Grey Line Propagation

Grey line propagation provides surprisingly long distance radio communications at dawn and dusk sometimes when other forms of ionospheric propagation may not be expected to provide signal paths of these distances.
This mode of propagation is widely used in ham radio, often providing signals from the other side of the globe at dawn and dusk.
When listening to signals coming in via grey line propagation, it is particularly interesting to hear how the signals come up in strength and then fall away again. It is also possible to hear how the propagation moves as the dawn or disk line changes and first stations in one area and the next are making contacts via the grey line.
Find out how grey line propagation arises and how it can best be used.

https://www.electronics-notes.com/articles/antennas-propagation/ionospheric/greyline-propagation.php