The magnetic field of the Earth consists of magnetic fields coming from several sources both internal and external source. The external magnetic field of the Earth is mainly from the interaction between the solar wind and geomagnetic field. The solar wind can be described as the movement of particles particularly H+, Helium and electron which are traveling from the Sun to the Earth and interacting with the geomagnetic field. This interaction leads to pressure to the geomagnetic field creating a cavity called the magnetosphere.

The solar wind can force the geomagnetic field out of the way at the distances larger than 10 RE, closer to the earth the solar wind is deflected and it flows around the geomagnetic field. This interaction results in the outer boundary of the magnetosphere called as the magnetopause. On the night side it also forms a comet shape area called geomagnetictail or magnetotail which has diameter around 40 RE. Another boundary called the bow shock forms in front of the dayside of the magnetopause as result of the motion of the Earth in the solar wind. The region between the magnetopause and the bow shock is called the magnetosheath which has a significant role in reducing speed of the solar wind particles to sub-magnetosonic speeds relative to the Earth. The cleft or polar cusp is the region in which some penetration of the solar wind is allowed to enter the ionosphere causing the slight kink to the magnetopause. In the central part of the magneto tail, when the field lines of different direction are located in very short distance causing they eliminate each other, there is a narrow region with zero field called the neutral sheet.

The plasmasphere region in the magnetosphere protected from the interference of the solar wind leads to ring currents related with the motion of ions in which the positive and negative ions move in opposite directions on the closed field lines. Out side the boundary of plasmaphere called plasmapause, the plasma is strongly influenced by the solar wind’s interaction. A few very energetic particle of the solar wind can also be able to penetrate into magnetosphere forming what is well known as the radiation belt or Van Allen Belt as result of the motion of the particle in the geomagnetic field experiencing an electromagnetic force which forces them to move around the field lines and send back and fore from pole to pole. Significant numbers of very energetic particles of the solar wind are also able to penetrate into magnetosphere under special condition i.e. the large increases of the solar wind’s intensity due to solar flares. They can travel and penetrate to the Ionosphere causing the magnetic storms and aurora.

There is evident from the magnetic observatory record that the geomagnetic field has a regular daily variation in which the solar daily variation and lunar daily variation plays a significant contribution in this daily variation. The solar daily variation, S, has a period 24 h while the lunar daily variation has period near 25 hour. During 24 h period, the additional field, D or the disturbance variation, can present and affect the regular daily variation. The solar quiet day variations, Sq, is a term where the geomagnetic field has a daily variation without extreme variations and it depends on the latitude and the local time. The solar disturbance daily variation, SD which is part of D, also present in the days with small disturbance or normal days. So, all of them can be formulated by using their relation with the difference between the total field and the average field (∆F) as

D = ∆F-Sq-L

The geomagnetic field sometimes shows the evident of the irregular disturbances on the magnetogram. The interaction of the solar wind with the geomagnetic field is the main factor in creating the irregular disturbances. When the intensity of these disturbances is very large, they are called a magnetic storm. The magnetic storm consists of three phases : the initial, main and recovery phases. The initial phase can be recognized as a sudden change called a sudden commencement. During the main phase, the horizontal component of the geomagnetic field experiences reduction in its intensity. The recovery phase is process in which the horizontal component increases significantly towards the auroral zone.
source