The Impact of Solar Flares on the Ionosphere During Geomagnetically Quiet Periods

The ionosphere is the ionized region of the atmosphere extending from 50 km to 1000 km. During solar flares, the near-Earth space environment is subjected to enhanced high-energy X-ray and EUV radiation, which significantly impacts ionospheric conditions. Variations in ionospheric parameters measured by ionosondes, specifically the fmin and foF2 values, were examined during solar flares occurring under geomagnetically quiet conditions (Dst > −40 nT, Kp < 4) between 2023 March and 2024 June . The required data were obtained from manually evaluated ionograms recorded by the Czech DPS4D ionosonde at Pruhonice (PQ052).

The degree of variation was determined by comparison with monthly mean values, allowing the calculation of deviations in the studied parameters (dfmin, dfoF2). Time series of these deviations were analysed. Furthermore, the relationship between the ionospheric deviations and a flare “geoeffectiveness” parameter was investigated. This parameter was defined by considering the X-ray flux, the solar zenith angle at the station at the time of the event, and the position of the flare on the solar disk. A positive correlation was found between dfmin and the flare geoeffectiveness parameter, which proved to be stronger than the correlation obtained for dfoF2. In addition, a cumulative dfmin parameter was introduced, and its correlation with integrated X-ray flux values was examined. In this case as well, the flares were separated by intensity classes, similarly to the non-integrated analysis. The strongest correlation was obtained for flares above M6, reaching a maximum correlation coefficient of 0.97. 

The relationship between EUV radiation and the ionospheric parameters was also investigated; however, these correlations were found to be considerably weaker and did not reach comparable levels of statistical significance.