It IS fascinating. The usual explanation doesn’t fully satisfy:
The “grey line” is a band around the Earth that separates daylight from darkness. Propagation along the grey line is very efficient. One major reason for this is that the D layer, which absorbs HF signals, disappears rapidly on the sunset side of the grey line, and it has not yet built upon the sunrise side. Ham radio operators and shortwave listeners can optimize long distance communications to various areas of the world by monitoring this band as it moves around the globe.”
To understand why this works the way it does I guess we would need live measurements of D-layer behaviour to compare with propagation results? For example, what do specific thicknesses of the layer allow for in terms almost of a temporary ducting? Boundaries are always where the interesting stuff is hidden.
Oh for a lab-sized D-layer to probe!