Grey line propagation enhancement is magic on the HF bands, for both ham radio and shortwave listening at long distances.
Time for my sunrise in Calgary. And, sunset in Madagascar. I am listening to the Eritrean Christen broadcaster Radio Maezer Samay on 11,705 MHz, transmitting from Talata Volonondry in Central Madagascar, east of South Africa.
As you can see from the map at the left, this high frequency radio signal is traveling along the great circle terminator, or the line path of sunrise-sunset. My reception of this distant signal (16,000 km) is quite strong, about S9. So, why is this distant signal so strong?
Two reasons. First, the transmitting antenna is beaming towards east Africa, which also means it is also beaming towards me in Calgary along the great circle path. Second, and more important, is grey line propagation enhancement.
As you can see in the lower-left propagation chart, two things are happening. First, you can see the maximum useable frequency (MUF, in yellow) rising to include 11.7 MHz. Second, you can see the lowest useful frequency (LUF, in blue) falling dramatically. This grey line propagation enhancement is typical whenever the transmitting and receiving stations are both near the terminator.
Grey Line Propagation – How It Works
You can understand this phenomena by considering ionospheric layers and spherical geometry. Simply put, as the ionosphere moves from darkness to light, the higher layers are illuminated differently than the lower layers. As you can see above right, the lower D layer drops off quickly (due to chemistry), while the higher F layer builds up.
Since the D layer is responsible for absorbing or attenuating HF signals, absorption falls rapidly. At the same time, the F layer gets better at propagation of signals. This is why grey line propagation gives you a sweet spot at local sunrise for long distance communications.
Check out this more detailed explanation at Electronics Notes.