Hysteresis of foF2 at European middle latitudes

The hysteresis of foF2 is studied for several European stations over the wh ole 24-hour diurnal interval for the equinoctial months of the years just b efore and just after the solar cycle minimum for solar cycles 20 and 21. Ba sed on previous results, the hysteresis is expected to develop best just fo r the equinoctial months and near the solar cycle minimum. The hysteresis i s generally found to be negative, i.e. higher foF2 for the rising branch co mpared to the falling branch of solar cycle. However, this is not the case in some individual months of some years. The noontime hysteresis represents the hysteresis at other times of the day qualitatively (as to sign) but no t quantitatively. The hysteresis appears to be relatively persistent from o ne solar cycle to another solar cycle in spring but not in autumn. A typica l value for springtime hysteresis is about 0.5 MHz. The inclusion of hyster esis into longterm ionospheric and radio wave propagation predictions remai ns questionable.