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External Origin Time Variations


It is known that the magnetic field observed at the Earth’s surface is by no means constant, but rather subject to variations on all time scales (from milliseconds to millions of years).
Of primary interest here are the fluctuations with periods from a few minutes to several days. These short-term variations are indeed of external origin and arise from the currents flowing in the ionosphere and magnetosphere. The high degree of complexity of the solar wind-magnetosphere-ionosphere coupling results in a large variety of magnetic signatures, which depend upon the state of the magnetosphere, and differ with the geographic and geomagnetic location of the observatory. Thanks to decades of observation, it is possible to delineate the main morphological features of the perturbations observed on the ground and to relate them to ionospheric and magnetospheric sources.
A fundamental starting point for the studies of these variations is their decomposition into regular and irregular parts.
Regular variations are all those variations that have both a smooth shape and a regular occurrence while irregular variations are characterized by a great variability in both shape and intensity and an irregular occurrence.

 

Figure 1 Schematic representation of the most important time variations observed in geomagnetic data and due to external sources.

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