What is Grey Line Propagation in Amateur Radio?

What is Grey Line Propagation in Amateur Radio?

Wiktionary definition of grey line is a transition line coinciding with twilight zone where a change in the propagation of radio waves is observed. Grey line propagation provides surprisingly long distance communications at dawn and dusk. I still remember how I used to work W-land (US) stations at dawn and dusk on CW using my homebrew setup on 40 m, nearly 4 decades back. I could never hear or work W-land at other times of the day on 40 and I did not have access to higher bands on my setup. That small time window corresponded to grey line propagation though I was not aware of it at that time. It is mentioned that grey line propagation occurs at frequencies below 10 MHz.

The D layer of the ionosphere which absorbs lower frequencies, fades at dusk as the illumination from the sun comes down at the Earths surface, and disappears at night. Ionization of the F layer is still better as it is at higher altitude, and gets illumination from the sun for a longer period. Moreover, as the air is thinner than D layer in the F layer, recombination of electrons with ions take longer period. For these reasons, F layer permits long distance communications at dawn and dusk. Grey line propagation is along the grey or twilight zone. That is how I was able to work W-land on 40 m during twilight, which is exactly on the other side of the globe for us!

During sunrise, F layer gets ionized earlier than the D layer. So frequencies which get absorbed by D layer gets access to F layer during dawn and gets reflected for a longer distance, permitting DX communications. The grey line propagation does not apply to higher frequencies which does not get absorbed by the D layer of the ionosphere. That is why it is more likely to occur on 40 m than 20 m. It is mentioned that grey line propagation is most noticeable at the 80 m amateur radio band. During my days of activity on HF nearly 4 decades back, I could not work any DX on 80 m, possibly due to the limitations of my homebrew setup.