At any moment, a band of twilight runs around the Earth: sunrise on one side, sunset on the other. Radio listeners call it the grey line, and for an hour or so either side of it, long-distance reception can be remarkable.
Why it works
In daylight the ionosphere’s lowest layer, the D layer, absorbs lower-frequency signals (see how skywave works). At night that layer disappears, but the higher F layer, which does the reflecting, also weakens over the hours.
Twilight gets the best of both. On the sunrise side, the D layer hasn’t formed yet; on the sunset side, it has just faded, while the F layer is still well charged. A signal travelling along the grey line meets little absorption and a good reflector, so it can go a long way.
How to use it
- Watch the map. The Skywave DX map shades night and day; the edge between them is the grey line.
- Look along the line, not across it. The best paths join two places that are both in twilight, such as your sunset and someone else’s sunrise.
- Try the lower bands. Grey line effects are strongest on medium wave and the lower shortwave bands, 90 to 41 metres (see the band guide).
- Be quick. The opening often lasts 30 to 60 minutes.
The seasons
The grey line tilts with the seasons. Around the equinoxes in March and September it runs nearly north–south; around the solstices it leans, linking different parts of the world. Paths that work in December may not work in June.
Catch something good at twilight? Log it: a DX log shows you which paths open at which times of year.