What Use Is An F-call?

What use is an F-call? Much of what Amateur Radio is about relates to the planet we live on. I realise that you might think that it's about radios, antennas, contacts and logging, but the vast majority of our hobby relates directly to the Ionosphere, the Sun, the Earth and all manner of other natural influences. One of these influences happens twice a day, Sunrise and Sunset. The transition from light to dark and from dark to light has a period during which there is twilight. This period in amateur radio is called the Grey Line, because if you were to map it on a map of the earth, there's a line that travels all the way around the globe where the separation between dark and light exists. A funny phenomenon occurs along this line. Propagation is very efficient along its path. You might recall that there are several layers of the Ionosphere. Closest to us is the D-layer, then up from that the E and F layer. You might also remember seeing satellites in the early evening, they are still in sunlight while you'