All about Geomagnetism

Geomagnetism 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 magne-tospheric 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 arche-ological artifacts and geologic structures.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 . 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.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.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.

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.
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 110s. The compass needle becomes less reliable in polar regions because the horizontal component H becomes smaller as the magnetic poles are approached.

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 microT 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.


Post a Comment