Sudden Enhancements ofF-Layer Ionization in Polar Regions

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Abstract
A study of the morphology of large sporadic enhancements of F-region electron density in the darkened portions of the polar cap reveals a close connection between “sporadic F” and ionospheric current systems. This connection arises from electric fields which are set up by these currents and then transferred to the F-region along an essentially vertical magnetic field. Under the influence of the electric and magnetic fields, a horizontal drift of ionization may become sufficiently strong to cause the development of sporadic F. Drift velocities obtained from synoptic analyses range between 2000 and 5000 kilometers per hour and agree with theoretical estimates. Criteria are presented for the development of sporadic F events, and the general diurnal and seasonal variations in the frequency of occurrence are inferred.

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