Because the external magnetic field perturbation observed on the ground is dominated by the contribution
from ionospheric currents, any ground magnetic perturbation to the main field is
commonly interpreted in terms of an "equivalent current". This is the infinite plane current that would
have to flow in the overhead ionosphere in order to produce the observed ground magnetic perturbation.
The magnetic perturbation due to an infinite plane current is independent of
distance from the current sheet, is parallel to the current sheet and is polarised at right angles
to the current. Thus if the magnetic perturbation is northward and eastward (i.e., dH > 0, dD > 0)
then the equivalent current is flowing eastward and southward.
An infinite plane current sheet is a very gross approximation to the spatially limited ionospheric current systems that actually exist. Nevertheless, the equivalent current method is a better approximation to the overhead current than one might think. This is because the total magnetic perturbation is an integral over all current elements in the current sheet. From the Biot-Savart law, the magnitude of the magnetic perturbation due to each individual current element is inversely proportional to the square of the distance from it. In the case of an infinite plane current sheet at 100 km altitude, 71% of the horizontal magnetic perturbation comes from the current within a 600 km x 600 km area centred on zenith.
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