Using data of Es parameters obtained at a high mid-latitude station in Germany (Juliusruh; geogr. lat.: 54º 38' daily and seasonal variations were derived and certain characteristics of these variations have been analysed during a period of changing solar activity. It has been confirmed that diurnal maxima appearing both in foEs and fbEs are generally higher around solar activity maximum than around the minimum. Seasonal maxima appearing in summer are also higher around solar activity maximum than towards the minimum. The daily variation of h'Es showed two maxima: one in the morning and another in the afternoon. The seasonal maximum of h'Es generally appeared in April or May while the minimum in October or November. All these results confirm earlier findings derived for a station in Hungary (Békéscsaba). The present study particularly intends to show changes in the diurnal variation of Es parameters associated with geomagnetic disturbances. The difference between foEs and fbEs might be used as indicator of the transparency of the Es layer, and this quantity seems to increase on days with strong geomagnetic storms. Analysing the transparency based on the maximum values appearing in the daily variations of foEs and fbEs, a rather high quantity has been derived for stormy days in the year 1971. Actually, in this year, the daily variation of h'Es was also characterised by an extremely high afternoon maximum. The wind-shear theory is generally suitable for explaining the generation of sporadic E layer at mid-latitude. The present results are not inconsistent with the points of this theory.
Bencze P 1969: Pure and applied Geophysics, 75, 167--174.