, the associated electronics uses this to create a signal exactly at the frequency which corresponds to the external field.

Both methods lead to high performance magnetometers.

Applications

The Caesium magnetometer is typically used where a higher performance magnetometer than the proton magnetometer is needed. In archaeology and geophysics, where the sensor is moved through and area and many accurate magnetic field measurements are needed, the Caesium magnetometer has advantages over the proton magnetometer.

The Caesium magnetometer's faster measurement rate allow the sensor to be moved through the area more quickly for a given number of data points.

The lower noise of the Caesium magnetometer allows those measurements to more accurately show the variations in the field with position.

Spin-exchange-relaxation-free (SERF) atomic magnetometers

At sufficiently high atomic density, extremely high sensitivity can be achieved. Spin-exchange-relaxation-free (SERF) atomic magnetometers containing potassium, caesium or rubidium vapor operate similarly to the caesium magnetometers described above yet can reach sensitivities lower than 1 fT/√Hz.

The SERF magnetometers only operate in small magnetic fields. The earth's field is about 0.5 Gauss. SERF magnetometers operate in fields less than 0.005 Gauss (5 miligauss)

As shown in large volume detectors have achieved 200 aT/√Hz sensitivity. This technology has greater sensitivity per unit volume than SQUID detectors.

The technology can also produce very small magnetometers that may in the future replace coils for detecting changing magnetic fields.

Rapid developments are ongoing in this area. This technology may produce a magnetic sensor that has all of its input and output signals in the form of light on fiberoptic cables. This would allow the magnetic measurement to be made in places where high electrical voltages exist.

SQUID magnetometer

SQUIDs, or Superconducting Quantum Interference Devices, measure extremely small magnetic fields; they are very sensitive vector magnetometers, with noise levels as low as 3 fT·Hz−0.5 in commercial instruments and 0.4 fT·Hz−0.5 in experimental devices. Until the advent of SERF atomic magnetometers in 2002, this level of sensitivity was unreachable otherwise.

These magnetometers require cooling with liquid helium (4.2 K) or liquid nitrogen (77 K) to operate, hence the packaging requirements to use them are rather stringent both from a thermal-mechanical as well as magnetic standpoint. SQUID magnetometers allow one to measure the magnetic fields produced by brain or heart activity (magnetoencephalography and magnetocardiography, respectively).

Early magnetometers

In 1833 Carl Friedrich Gauss, head of the Geomagnetic Observatory in Göttingen, published a paper on measurement of the earth's magnetic field. It described a new instrument that Gauss called a "magnometer" (a term which is still occasionally used instead of magnetometer) It consisted of a permanent bar magnet suspended horizontally from a gold fibre A magnetometer is also called a gaussmeter.

See also

References

External links

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