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

[摘要] 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 d f 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 (>  M 6). 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 <  M 6).

[学科分类] 化学工程（综合）