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Magnetic variations and magnetic storms

Short-period magnetic variations were first observed in the Eighteenth Century, by observing a compass needle with a microscope. It was soon realised that there was a fairly regular daily variation, and also occasional larger irregular variations. These irregular variations were found to coincide with the occurrence of auroras.

By the Nineteenth Century, it was realized that both phenomena followed the occurrence of disturbances on the sun, with a delay of about 18 hours between the observation of a solar flare and the related aurora and magnetic field disturbances. This indicated that something was travelling at a speed greater than 1000 km/s from the Sun to the Earth which disturbed the magnetic field as it arrived.

In the early Twentieth Century, it was realized that a cloud of ionized particles would act as a conductor. As such a cloud approached the Earth, the interaction between the moving conductor and the static magnetic field would produce circulating currents, producing a variable magnetic field. Our models of these upper atmospheric processes have improved greatly since satellite observations began in 1958.

The level of activity of the sun follows the well-known 11-year sunspot cycle.