A summer, dayside, mid-latitude trough detected by a digital ionosonde located at Halley (76°S, 27°W, L = 4.2) is described. The trough is found to be present in the F2-region only and its movements are found to conform to known trough dynamics. The F1-layer shows a greater degree of development within the trough; slant type sporadic E reflections are present underneath the trough minimum. Satellite data from the northern hemisphere show a conjugate trough, with rapid ion flow occurring within it. Possible formation processes for the trough are examined. It is unlikely that depleted nightside plasma could have contributed to the trough. The trough is formed by the effect of enhanced F2 recombination rates combined with a differing solar production term for the plasma associated with the trough minimum and equatorial edge.