The Twilight Zone: just what is 'grey-line' propagation?

摘要

World-wide communications using the MF and HF bands are dependent on radiation coming from the sun. But twice a day, at sunrise and sunset, the ionosphere undergoes dramatic changes, giving enhanced propagation in some directions. In terms of radio propagation, the D and E layers are responsible for most of the absorption of radio waves that pass through them, but the absorption is frequency dependent. The D layer can completely absorb signals on 160, 80 and 40 metres during the day, and can attenuate signals on 20m too. Hence the reason you don't hear much, if any, DX on the low bands during the day as sky-wave signals are absorbed before they can reach the reflective (more correctly, refractive) E and F layers. The ionosphere undergoes a dramatic change in ionisation at the transition from day to night. The electron (and ion) density in the E-layer decreases by a factor of 200 to 1 and the F1 by nearly 100 to 1 (see Fig 1). At sunset, the D layer disappears rapidly.