What is a likely cause of irregular fading of signals propagated by the ionosphere?

When radio waves travel through the ionosphere, they are refracted and scattered along multiple different paths before returning to Earth. Because these paths have slightly different lengths, the signals arrive at the receiving antenna at slightly different times. Because the ionosphere is constantly shifting and changing, these multiple arriving signals will randomly combine. Sometimes they combine in phase and make the signal stronger, and sometimes they combine out of phase and cancel each other out, resulting in irregular fading (multipath fading). Thunderstorms create static noise, not fading, and intermodulation is a localized receiver issue.