magnetic field (origin of the Earth's internal field) Although most of us have used a magnetic compass to orient ourselves, we do not often think in terms of the source of the energy that powers the compass needle. This energy is provided by the Earth's magnetic field. The origin of the field is a complex subject which has enthralled a large number of the best scientists for the past four centuries; Albert Einstein even referred to it once as one of the three most important unsolved problems in physics. Certain aspects of how the field is generated are, however, now fairly well understood. Earth's magnetic field originates in the fluid outer core and is generated by electrical currents resulting from convective fluid motions of the highly conducting iron– nickel alloy of which the core is made. Even though the fluid motions in the core are quite complex, the magnetic field at the Earth's surface (roughly 3000 km away from the core) is nearly as simple as the dipolar magnetic field of a simple bar magnet (Fig. 1). Unfortunately, this simplicity inhibits our ability to deduce the precise mechanisms that could generate such fields; neither the exact mechanism that generates the field nor the patterns of the convection cells are known precisely. As a result, scientists who investigate the physical origin of the field have to rely on peculiarities that the field exhibits, as well as its gross features. In general, any reasonable physical model of the field must explain the secular variations of the field as well as its capability to reverse itself on a timescale of thousands of years (see geomagnetism: main field, secular variation, and westward drift; geomagnetism: polarity reversals). Under these constraints, the overall mechanism that generates the field is thought to be similar to an ordinary electrical dynamo. A dynamo converts mechanical energy from a moving electrical conductor into electromagnetic energy and thus generates current. But the fluid motions in the highly conducting outer core could not, by themselves, generate the electric currents in the outer core and the consequent magnetic field. An initial (seed) magnetic field is required to start a dynamo. For the Earth, this seed magnetic field may have been supplied by the Sun when the metallic iron–nickel core first formed within the Earth. Since that time the geometry of the convection cells and also the characteristics of the field might have changed as the inner core began solidify and grow in size. When all the core has solidified, the internally generated magnetic field will cease to exist (as on the present-day Moon).

The magnetic field cannot, however, be sustained simply by the presence of highly conducting fluid in the outer core. (Because of the high temperature, this fluid is as mobile as is water on the Earth's surface.) Thermo-gravitational convection acts to sustain the field: if the convection is turned off and the fluid remains static, the field decays, the inferred characteristic decay time being about 10 000 years. The direction of the field comes from the Earth's rotation, which shapes the patterns of the convection. Because of the Earth's rotation, the convecting fluid experiences the Coriolis effect, this causes clockwise circulation in the downwelling limbs of the convecting fluid in the northern hemisphere, and counterclockwise circulation in the southern hemisphere (Fig. 1).

It is clear that the magnetic field has its origin in electrical currents in the outer core and some sort of dynamo-like process resulting from interactions of conductive fluids, gravity, Earth's rotation, temperature differences between the upper and lower boundaries of the outer core, convective fluid motions, etc. Developing physical models of such a dynamo is, however, extremely complex. A mathematical treatment requires simultaneously dealing with about ten formidable equations containing physical parameters whose values are very uncertain. Despite their complexity, dynamo models are attractive because magnetic fields can be modelled which more or less parallel the Earth's axis of rotation, as well as reversals of the field.

There are many dynamo models and it is not possible even to mention all of them here; the following short list is intended to give only an overview. In 1919, J. Larmor suggested a rotating disc dynamo model to explain the origin of the solar and the Earth's magnetic field (Fig. 2a). In a disc dynamo, an initial magnetic field induces in a rotating conductive disc an outwardly directed electric current which can be tapped with an assembly of a brush and a wire. The wire can be wound around the axis of the disc to reinforce the initial field. Reversals in the direction of the magnetic field cannot occur in the single-disc dynamo, but chaotic reversals, such as are observed on the Earth, can occur in a coupled double-disc dynamo (Fig. 2b), as was discussed by Rikitake in the late 1950s. (Because reversals of the Earth's magnetic field were firmly established only in the late 1950s, Larmor had no reason to propose a reversing disc dynamo model.) An important transition from the mechanical analogues to the self-sustaining magnetic field involving fluid motions was made in the 1940s by W. M. Elsasser in the United States and E. C. Bullard in England. This led to the first testable numerical computations of the Earth's magnetic field. In 1955, E. N. Parker was able to show heuristically that by assuming a certain velocity field for the fluid motions in the outer core and the frozen-influx concept of H. Alfvén (see magnetohydrodynamic waves in the Earth), one can reinforce the original magnetic field (an av dynamo). In a kinematic model such as this, the magnetic field may continue to grow indefinitely. But the magnitude of the field can be maintained, and even reversals can be produced, by calling upon instabilities created by preferentially located cyclonic activity in the outer core ( Levy–Parker models). Instabilities can also play an important role in the magnetohydrodynamic models proposed by S. I. Braginsky, where magnetic forces (Lorentz forces) become important in shaping the fluid motions. In another set of models, called hydromagnetic models, a velocity field, is derived by appropriately coupling the dynamo equations. In the hydromagnetic models constructed by F. H. Busse in the 1970s, convection can occur in columns parallel to the rotation axis. A columnar convection pattern has also been inferred by J. Bloxham and D. Gubbins (Fig. 1) by mathematically projecting the observed magnetic field at the Earth's surface to the surface of the core–mantle boundary.

