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What is Geomagnetism
HISTORICAL PERSPECTIVE
The use of magnetic compass for navigation was known in by Chinese in 11th century and in 12th century in Europe. Columbus on his voyage to America in 1492 reported the change in Declination with longitude. In 1510, George Hartmann in Rome discovered the Inclination in the magnetic needle. William Gilbert in 1600 in his publication De Magnete described that the earth’s magnetic field has its origin inside the earth. During 16th and 17th centuries, measurements of Declination were conducted at various places around the globe. The English scientist , Graham in 1722 was the first to note that the magnetic field undergoes a regular daily variation. Celsius and Hiorter could even observe the swing of the compass needles during Auroral brightening.
In the historical evolution of magnetic measurements, Alexander von Humbodt and Carl Friedrich Gauss with Wilhelm Weber has left a key place in establishing network of magnetic observatories, in the development of instrumentation for measurements in absolute units and in the theory of geomagnetism. Humbolt was in the pioneering era of setting up magnetic observatories. The German mathematician , Gauss in 1830s devised the method of Spherical harmonic analysis to model the geomagnetic field and postulated that the major part of the field originated within the earth. The improvement in the measurements of the magnetic field in Absolute units by Gauss made it possible to conduct magnetic measurements around the globe for long duration.
GEOMAGNETIC VARIATIONS AT NEAR AND FAR SPACE ENVIRONMENT OF EARTH
To a first approximation, properties of the earth’s magnetic field resemble that of a dipole like magnet with its origin from the dynamo mechanism within the earth’s outer core. The absolute measure of the magnetic field at any location on the Earth is the combination of the sources in the Earth’s core, in the Earth’s crust and in the near and far space environment of the earth such as ionosphere and magnetosphere. The main magnetic field contributes to almost 99% of the observed magnetic field, changes associated with this field is the secular variation or long-term variation. In addition to the main field variations, as suggested by the hypothesis of Stewart in 1882, daily oscillations are due to the currents in the ionized atmosphere of the earth.
Stewart’s ideas on these variations were attributed to the currents flowing in the upper atmosphere due to periodic air motions in the presence of magnetic field. Later this proposition was referred as atmospheric dynamo theory of solar quiet ( Sq) and lunar (L) variations .
Schuster confirmed the inference of Stewart and Breit and Tuve and Appleton and Barnett confirmed this proposition experimentally in 1924. Thus the measurement of the magnetic field at any point on the surface of our planet Earth, is considered as a less expensive experimental basic tool to study and infer the geophysical processes taking place inside the Earth and its near and far space environment.
Chapman and Bartles (1940) and Vestine (1960) represented the source of the regular magnetic variation in terms of a current sheet at an altitude of ~ 100 km above the earth’s surface with two vertices, at around 35 deg . latitudes. Regular and systematic variations of the magnetic components at many observatories around the globe have subsequently brought out by researchers in 1950s the important finding of the equatorial electrojet , a phenomenon representing the enhanced flow of east-west current over the magnetic equator.
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