Magnetometer

A magnetometer is a scientific instrument used to measure the strength of magnetic fields. Earth's magnetism varies from place to place and differences in the Earth's magnetic field (the magnetosphere) can be caused by a couple of things:

1. The differing nature of rocks
2. The interaction between charged particles from the sun and the magnetosphere

Magnetometers are used in geophysical surveys to find deposits of iron because they can measure the magnetic pull of iron. Magnetometers are also used to detect archeological sites, shipwrecks and other buried or submerged objects.

A magnetometer can also be used by satellites like GOES to measure both the magnitude and direction of the earth's magnetic field.

Magnetometers are very sensitive, and can give an indication of possible auroral activity before one can even see the light from the aurora.

Contents

1 Proton precession magnetometer 2 Overhauser magnetometer 3 Caesium vapour magnetometer 4 External links

Proton precession magnetometer

One type of magnetometer is the proton precession magnetometer, which operates on the principle that protons are spinning on an axis aligned with the magnetic field.

An inductor creates a strong magnetic field around an hydrogen-rich fluid, causing the protons to align themselves with the newly created field. The field is then interrupted, and as protons are realigned with Earth's magnetic field, spinning protons precess at a specific frequency. This produces a weak magnetic field that is picked up by the same 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).

Overhauser magnetometer

The Overhauser effect takes advantage of a quantum physics effect that applies to the hydrogen atom. This 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 a 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.

Overhauser magnetometers achieve some 0.01 nT/Hz^1/2 noise levels, depending on particulars of design, and they can operate in either pulsed or continuous mode.

Caesium vapour 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 sensative and accurate device employer across a wide range of fields. Although it relies on some interesting quantum mechanics to operate it's basic principles are easily explained. The device broadly consists of three items, a photon emitter containing a caesium emitter, a chamber containing caesium vapour together with a 'buffer [gas]' through which the emitted photons and a photon detector, arranged in that order.

Calibration:

The basic principle which allows the device to operate is the fact that a caesium atom can exist in any of 6 energy levels (the placement of electron 'orbits' around the atomic nucleus). When a caesium atom within the chamber encounters a photon from the emitter 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 only 'sensative' to the photons from the emitter in three of it's six energy states and therefore eventually (assuming a closed system) all the atoms will fall into a state in which the all the photons from the emitter will pass through unhindered and be measured by the photon detector. At this stage the device can be said to be perfectly calibrated.

Detection:

Given that our theoretical magnetometer is now calibrated we can expose it to the environment. It is easy to imagine that the environment is constantly emitting quanta of energy and that some of these will pass through our chamber. When they do they may hit one of our caesium atoms and cause it to jump into a new energy state, which may in turn be one in which it can absorb a photon from out caesium emitter. If this is the case it will cause a decrease in the number of photons reaching our detector and this can be easily recorded. Scaling from this simple example to account for the vast number of energy transactions occuring within the caesium vapour it is easy to see how the system works.

Real World:

Obviously when removed from an isolated environment the caesium vapour can never be 'perfectly' calibrated and the system is subject to much environmental interferance, however, by the application of feedback systems and an averaging of the detection rates seen in a benign environment we can calibrate the instrument sufficiently will in a real world environment to make it accurate and useful for detecion.

External links

Dan's Homegrown Proton Precession Magnetometer Page: http://www.portup.com/~dfount/proton.htm

• USGS Geomagnetism Program (http://geomag.usgs.gov)

GEM Advanced Magnetometers Website: http://www.gemsys.ca