The role of ion drift in the formation of ionisation troughs in the mid- and high-latitude ionosphere—a review

F-layer ionisation troughs are frequently observed in the sub-auroral and high-latitude ionospheres. We define the mid-latitude trough as the region of low plasma concentration at F-region altitudes that occurs near the equatorward side of the low latitude edge of the energetic electron precipitation boundary of the auroral oval. High latitude troughs are simply defined as troughs that occur in the auroral oval and polar cap. We review the progress that has been made in describing the phenology and morphology of the mid-latitude trough since the review by Moffett and Quegan [(1983), J. atmos. terr. Phys. 45, 315]. We also provide the first summary of observations of the high latitude trough. We then go on to describe the physical processes which can lead to trough formation. We discuss separately the importance of production, loss and both vertical and horizontal transport for the formation ofF-region troughs. We conclude that the consequences of ion velocity in the rest frame of the neutral particles is of paramount importance for trough formation through dynamical and chemical processes. We consider the geophysical conditions and the locations where trough formation is most likely both during relatively quiescent geomagnetic periods and during periods when high latitude electric fields are large and varying rapidly with time. We describe the characteristics of the resultant troughs, such as electron and ion temperatures and ionic composition. We propose a new, more rigourous definition for sub-auroral ion drift events (SAIDs) based upon ion motion in the neutral particle rest frame. Sophisticated computer modelling of several situations is provided to support the tenets of the trough formation presented in the paper. Despite the unifying theory of trough formation presented here, several areas for further theoretical, computational and observational study are identified.

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