The earth behaves like a magnet. The magnetic field at a point on the surface of the earth is such as would be caused by a dipole magnet placed near its centre. About 97% of the earth's main magnetic field is of internal origin in the inner molten core of the earth and the rest is produced by the atmospheric electric current system attributed to extra-terrestrial causes like the sun and etc. Thus "Geomagnetism is that branch of science which deals with the study and characteristics of geomagnetic field".

The earth's magnetic field or, in modern usage, the geomagnetic field resembles, at a first approximation, the field produced by a magnetic dipole situated at the earth’s centre. The axis of the magnetic dipole makes and angle of 1.5° with the earth’s axis of rotation and cuts the earth’s surface at the Geomagnetic poles.

ACTUAL MAGNETIC FIELD

The actual magnetic field of the earth departs from the above model. The actual magnetic poles are situated where the lines of force are perpendicular to the Earth’s surface as observed by the Dip needle. Therefore, the magnetic poles are also called dip poles. The Dip equator or magnetic equator is situated where the lines of force are horizontal as observed with the dip needle. The magnetic meridian is the projection of the actual line of force on the horizontal plane. The two magnetic poles are some 500 to 1000 miles distant from the respective geomagnetic poles. The field intensity is about 0.65< near the poles and 0.25< near the equator. Decrease of intensity with height above ground proceeds approximately as the cube of the distance from the earth’s centre.
 
 

1. INCLINATION

The angle, which the line of force at a point of observation makes with the horizontal plane is the inclination I, also called dip. I is positive when the geomagnetic field vector is directed down ward (i.e. in the northern hemisphere) and Z varies +90° at the north magnetic pole and –90° at the south magnetic pole.

The scalar magnitude of the field vector is called the total intensity F, which is always positive and varies between 0.25 gauss (<)in South America and 0.80 gauss near the north magnetic pole.

3. HORIZONTAL INTENSITY

The projection of the geomagnetic field vector on the horizontal plane is the horizontal intensity H which is always positive and can assume values between zero at the magnetic poles and 0.40 gauss at the magnetic equator.

4. VERTICAL INTENSITY

The projection of the geomagnetic field vector on the vertical is the vertical intensity Z which is zero at the magnetic equator, +0.60 gauss at the north magnetic pole and –0.70 gauss at the south magnetic pole.

5. DECLINATION

The angle made by the magnetic meridian and the true north meridian is the magnetic declination D, which is positive or east when the magnetic meridian is east of the true north meridian.
Near the magnetic poles, D may vary from 180° to –180°. A t lower latitudes D is confined to ± 30°.

6. COMPONENT X

The projection of H on the true meridian is the geomagnetic north component X, which is always positive except for small areas near the magnetic poles.

7. COMPONENT Y

The projection of H on the true east direction is the geomagnetic east component Y. The geomagnetic east component is positive when directed eastward and is of the same algebraic sign as the magnetic declination.

8. MATHEMATICAL RELATION
 
 

At times, the diurnal variation of the Geomagnetic field is disturbed by increased particle radiation originating from Active areas of the sun. The particles penetrate into the ionosphere and upset the current system maintained by the sun’s electromagnetic radiation. The disturbances, called magnetic storms, are felt most strongly in the aurora belts and least at the geomagnetic equator, but start practically simultaneously all over the globe. During a magnetic storm the general level of H will be appreciably lowered. The recovery will take several days.

This change is attributed to the ring current, a current flowing around the globe from east to west at a distance of several earth radii. Magnetic storms are most frequent and violent during the maximum of the sunspot cycle.
 
 

Solar flares are associated with regions of sunspot group and is complicated phenomena. So much is certain that it begins with an outburst of ultra violet radiation. These eruptions are many times brighter than normal and their temperature may be as high as 20,000°K. A flare develops rapidly and may last several minutes and in exceptional cases up to 1 to 2 hours. Due to the out burst of ultra violet radiation of 1 to 15 A° (1A° = 10^-8 cm wave length) from such an eruption, the ionization in the lowest D-layer (40 km to 60 km) quickly increases by many times. This leads to the absorption of the whole range of short waves resulting incomplete fade-out of short wave radio communication.

In F2-Layer the effect of these eruptions will be the increase of ionization for several hours or in some cases several days and the general level of critical frequency of F2-Layer will also be raised higher than normal.