GEOMAGNETIC FIELD

magnetic field associated with the Earth. It primarily is dipolar (i.e., it has two poles, these being the north and south magnetic poles) on the Earth's surface. Away from the surface, the dipole becomes distorted. In the 1830s, the German mathematician and astronomer Carl Friedrich Gauss studied the Earth's magnetic field and concluded that the principal dipolar component had its origin inside the Earth instead of outside. He demonstrated that the dipolar component was a decreasing function inversely proportional to the square of the Earth's radius, a conclusion that led scientists to speculate on the origin of the Earth's magnetic field in terms of ferromagnetism (as in a gigantic bar magnet), various rotation theories, and various dynamo theories. Ferromagnetism and rotation theories generally are discreditedferromagnetism because the Curie point (the temperature at which ferromagnetism is destroyed) is reached only 20 or so km (about 12 miles) beneath the surface, and rotation theories because apparently no fundamental relation exists between mass in motion and an associated magnetic field. Most geomagneticians concern themselves with various dynamo theories, whereby a source of energy in the core of the Earth causes a self-sustaining magnetic field. In the dynamo mechanism, fluid motion at the Earth's core involves the movement of conducting material across an existing magnetic field, thus creating a current. The geomagnetic dynamo is by far the most important source, for without the field that it creates, other sources could not exist. Other sources of the geomagnetic field include crustal magnetization (which occurs when the solid materials that form the Earth's crust become magnetized by the Earth's main field and cause detectable anomalies); ionospheric dynamo (the flow of charged particles across magnetic field lines that is caused by the Sun's heating of the ionosphere); the ring current (the current produced by the drift of charged particles far above the Earth's surface); magnetopause current (the sheetlike flow of particles that form a boundary between the Earth's magnetic fields and solar wind); tail current (similar to magnetopause current); field-aligned currents (caused by the circulation of magnetic field lines); and auroral electrojets (broad sheets of electric current that flow in the northern and southern auroral ovals).