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