Gray line
| Gray line | |
|---|---|
| Also called | Grayline, the terminator, twilight propagation |
| What it is | The band of twilight that circles Earth between day and night |
| Main effect | Low D-layer absorption while the F layer is still ionized |
| Bands most affected | 160, 80 and 40 m; also 30 and 20 m |
| Timing | |
| When | Around local sunrise and sunset |
| Window | Usually short; on 160 m it can last only minutes |
| Speed | Sweeps west at roughly 1,000 mph at the equator |
| Seasonal change | Runs north–south at the equinoxes; tilted by up to about 23.5° near the solstices |
The gray line is the band of twilight that circles Earth, dividing the half lit by the sun from the half in darkness. Astronomers call the boundary the terminator. For radio amateurs it is one of the most useful features of propagation: around sunrise and sunset, signals on the lower HF bands often travel farther and arrive stronger than at any other time of day, and paths that run along the twilight band can be remarkably good. Low-band DXers, who chase long-distance contacts on the lowest bands, plan much of their operating around it.
Why it works
Two layers of the ionosphere behave very differently at dawn and dusk.
The D layer, about 60 to 90 km up, absorbs HF signals rather than returning them, and it absorbs the lower frequencies most. It exists only while the sun shines on it. The air at that height is relatively dense, so the free electrons recombine quickly, and the layer fades soon after sunset and does not rebuild until after sunrise.
The F layer, roughly 150 to 400 km up, does the long-distance work. The air there is so thin that recombination is slow, and the layer keeps much of its ionization through the night.
At dusk, then, D-layer absorption drops away while the F layer is still well ionized from the day. At dawn the absorbing layer has not yet formed, and the F layer begins to strengthen as sunlight reaches it, which happens high above the ground before the sun rises at the surface. For a short time at each end of the day, the ionosphere offers strong refraction with little absorption, the best combination for the 160, 80 and 40-meter bands. A path that lies along the twilight band enjoys those conditions for much of its length.
Using the gray line
Hams use the gray line in two main ways.
- Along the line. Two stations that are both in twilight, one at sunset and the other at sunrise, can often work each other on the low bands even when the path is very long. At the equinoxes, for example, sunset on the US East Coast comes at about the same moment as sunrise in Southeast Asia and Indonesia, and low-band signals between those regions often peak then.
- At one end. Even when the far station is in full darkness, a station at its own sunrise or sunset often finds its signals noticeably stronger. Many 160-meter DXers report a short burst of excellent signals right around local sunrise, followed by a quick fade as the D layer forms.
The effect is strongest on the lowest bands but reaches the 30 and 20-meter bands too, where the gray line can bring openings to unusual places. It also helps explain many long-path contacts, in which a signal travels the long way around the world along the twilight band; see Long path and short path.
Seasons and geometry
Because Earth's axis is tilted by about 23.5 degrees, the terminator's position changes through the year. At the March and September equinoxes it runs straight north–south through both poles, so sunrise comes at about the same time all along a line of longitude. Toward the June and December solstices it tilts, reaching about 23.5 degrees from north–south at the solstice itself. As a result, the places that share twilight at any moment shift from day to day, and a gray-line path that works well in December may not line up at all in June.
Latitude matters as well. Near the equator the sun rises and sets steeply, so twilight is brief. At high latitudes it rises and sets at a shallow angle, twilight lasts much longer, and near the summer solstice places far enough north have no true night at all. Low-band DXing as a whole favors winter, when nights are long and summer thunderstorm static is much lower.
Tools
Many logging programs, DX cluster displays and websites show a live map of day and night with the terminator drawn in, and some can list sunrise and sunset times for any location. A common method is simply to compare local sunrise and sunset with those of the DX station's country and be on the air when the two coincide. Watching the gray line move across a map also makes it clear why the best time to work a given place changes with the season.