What makes HF scatter signals often sound distorted?

When HF signals propagate via scatter mode, they bounce off numerous, unevenly distributed irregularities in the ionosphere. Instead of a single, clean reflection, the signal breaks apart and scatters in many directions. Consequently, the scattered radio waves travel along several different paths of varying lengths to reach the receiver's antenna. These multiple signals arrive at slightly different times, causing rapid phase cancellations and additions. This multipath reception is the primary reason scatter signals sound fluttery or distorted. While auroral activity can also cause distortion, it is a distinct mode of its own; the fundamental cause of distortion in standard scatter propagation is the combination of multiple path lengths.