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. We have investigated the solar flare effects onionospheric absorption with the systematic analysis of ionograms measured atmidlatitude and low-latitude ionosonde stations under different solar zenithangles. The lowest recorded ionosonde echo, the minimum frequency ( fmin , aqualitative proxy for the “nondeviative” radio wave absorption occurringin the D-layer), and the d fmin parameter (difference between the value ofthe fmin and the mean fmin for reference days) have been considered. Datawere provided by meridionally distributed ionosonde stations in Europe andSouth Africa during eight X - and M -class solar flares in solar cycle 23.Total and partial radio fade-out was experienced at every ionosphericstation during intense solar flares (>  M 6). The duration of thetotal radio fade-out varied between 15 and 150 min and it was highlydependent on the solar zenith angle of the ionospheric stations.Furthermore, a solar-zenith-angle-dependent enhancement of the fmin (2–9 MHz) and d fmin (1–8 MHz) parameters was observed at almost every station.The fmin and d fmin parameters show an increasing trend with the enhancementof the X-ray flux. Based on our results, the d fmin parameter is a goodqualitative measure for the relative variation of the “nondeviative”absorption, especially in the case of the less intense solar flares, which donot cause total radio fade-out in the ionosphere (class  M 6).

Effects of solar flares on the ionosphere as shown by the dynamics of ionograms recorded in Europe and South Africa