What Use Is An F-call?

What use is an F-call? In the past I've mentioned the Grey Line. If you recall, this is a twice daily phenomenon. During dawn and dusk, and often slightly before and after, there is an enhanced level of DX propagation. That is, if you're listening while it's happening, you'll notice far-off stations you didn't hear before, clear as day, then five or ten minutes later they're gone. Today I'm not going into the actual process that makes this happen, absorption, D and F layers and the like. The more you read about the physics of this, the more you'll get bamboozled with different and vociferous views. Instead I want to talk about something that hadn't occurred to me until earlier in the week. Once I tell you the response is likely going to be the same as mine was: Duh! So, propagation along, not across, along the Grey Line, is enhanced. So, the line is North-South, right? Well, yes and no. Twice a year it is, at the equinox, when the length of the day and the length of the night is the same, but otherwise, i