Let's talk about greyline propagation.
Greyline propagation, or propagation along the earth's sunrise/sunset terminator, is very noticeable on the 160, 80, and 40 meter ham bands and lower shortwave bands. Mediumwave signals can and do travel that path as well.
What does the greyline look like on a map?
There's a great solar clock map I've found which I use called Simon's World Map. You can start it and leave it on your desktop.
It's freeware. The creator (Simon) is the guy involved in the HF+ Discovery SDR project, competitors to the SDRPlay and SDRUno, and others.
A screenshot of mine in use right now:
The night/day overlay over the world map shows the width of the earth's terminator and its transition from total darkness to the sunrise and sunset edge. The actual sunrise or sunset point is the edge where the overlay disappears. The lightest band of the overlay is civil twilight, where the sun is just below the horizon to a point 6 degrees below the horizon. The next darkest band is nautical twilight, where the sun is between 6 degrees and 12 degrees below the horizon. The next darkest band is astronomical twilight, where the sun is between 12 degrees and 18 degrees below the horizon. The darkest area is where total darkness exists, where the sun's position is greater than 18 degrees below the horizon. Medium wave and other signals in the lower shortwave bands, as described above, can propagate along the terminator at greatly reduced signal attenuation - often received at astonishing distances.
Space Weather Specialists define it this way on their Real-Time Maximum MUF page, found here:
Their definition puts the greyline stripe exactly between the rising or setting sun and the 12 degree point of darkness below the horizon, also known as the Nautical Twilight point. However, they seem to ignore any greyline existence on the daylight side of SR/SS.
Civil twilight is halfway to Nautical Twilight, or 6 degrees below the horizon.
Published sunrise/sunset times in newspapers and charts almost always base their times on "Official" sunrise/sunset which is 90 degrees 50 minutes, or 90.833333 degrees from solar zenith, making their times several minutes different from actual SR/SS. Actual SR/SS is based on 90 degrees from solar zenith.
I would argue that greyline conditions of course exist for a time past sunrise and before sunset, straddling the actual SR/SS. How much? I would tend to split it at about 6-9 degrees on either side of SR/SS, but it depends on the frequency too. 6 degrees rotation of the earth is 24 minutes of clock time.
12 degrees below the horizon seems a bit much to me. 12 degrees of earth curvature is 828 miles or 48 minutes of sun travel which is quite a departure from actual sunrise and sunset. Again, I would tend to split it at 6-9 degrees on either side of SR/SS. 7.5 degrees is 30 minutes, which seems about right.
VOACAP has a nice calculator which gives their representation of greyline start and end times by location and date:
VOACAP uses the 6 degree below the horizon (darkness side) and 3 degree above the horizon (daylight side) points for their greyline calculations.
Their greyline notes can be read here.
My mediumwave broadcast pattern map set for 2022 is almost ready for publication, coming mid-October. This year it will be available exclusively on The Medium Wave Circle. Many changes and updates have been implemented over the last 18 months. One is a greyline day/night overlay, similar to Simon's World Map. Coming soon!
Greyline is a good topic for discussion. Maybe others have thoughts.
Bill
