Besides the ever-present geomagnetic main field that varies a little from year to year,
there are small magnetic field perturbations that come and go in anything from seconds to days. It is these
changes that we are interested in because they are caused by external electrical currents that flow in
There are a number of external current systems that each contribute something to the magnetic field perturbation that we observe on the ground. In order of increasing perturbation magnitude, they are the magnetopause current, the ring current, and the ionospheric current.
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