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The figure illustrates the Cowling effect which explains how a patch of enhanced conductivity
feeds back on the imposed electric field to produce an intense ionospheric electrojet. The westward
electric field drives a westward Pedersen current and a poleward Hall current. At the poleward edge
of the high conductivity strip there is a divergence of Hall current and an excess positive charge
would accumulate. This gives rise to an equatorward polarisation electric field. The polarisation
electric field drives an equatorward Pedersen current that in part cancels the poleward Hall current.
The polarisation electric field also drives a westward Hall current that enhances the westward Pedersen
current. As a result an intense westward electrojet is established. This is the DP1 current system.
More detailed models are needed to explain some DP1 observations (Baumjohann et al., 1981).
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