The poleward edge of the mid-latitude trough—its formation, orientation and dynamics

Data from the Advanced Ionospheric Sounder (AIS) deployed at Halley, Antarctica (76°S, 27°W; L = 4.2) and the Dynamics Explorer-2 spacecraft (DE-2) are used to investigate several aspects of the formation processes and dynamics of the poleward edge of the mid-latitude electron density trough. These include a study of the flux and energy of charged particles precipitating into the F-region as a function of Magnetic Local Time. It is found that local energetic electron precipitation is a major source of ionisation of the poleward edge in the evening sector, but after magnetic midnight transport processes become more important. Occasionally a significant increase in the flux of conjugate photo-electrons is co-located with the poleward edge of the trough in the morning sector. Some possible mechanisms are discussed but no firm conclusions are drawn. The combination of AIS and DE-2 data has allowed identification of significant longitudinal structure on the poleward edge of the trough that may be the result of substorm activity. It is found that the orientation of the poleward edge of the trough and the locus of the plasmapause predicted from the ‘tear-drop’ model vary in rather a similar manner with local time, though no close physical link between the two features is inferred from this coincidence. A comparison of the equatorward motion of the poleward edge on many nights is used to show that Kp is a poor index to use in any empirical model for predicting the temporal variations of the location of the trough. It is suggested that a more thorough understanding of the processes controlling the variability of the magnetospheric convection electric field is required before any significant improvement to the empirical models is likely to occur.

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