Impact of Solar Flares on the Ionosphere During Geomagnetically Quiet Periods Based on Ionosonde Data

The ionosphere is the ionized region of the atmosphere extending from 50 km to 1000 km. During flares, the Earth’s surrounding space is subjected to high-energy X-ray and EUV radiation, which also impacts the ionosphere. The changes of the ionospheric parameters measured by ionosondes, namely the fmin, foE and foF2 values, were examined during solar flares that occurred in geomagnetically quiet conditions (Dst > -40 nT, Kp < 4). The necessary data were derived from manually evaluated ionograms recorded by a DPS4D ionosonde at Pruhonice station in Czechia (PQ052). The degree of variation was compared to quiet reference days, allowing the determination of the deviations in the required values (dfmin, dfoE, foF2). Time series of the deviations were investigated. Furthermore, the relationship between the deviations and a “geoeffectiveness” parameter of the solar flare defined by some important properties of the event was also examined. The X-ray flux, the solar zenith angle of the station at the time of the event, and the position of the flare on the solar disk were also taken into account for the determination of the "geoeffectiveness" parameter. A positive correlation was observed between dfmin and the "geoeffectiveness" parameter of the flare, which was more significant than the correlation between the dfoF2 and the "geoeffectiveness" parameter.