Geomagnetism

(geophysics) The magnetism of the earth. Also known as terrestrial magnetism. The branch of science that deals with the earth's magnetism.

The magnetism of the Earth; also, the branch of science that deals with the Earth's magnetism. Formerly called terrestrial magnetism, geomagnetism involves any topic pertaining to the magnetic field observed near the Earth's surface, within the Earth, and extending upward to the magnetospheric boundary. Modern usage of the term is generally confined to historically recorded observations to distinguish it from the sciences of archeomagnetism and paleomagnetism, which deal with the ancient magnetic field frozen respectively in archeological artifacts and geologic structures. See also Paleomagnetism; Rock magnetism.

The primary component of the magnetic field observed at the Earth's surface is caused by electric currents flowing in its liquid core, and is called the main field. Vectorially added to this component are the crustal field of magnetized rocks, transient variations imposed from external sources, and the field from electric currents induced in the Earth from these variations.

The geomagnetic field is specified at any point by its vector F. Its direction is that of a magnetized needle perfectly balanced before it is magnetized, and freely pivoted about that point, when in equilibrium. The north pole of such a needle is the one that at most places on the Earth takes the more northerly position. Over most of the Northern Hemisphere, that pole dips downward (see illustration). The elements used to describe the vector F are H, the component of the vector projected onto a horizontal plane; its north and east components X and Y, respectively; Z the vertical component; F the magnitude of the vector F; the angles I, the dip of the field vector below the horizontal; and D the magnetic declination or deviation of the compass from geographic north. By convention, Z and I are positive downward, and D is positive eastward (or may be indicated as east or west of north). These elements can be related to each other by trigonometric equations. See also Magnetic compass.

inclination, H = horizontal intensity, X = north intensity, Y = east intensity, Z = vertical intensity, F = total intensity.">
Elements of the geomagnetic field. D = declination, I = inclination, H = horizontal intensity, X = north intensity, Y = east intensity, Z = vertical intensity, F = total intensity.

A magnetic pole is a location where the field is vertically aligned, H = 0. Due to the presence of sometimes strong (for example, >1000 nanoteslas) magnetic anomalies at the Earth's surface, there are a number of locations where the field is locally vertical. However, those field components that extend to sufficient altitude to control charged particles can be accurately located by using the computations from a spherical harmonic expansion using degrees up to only about n = 10. Indeed, a pole can be defined by using only the main dipole (n = 1), or many terms. See also Aurora.

The n = 1 poles are sometimes referred to as the geomagnetic poles, and those computed using higher terms as dip poles. The term geomagnetic could also refer to the eccentric geomagnetic pole, which can be computed from n = 1 and n = 2 harmonics so as to be the best representation of a dipole offset from the center of the Earth. The latter has been used as a simplified field model at distances of 3 or 4 earth radii. Due to the more rapid fall-off of the higher terms with distance from the Earth, the two principal poles approach those of the n = 1 term with increasing altitude, until the distortions due to external effects begin to predominate. See also Magnetosphere.

The distribution of the dip angle I over the Earth's surface can be indicated on a globe or map by contours called isoclines, along which I is constant. The isocline for which I = 0 (where a balanced magnetized needle rests horizontal) is called the dip equator. The dip equator is geophysically important because there is a region in the ionospheric E layer in which small electric fields can produce a large electric current called the equatorial electrojet. See also Geomagnetic variations; Ionosphere.

A magnetized compass needle can be weighted so as to rest and move in a horizontal plane at the latitudes for which it is designed, thus measuring the declination D. The lines on the Earth's surface along which D is constant are called isogonic lines or isogones. The compass points true geographic north on the agonic lines where D = 0. At nonpolar latitudes, D is a useful tool for marine and aircraft navigational reference. Indeed, isogones appear on navigation charts, electronic navigational aids are referenced to D, and airport runways are marked with D/10. A runway painted with the number 11 indicates that its direction has a compass heading of 110°. The compass needle becomes less reliable in polar regions because the horizontal component H becomes smaller as the magnetic poles are approached. See also Navigation.

The intensity of the field can also be represented by maps, and the lines of equal intensity are called isodynamic lines. The dipole dominates the patterns of magnetic intensity on Earth in that the intensity is about double at the two poles compared to the value near the Equator. However, it can also be seen that the next terms of the spherical harmonic expansion also have a significant effect, in that there is a second maximum in Siberia, and an area near Brazil that is weaker than any other. This so-called Brazilian anomaly allows charged particles trapped in the magnetic field to reach a low altitude and be lost by collisions with atmospheric gases. The highest intensity of this smooth field is about 70 microteslas near the south magnetic pole in Antarctica, and the weakest is about 23 μT near the coast of Brazil.

The term magnetic anomaly has become clearer than it was previously because it is recognized that the geomagnetic field has a continuous spectrum but with two distinct contributors. Originally, the term meant a field pattern that was very local in extent; the modern definition is that portion of the field whose origin is the Earth's crust. The sizes of the strong and easily observable features are generally up to only a few tens of kilometers. Their intensity ranges typically from a few hundred nanoteslas up to several thousand, and they are highly variable depending on the geology of the region.

The main or core component of the geomagnetic field undergoes slow changes that necessitate continual adjustment of the model coefficients and redrawing of the isomagnetic maps. In any magnetic element at a particular place, the variation may be an increase or a decrease and is not constant in either magnitude or sign. This distribution of the rate for any element can be indicated on isoporic maps by lines (isopors) along which the rate is constant. Typically, the pattern of isopors is more complex than that of the isomagnetic lines for the same element, partly because the spectrum of such change is not dominated by the dipole as is the case of the static field.

Studies indicate that the dipole component of the field 2000 years ago was about 50% stronger than the present. Its average decay rate has averaged about 0.05% per year (15 nanoteslas per year) since about 1840 when absolute measurements were first begun, but accelerated from 1970 to its 1994 value of 0.08% per year (24 nT/yr). However, there is also evidence that the decade of the 1940s showed a rate of only about 10 nT per year. A linear projection of the present rate would have the dipole decreasing to zero in less than 1500 years. Although archeomagnetic evidence indicates that the field has indeed decayed to near zero level within the last 50,000 years with a subsequent return to the present polarity, and paleomagnetic results show that the field has reversed its polarity many times since the Earth's formation (the last time, about a million years ago), there is no model that can predict the future course of field change.

Deriving a suitable model that explains the source of the Earth's magnetic field has been one of the most frustrating problems that theoreticians have faced. Starting with the physical laws that should govern the behavior of a highly conducting, rotating, spherical fluid and coming up with a model of the geomagnetic field is exceedingly difficult. Dynamo means that a current is generated as an electrical conductor is moved through a magnetic field, as in a dynamo supplying electrical power. See also Geodynamo.

The main source of data for magnetic maps and models before the advent of satellites was fixed magnetic observatories. These stations, numbering about 140, provided the continuous record of changes in the magnetic field at their location. Their data are generally accurate and an excellent indicator of both secular change and the transient variations, but their global coverage is too sparse for a determination of the whole field. Spherical harmonic analyses based only on such data produce distorted results because of the large gaps in coverage, especially because of the sparseness of observing locations in southern oceanic regions. See also Magnetism.