Gray-line propagation is long distance propagation on three or four HF bands, sometimes up to 8,000 to 10,000 miles at sunset or sunrise, (twilight when the light is kind of gray). A picture of the gray-lline, (or terminator between daylight and darknes), where sunrise and sunset are occurring on the Earth is shown in the figure below generated by the Navy’s very cool ‘Day and Night Across the Earth’ calculator.
At sunrise and sunset, the solar absorption in the caused by ionization in the D layer of the ionosphere is reduced while the ionization in the F2 layer that is responsible for skip communications is not.
Exam questions and answers on this topic are below the picture.
What type of propagation is probably occurring if radio signals travel along the terminator between daylight and darkness?
At what time of day is gray-line propagation most prevalent?
A. At sunrise and sunset
B. When the sun is directly above the location of the transmitting station
C. When the sun is directly overhead at the middle of the communications path between the two stations
D. When the sun is directly above the location of the receiving station
What is the cause of gray-line propagation?
A. At midday, the sun, being directly overhead, superheats the ionosphere causing increased refraction of radio waves
B. At twilight, solar absorption drops greatly, while atmospheric ionization is not weakened enough to reduce the MUF
C. At darkness, solar absorption drops greatly, while atmospheric ionization remains steady
D. At mid afternoon, the sun heats the ionosphere, increasing radio wave refraction and the MUF
What communications are possible during gray-line propagation?
A. Contacts up to 2,000 miles only on the 10-meter band
B. Contacts up to 750 miles on the 6- and 2-meter bands
C. Contacts up to 8,000 to 10,000 miles on three or four HF bands
D. Contacts up to 12,000 to 15,000 miles on the 2 meter and 70 centimeter bands