Geomagnetists deciphering the mysteries of the origin of the Earth's magnetic field constrain their models with snapshots of the magnetic field obtained from a variety of sources: satellites such as Magsat (c. 1980), which measure the near-Earth magnetic field; the POGO series satellites flown in the mid-1960s; surface geomagnetic observatories scattered round the world; and historical navigational charts (going back to the time of Halley); they also use information about the strength and direction of the field preserved in archaeological artefacts such as baked pottery and kilns (over thousands of years) and rocks formed over geological timescales (millions to hundreds of millions of years). As mentioned above, models of the physical processes in the outer core must be able to replicate the nature and form of variations that are observed (for example, periods of frequent reversals, periods of quiescence in reversals, rate of change of dipole moment, rate of secular variation of the dipole field, westward drift of the non-dipole field, etc.).

Dhananjay Ravat

Bibliography

Bloxham, J. and and Gubbins, D. (1989) The evolution of the Earth's magnetic field. Scientific American, 255, 68–75.
Merrill, R. T. and and McElhinny, M. W. (1983) The Earth's magnetic field: its history, origin and planetary perspective. Academic Press, London.

Magnetic field

Earth acts as though it were a huge dipole magnet with the positive and negative poles near the North and South Poles. This does not mean that Earth is literally a dipole magnet—there are too many variations in the field—but that the best fit for a model of the field is two poles of a magnet, rather than a quadrupole, or other shape. The magnetic field of the earth allows magnetic compasses to work, making navigation much easier. It also molds the configuration of Van Allen belts, bands of high-energy charged particles around the earth's atmosphere.

Most of Earth's magnetic field (90%) occurs below the surface and possibly exists because Earth's core doesn't move at the same rate as the earth's mantle (the layer between the earth's core and its crust ). The external 10% of the field is generated by movement of ions in the upper atmosphere.

The earth's magnetic field may help some animals navigate as they migrate. People have been using magnetic compasses for navigation since the fifteenth century. Because it has been so important for navigation, the magnetic field has been mapped all over the surface of the earth.

The magnetic field can also be used in other ways. For example, an instrument called a geomagnetic electrokinetograph determines the direction and speed of ocean currents while a ship is moving by measuring the voltage induced in the moving conductive sea water by the magnetic field of the earth.

The earth's magnetic field can change quickly and temporarily or slowly and permanently, depending on the cause of the change. The magnetic field can change very quickly, within an hour, in magnetic storms . These occur when the magnetic field is disturbed by sunspots, which send clouds of charged particles into Earth's atmosphere. (These same protons and electrons excite oxygen , nitrogen, and hydrogen atoms in the upper atmosphere, causing the aurora borealis and aurora australis.) These disturbances can be measured all over the globe and can cause static on radio stations.

The orientation of the magnetic field also changes slowly over centuries. In the planet's lifetime, the magnetic field has changed and even reversed (north pole becomes south and vice-versa) several times. Evidence for this is seen in reversed paleomagnetism of some sedimentary and igneous rock . In the 1960s, scientists showed that rocks formed at a particular interval in geologic time all indicate a magnetic field with the same orientation; older or younger rocks may show a reversed orientation. The cause of these paleomagnetic reversals is not yet known.

Today, the magnetic poles are not at the same place as the poles of the earth's rotational axis. Therefore, "magnetic north" is not quite the direction of "true north." The difference is known as the magnetic declination. Accordingly, scientists have established a series of geomagnetic coordinates, including latitude and longitude . These are centered on the magnetic dipole of the earth and designed (like geographic latitude and longitude ) as though the earth were a perfect sphere.

See also Earth, interior structure; Earth (planet); Polar axis and tilt

magnetic field Region surrounding a magnet, or a conductor through which a current is flowing, in which magnetic effects, such as the deflection of a compass needle, can be detected. A magnetic field can be represented by a set of lines of force (flux lines) spreading out from the poles of a magnet or running around a current-carrying conductor. The direction of a magnetic field is the direction a tiny magnet takes when placed in the field. Magnetic poles are the field regions in which magnetism appears to be concentrated. If a bar magnet is suspended to swing freely in the horizontal plane, one pole will point north; this is called the north-seeking or north pole. The other pole, the south-seeking or south pole, will point south. Unlike poles attract each other; like poles repel each other. The Earth's magnetic poles are the ends of the huge ‘magnet’ that is Earth.

mag·net·ic field • n. a region around a magnetic material or a moving electric charge within which the force of magnetism acts.

magnetic field See GEOMAGNETIC FIELD.