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

We have investigated the solar flare effects on ionospheric absorption with the systematic analysis of ionograms measured at midlatitude and low-latitude ionosonde stations under different solar zenith angles. The lowest recorded ionosonde echo, the minimum frequency (f(min), a qualitative proxy for the "nondeviative" radio wave absorption occurring in the D-layer), and the df(min) parameter (difference between the value of the f(min) and the mean f(min) for reference days) have been considered. Data were provided by meridionally distributed ionosonde stations in Europe and South Africa during eight X- and M-class solar flares in solar cycle 23. Total and partial radio fade-out was experienced at every ionospheric station during intense solar flares (> M6). The duration of the total radio fade-out varied between 15 and 150 min and it was highly dependent on the solar zenith angle of the ionospheric stations. Furthermore, a solar-zenith-angle-dependent enhancement of the f(min) (2-9 MHz) and d f(min) (1-8 MHz) parameters was observed at almost every station. The f(min) and d f(min) parameters show an increasing trend with the enhancement of the X-ray flux. Based on our results, the d f(min) parameter is a good qualitative measure for the relative variation of the "nondeviative" absorption, especially in the case of the less intense solar flares, which do not cause total radio fade-out in the ionosphere (class < M6).