See for example the following timelapse animation of an approaching sunrise, made from photo sequence shot in Namibia by Georg Zotti from the Stellarium team. Exposure time gradually decreases to account for sky brightness.

Below is a chart that shows time dependence of a middle vertical line of pixels from the animation above:

At first I thought this glow was due to some kind of very uniform cloud cover, but then I noticed that cirrus clouds, if a few are present near the horizon, stop being directly lit when the Sun is about 4.5° below the horizon, before the glow disappears, while the glow remains present until the Sun is somewhere about 5.5° below the horizon (these figures are from observations at 60°N,30°E). So I suppose the glow originates somewhere in the stratosphere, or at the very least several kilometers above the cirrus clouds.

I've been able to find out that there exist stratospheric sulfur aerosols, but I don't know whether they are responsible for this: whether they are so homogeneous horizontally across hundreds of kilometers, whether they are dense enough to cause this glow etc..

So, what exactly is the origin of this glow? What kind of particles, what size distribution and altitude profile do they have?