Simultaneous ground-based observations of the plasmapause and the F-region mid-latitude trough

The plasmapause and the mid-latitude ionospheric trough have been observed simultaneously from two Antarctic stations, Halley and Faraday, during five winter nights covering a range of geomagnetic disturbance conditions. The equatorial radius of the plasmapause was measured using whistlers recorded at Halley, whilst the poleward edge of the trough was located from ionospheric soundings at one or other of the stations. Before midnight the trough was well poleward of the plasmapause (by 1–2 L) when first observed (typically at ~21 LT), but then moved rapidly equatorwards. After local magnetic midnight the two features were roughly coincident, and in general moved slowly to lower L-shells with increasing local time. At no time were there simultaneous and identical movements of the two features, suggesting a lack of coupling between them. Agreement of the observations with statistical studies and models was fair, given the considerable variability among the five cases studied. For the geomagnetically quieter nights the trough data fit the Spiro model predictions, whereas in the most disturbed case, agreement is better with the Quegan et al. model. The latter model predicts a difference in L between the two features which would fit the data better if shifted 1–2 h later in local time.