What effect does a sudden ionospheric disturbance have on the daytime ionospheric propagation?

A sudden ionospheric disturbance (SID) is caused by an intense burst of X-rays and ultraviolet radiation from a solar flare. This radiation penetrates deep into the lower D layer of the ionosphere on the sunlit (daytime) side of the Earth, rapidly increasing its ionization. Unlike the higher F layer which refracts signals, a highly ionized D layer acts like a sponge and absorbs passing radio signals. Because absorption in the D layer is inversely proportional to the square of the frequency, lower frequencies are absorbed much more heavily. As a result, lower-frequency HF bands (like 80m or 40m) are severely disrupted or completely blacked out, while higher frequencies (like 10m or 15m) might still pass through with less attenuation.