Earth’s terminator or Twilight Zone, is the line drawing the separation of day and night.
The twilight or change in atmospheric color is caused due to the scattering and refraction of light in gaseous layer of the atmosphere.
Grey-Line radio propagation is the strengthening of radio waves along the terminator in the ionosphere. Amateur radio operators take advantage of this phenomenon to perform long distance communications that are difficult under normal conditions.
I rendered this in 4k at 500 samples on a RTX2080 and used D-Noise. The render took approximately 5hr and 30mins. For anyone using a NVIDIA card I recommend D-NOISE.
Work in progress pictures