The influence of solar X-ray flares on the SR

Solar X-ray flares emit a sudden burst of hard radiation, which penetrates into the upper part of the D-region of the ionosphere on the day side of the Earth and increases the ionization there. The higher conductivity shifts the effective upper boundary of the cavity lower and improves the reflection of waves, which leads to an increase in the frequency of the first mode of the SR.

Observed increases in the frequency of the 1st mode are on the order of ~8-11 % during strong flares, as recorded at a low-latitude station and during solar flashes at other observatories Singh 2014Satori 2016Ikeda 2018. The effect is most pronounced on the day (illuminated) side of the cavity, where the X-ray radiation falls, and it correlates in time with the course of the flare.

For data interpretation this means that short-term jumps in the frequency of the 1st mode need not reflect a change in lightning activity, but rather the reaction of the ionosphere to solar radiation. Distinguishing the two causes requires comparison with independent indices of solar activity.