What effect does a sudden ionospheric disturbance have on the daytime ionospheric propagation?
Why A sudden ionospheric disturbance (also called a radio blackout) comes from the X-ray burst of a solar flare, which slams extra ionization into the D layer on the sunlit side of Earth. The D layer absorbs rather than refracts, and absorption goes roughly as 1/f^2, so the lower the frequency the harder the hit. During a strong SID, 80 and 40 meters can go completely dead while 15 or 10 meters may still support some contacts.
Watch out The idea that nothing happens because only the night side is affected has it backwards: the X-rays travel in a straight line from the Sun, so only the daylight hemisphere is ionized and disrupted. Satellite links at VHF and above punch through the D layer and are largely unaffected.
D layer absorption goes as 1/f^2: a SID kills the low bands first, and only where the Sun is shining.