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why earth's magnetic field vary with time?

The geomagnetic field varies in time. The spectrum of variations is very large, from fractions of a second up to millions of years. There are variations of both internal and external origin. The diurnal, seasonal and annual variations are primarily of external origin and are caused by the solar activity. The sun emits particles and radiation; these are responsible for ionisations in the ionosphere with associated electrical currents and magnetic fields. The longest known cycle of external origin known is about 11 years linked to the 22 year cyclicity of the solar activity. Variations of internal origin have much greater periods from a few years to millions of years. By definition the variations of the main field of internal origin are termed secular variations. For practical reason, the secular variation is the variation from year to year. Consequently, this variation is not completely of internal origin and still contains components of external origin.


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