The noon and midnight mid-latitude trough as seen by Ariel 4

The electron density data returned by the polar orbiting satellites Ariel 3 and Ariel 4 revealed that the mid-latitude trough is one of the distinct large-scale features of the ionosphere at about 550 km. Recent work (e.g. Tulunay and Grebowsky, 1975) on the data included the investigation of the temporal development of the latitudinal position of the mid-latitude electron density trough at dawn and dusk during the large magnetic storms of May 1967 and May 1972. Model calculations which assumed that the equatorial convection E-field varies in step with the K_p index reproduced on the average the observed behaviour. In the present paper, trough observations made at noon and midnight during the period, 12–21 December 1971 which encompassed a relatively large magnetic storm have been discussed. In this context, model calculations have been employed as a guide of average approximations of the actual situation in predicting the plasmapause location. It is also shown that the trough observed on the noon passes is not generally plasmapause related as we expect the nightside troughs to be.