What is the difference between the Greyline and the Solar Terminator?
The greyline, also known as the "twilight zone" or "terminator line," is a narrow band that separates the illuminated day side and dark night side of the Earth. It is the transition zone between daylight and darkness, and it moves around the Earth as the planet rotates.
The greyline appears as a grey area on a map that shows the day and night sides of the Earth. This line is important in radio communication because it provides an opportunity for long-distance communication using high-frequency (HF) radio waves. HF radio waves can travel long distances by bouncing off the ionosphere, and the greyline is where the ionosphere is most active, making it easier for radio waves to propagate across long distances.
In addition to its importance in radio communication, the greyline is also a popular spot for amateur radio enthusiasts who engage in a practice known as "greyline DXing." This involves attempting to make long-distance contacts with other radio operators around the world using the propagation-enhancing properties of the greyline.
What is Solar terminator?
While greyline refers to Earth the solar terminator is the line or boundary between the illuminated and dark regions of a planet or moon that is experiencing sunlight. Specifically, it's the boundary or the line that separates the day side and night side of a celestial body. It's the line or curve that marks the edge of the shadow on the planet or moon where the sun is rising or setting.
The solar terminator is important in space observations, as it is where the day and night side meet, and it provides an opportunity for observation and study of the interactions between the two sides of the planet or moon.
On the Earth the solar terminator moves west to east, and it takes about 24 hours to complete one rotation around the Earth.
You may find complementry information here. See also an index for HF Radio Propagarion.