The analysis of 3 months’ observations of Faraday rotation observed on S‐66 transmissions made at the Kiruna Geophysical Observatory (ϕ = 67°N, A= 20.3°E) is presented. The ionospheric electron content along the subionospheric path of the satellite was computed for almost 400 transits. Effective slab thicknesses were calculated using critical frequency values from six Scandinavian stations, covering an interval of 10 deg of latitude. Latitudinal and diurnal variations of the ionospheric electron content and the effective thickness were studied. The most striking feature of the latitudinal variation of the total electron content was a distinct minimum at the southern boundary of the auroral zone during winter nighttime. This minimum coincides with the “trough” observed by topside sounding satellites.
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Ludwik Liszka (1966) studied this question.