On August 21, 2017, I had the pleasure of witnessing a total solar eclipse. One of the main attractions was the 360 degree "sunset". But I am now wondering what does cause this phenomenon to happen since the redness aspect of a sunset is due to the scattering of the Sun's light over thousands of miles of atmosphere?
1$\begingroup$ Really, that "360 sunset light" in a solar eclipse is always there, it's just normally undetectable due to the overwheling effect of the overhead sunlight. $\endgroup$– Señor OSep 27, 2017 at 21:33
Although the Earth's atmosphere extends up to about 10,000 km above the surface, the atmosphere is much denser at lower altitudes, such that 50% of the mass of the atmosphere is contained in only the bottom 5.6 km. In contrast, the umbra of a total solar eclipse (the path of totality) is typically more than 100 km wide, and there's partial darkening (the penumbra) well beyond that. So if you're in the middle of the umbra, the sunlight that reaches you indirectly via Mie scattering from distant clouds needs to pass through many times as many particles in the atmosphere to reach you than would be the case from direct overhead sunlight, very similar to how light during a sunset needs to pass through many times as many particles in the atmosphere as overhead sunlight does.
Just as with a sunset, the Rayleigh scattering of light though that larger-than-usual number of particles mainly scatters away the shorter wavelengths of light, leaving predominantly the longer (redder) wavelengths to reach you.