What is the difference between the Greyline and the Solar Terminator?
The "terminator" and "greyline" separate the daylight and dark sides of a planetary body, but their definitions differ. The terminator is a line tangent to the parent star, while the grayline is the twilight zone.
Why is radio propagation better along the greyline?
Propagation is ehanced along the "greyline" because there is no absorbing D-region there. Read a reasoned explanation.
The project "Understanding HF Propagation" provides a detailed overview and tutorials on HF propagation.