Current systems are important in an understanding of ionospheric perturbations associated with the onset of geomagnetic storms and the progression of these events. There are four principal current systems in the ionosphere that give rise to relatively rapid fluctuations in the geomagnetic field. These systems include: the ring current, the magnetopause current system, the atmospheric dynamo, and various high latitude current systems. The first two are associated with magnetic storms and occur at magnetospheric distances. The atmospheric dynamo is important in an understanding of tidal-driven forces, which interact with the ionospheric plasma causing a vertical drift of the F-region ionization. Descriptions of ionospheric current systems and dynamo theory may be found in a monograph co-authored by Rishbeth and Garriott  and in a book by Ratcliffe . High latitude currents (i.e., polar and auroral) and atmospheric dynamo currents are observed at lower ionospheric heights in the vicinity of the E-layer.
Brekke  provides a good treatment of relevant high latitude current systems. There is also a current system within the neighborhood of the magnetic equator, the equatorial electrojet, which flows along the geomagnetic equator, eastward by day and westward by night. It is associated with a class of discrete ionospheric formations that are termed equatorial sporadic E.
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