Geomagnetism is the science dealing with the magnetic field of the Earth. The Earth behaves as if a bar magnet were embedded it i.e. lines of magnetic force originate from within and stretch out deep into the space. Owing to the pressure of the solar plasma continuously flowing from the Sun in the form of Solar Wind, the earth's magnetic field gets compressed on the dayside, and is drawn out into a long tail on the nightside, to form the structure called the Magnetosphere.
If a bar magnet is freely suspended at a point on earth in the northern mid-latitudes it would take up a position along the geomagnetic N-S direction. The magnetic pole of the earth in the north hemisphere is a south pole. Similarly the earth's magnetic pole in the southern hemisphere is a north pole. The plane in which the magnet sets itself is called the magnetic meridian. It differs slightly from the geographic meridian at most places on earth mainly because the magnetic poles of the earth are located slightly away from the geographic poles. The line joining the magnetic poles is called the magnetic axis. The magnetic poles of the earth are not fixed, but wander in time.
Why, undertake Geomagnetic Studies? In the vast realms of our Solar system, our Earth is just a point in Space. It is swamped by emissions from the Sun, of both electromagnetic and charged, particle nature. The electromagnetic radiations penetrate up to the earth's surface, with different radiations being selectively absorbed by different atmospheric constituents at varying altitudes, giving rise to an ionised environment in the atmosphere called the Ionosphere, which carries electric currents.
The charged particle from the Sun with low energy of a few eV (electron volts) for the protons and electrons constitute the Solar Wind. These particles and other higher energy ones are prevented from reaching ground level by the barrier constituted by the earth's magnetic field and its atmosphere, protecting life on earth, from Harmful solar radiations. To an observer in space the earth lies cocooned in the 3-dimensional environment of magnetic field and atmosphere called the Magnetospheric cavity. The charged particles o solar origin trapped by the earth's magnetic lines of force, generate currents in various regions called Magnetospheric currents.
These external current systems cause perturbations and variations in the earth’s main magnetic field. The latter originates in current flowing within the earth’s interior.
Variations in the geomagnetic field, when interpreted skillfully can give clues to physical processes at work in: (i) the entire magnetospheric cavity (ii) in interplanetary space (iii) on the Sun, and (iv) within the earth. The geomagnetic variations can also, used for prospecting for natural resources on earth. The entire volume of the .magnetospheric cavity and interplanetary space constitutes a giant, natural plasma laboratory with Complexes processes that cannot be easily reproduced on earth. To unravel these complexities we require a long, unbroken series of  geomagnetic measurements.
Today the Colaba-Alibag Observatories form an autonomous research Institute called the Indian Institute of Geomagnetism (IIG located at Colaba, Bombay. The Institute functions under the Department of Science and Technology, Government of India. IIG operates nine standard magnetic observatories, and also a magnetic observatory at the Indian station MAITRI in Antarctica. IIG is also recognised as a World Data Centre (WDC-C2) for Geomagnetism and is engaged in research in many of the frontal areas of Geomagnetism.
What is Space Geomagnetism? There are magnetic fields of a time-varying nature, associated with electric currents which flow in the earth's electrically-conducting upper atmosphere. The currents can flow in the Ionosphere or the Magnetosphere and the magnetic field associated with these currents leave recognisable signatures on the geomagnetic records (magnetograms) which can be interpreted in terms of the physical processes at work in these regions.
The variations in the geomagnetic field exhibit many periodicities ranging from millions of years for reversals of magnetic polarity to a few seconds for magnetic pulsations. Geomagnetic pulsations thus occupy the lowest end of the spectrum. Pulsations are. continuous (Pc), or irregular (Pi) trains of variations in the magnetic field components, with time periods ranging from 30 min to 0.2 ~. The presence of pulsations on magnetograms indicates that certain physical processes are at work in the earth's space environment.
Further details can be had from the Indian Institute of Geomagnetism, Colaba, Bombay 400005 (India)