High-latitude Sporadic-E and other Thin Layers – the Role of Magnetospheric Electric Fields

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Observations of high-latitude sporadic-E (Es) layers and theories of their formation are reviewed. The layers are found to be composed of metallic ions, they are at times formed by tidal wind shear, and they are more common in summer than in winter. All of these properties are common to Es layers at mid-latitudes. However, the high-latitude layers are rather often formed, modified or transported by the action of magnetospheric electric fields. Taking into account the action of both tides and electric fields leads to an understanding of the daily variation of Es occurrence, the daily variation of Es heights and the occasional appearance of upward migrating Es layers. Correlations between Es and neutral metallic layers at low altitudes can be explained by neutralisation of the metallic ions in the Es layers, but joint Es and neutral layers at higher altitudes are still unexplained. The action of electric fields and the interaction with neutral layers can explain the formation of multiple Es layers and may provide an explanation for the seasonal variation of Es occurrence. Uncertainties remain as to whether the narrowness of Es layers is fully compatible with formation by electric fields, to whether neutralisation at low altitudes can provide a sufficient explanation of the seasonal variation, and to whether the quasi-periodic fine structure observed in mid-latitude Es also appears at high latitudes. The understanding of ion transport by electric fields which results from the study of Es layers leads to new insights and new questions related to other plasma layers (polar mesosphere summer echoes and winter ion layers) which appear in the high-latitude winter ionosphere.