E3B10 (B)
What is the cause of gray-line propagation?
A. At midday, the Sun super heats the ionosphere causing increased refraction of radio waves
B. At twilight and sunrise, D-layer absorption is low while E-layer and F-layer propagation remains high
C. In darkness, solar absorption drops greatly while atmospheric ionization remains steady
D. At mid-afternoon, the Sun heats the ionosphere decreasing radio wave refraction and the MUF
