Spatial structure of high latitude sporadic E

Observations of high latitude types of sporadic E, Es, have been made at Halley, Antarctica (76°S, 27°W; L = 4.2) using a modern digital ionosonde with a direction finding capability. It is found that two types of Es, namely auroral and retardation, often exhibit a meridional structure in which the echoing region appears to be poleward of Halley by an amount which increases as the frequency of the probing radio wave increases. The behaviour of these layers is described in comparison with layers of similar appearance which are known to be located overhead. The observations are interpreted in the framework of a model consisting of a low density homogeneous Es layer which has field-aligned enhancements embedded in it. A simple analytical implementation of this model is found to fit the observations well. The meridionally structured layers cannot be distinguished from overhead layers simply by their appearance on ionograms. Thus high latitude Es data obtained from routine scaling of ionograms should be used with caution.