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

Irregular fading of signals propagated by the ionosphere can often be attributed to the random combining of signals arriving via different paths, a phenomenon known as multipath propagation. As radio waves bounce off the ionosphere, they can take various paths to reach a receiver. When signals from these different paths arrive at the receiver, they can either constructively or destructively interfere with each other, leading to fluctuations in signal strength. This unpredictability in signal combinations results in the irregular fading experienced in some communication situations.

Other factors, like frequency shifts from Faraday rotation or interference from thunderstorms, can affect signal quality as well, but these do not typically result in the kind of random, irregular fading associated with multipath propagation. Intermodulation distortion involves unexpected mixing of signals rather than direct signal fading, which further distinguishes multipath impacts on signal transmission.