Abstract. Systematic analysis of ionosopheric parameters measured at mid- and low-latitudes was performed to study the ionospheric response to solar flares. The lowest recorded ionosonde echo, the mimimum frequency (fmin, a qualitative proxy for the nondeviative radio wave absorption occurring in the D-layer), furthermore the dfmin parameter (difference between the value of the fmin and the mean fmin for reference days) have been investigated. The time series of the fmin and dfmin parameters recorded at meridionally-distributed ionosonde stations in Europe and South Africa were analyzed during eight X and M class solar flares during solar cycle 23. The solar zenith angles of the observation sites at the time of the selected flares have been also taken into account. Total and partial radio fade-out was experienced at every ionospheric stations 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 fmin (2–9 MHz) and dfmin (1–8 MHz) parameters was observed at almost every stations. The fmin and dfmin parameters show an increasing trend with the enhancement of the X-ray flux. Based on the results, the dfmin 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
Comments on manuscript “Investigation of the ionospheric absorption response to flare events during the solar cycle 23 as seen by European and South African ionosondes”
