Geomagnetic phenomena are closely related to those in other fields of geophysics and the modern advancements in geomagnetism are interdependent on those in other disciplines.
1.- Historical remarks
2.- The earth's magnetic field
4.- The earth's radiation belt
1.- Historical remarks
Nearly seven hundred years ago (1269), Petrus Peregrinus, a Frenchman, published the first scientific paper on magnetism recognizing that a spherical lodestone has two poles at which a small magnet orients itself parallel to the axis connecting the poles. However, Peregrinus supposed, according the medieval traditions of philosophy that the poles of the magnet received their power from the heavens.
In 1600, William Gilbert explored and charted the surface field of a small spherical lodstone by placing tiny magnets on its surface. Gilbert found that the inclination of the magnets matched with observations at several points on the globe. A slow variationin declination was first reported by Henry Gellibrand in 1635; this is probably the beginning of the study of the geomagnetic secular variation.
The daily variation of declination was discovered in 1722 by George Graham, and was confirmed in 1740 by Andreas Celsius, who together with his colleague Hiorter first noted in 1741 the close ralation between the aurora and the magnetic disturbance. The study of the polar disturbance dates back to that time. One of the important early contribution in the study of the aurora was the publication of the first drawing of the auroral zone by Elias Loomis of Yale in 1860.
Balfour Stewart predicted in 1882 that the daily magnetic variations are due to electric currents flowing in a conducting layer in the upper atmosphere. The existence of such a conducting layer -the ionosphere- was discovered in 1902 by Heaviside and Kennelly independently.
The era of satellites and rockets has allowed direct measurements and observations of solar radiation which at the same time, have greatly advanced our knowledge of the physics of the earth and its immediate vicinity.
On the other hand, the modern progresses made in plasma physics have made it possible to interpret many complex geomagnetic and related phenomena in the ionosphere, in the exosphere and in interplanetary space.
2.- The earth's magnetic field
The earth possesses two fundamental fields, magnetic and gravitational. The conception of magnetism of the earth as a planetary property is due to William Gilbert, who wrote (in Latin) the great treatise De magnet
It was apparent from the early maps of declination compiled primarily from ship logs of the compass direction that the earth's field was not a true dipole. As the earth's magnetic field has a secular variation it become a common practice redraw the charts of declination each 5 to 10 years. The charts were prepared as a navigaation aid. Nowadays satellite measurements of the magnetic field are an important part of this effort.
From the existence of magnetic disturbances, we know, that the field does not originate entirely within the interior of the earth, but there are also sources of magnetic fields in ionospheric and magnetospheric regions. Theoretically, a separation of internal and external sources can be achieved through the spherical harmonic analysis
Two types of permanent external sources that are known to exist are the quiet day ring current caused by the drift motion of trapped particles and the deformation of the magnetic field caused by the solar wind.