Greyline and the terminator

The terminator is the moving line on Earth's surface that separates day from night. The grey line is the band of twilight that follows it, typically a few hundred kilometers wide. For ham operators, the grey line is special because it produces a brief window where HF propagation is unusually good for long-distance contacts, especially on the lower bands (160m, 80m, 40m).

Why HF opens up along the grey line

Most HF bouncing happens off the F-region of the ionosphere, but the lower D-region absorbs HF energy during daylight, especially on the lower bands. At night the D-region recombines and disappears, so 80m and 40m become long-haul bands. During the day, the same bands are limited to a few hundred miles.

At dawn and dusk, the path between two stations on opposite sides of the terminator passes through the F-region (which is still ionized from daylight) without crossing the absorbing D-region (which has dissipated). The result: low-band signals that would be absorbed in full daylight or attenuated during the night get a brief window of low-loss long-haul propagation.

The window typically lasts 15 to 45 minutes per station, but it can be the only time of day when a particular long-haul path is workable. Pacific to Europe via 80m is the classic example, only viable during the few minutes when both ends are on the grey line.

How the terminator is computed

The terminator is the great circle perpendicular to the line from Earth's center to the sun. Its position depends on:

• Time of year (solar declination, oscillating ±23.5° over the

year). Determines whether the terminator tilts north or south.

• Time of day (Earth's rotation). Determines the longitude of

the terminator.

The /greyline tool computes the subsolar point from the NOAA Solar Position Algorithm and then draws the terminator as a great circle 90° from it. The twilight band is a second offset (typically 6° for civil twilight, 12° nautical, 18° astronomical) that brackets the terminator.

The greyline operator's checklist

1. Find your local sunrise or sunset time.
2. Use the /greyline path planner: enter your QTH and a target

DXCC entity. The tool shows when your terminator and theirs are on the same great circle.

1. Pick a band the conditions suggest is open (low bands at dusk

for trans-oceanic; higher bands at dawn for trans-equatorial).

1. Listen first. The grey line opening is brief; if a band is full

of DX it is open; if it is quiet you missed the window.

See also

• NOAA Solar Position Algorithm

(Reda and Andreas, NREL TP-560-34302) for the underlying solar geometry math.

• Natural Earth coastlines

used by the /greyline globe visualization.

• [[hf-propagation]] for the broader picture of why solar geometry

matters for HF.