Regardless of setting, I get this band of brown haze at the horizon.
That's the 'Belt of Venus' - the shadow the Earth casts into its own atmosphere. In FG, it is somewhat darker than in most photographs for a couple of reasons:
* the O'Neil shader has a single scattering approximation, but air inside the shaded region is really illuminated by multiple scattering processes (which is complicated to implement) - the other shaders basically follow in brightness to blend with O'Neil part (anything else looks really silly). so they're also a bit darker than reality
* in photography, one typically brightens the scene by opening the aperture or prolonging the exposure time - many sunset photographs are not what you would see with bare eyes
* then there's the question of eye adaption of course - with the sun on one horizon and darkness on the other, it matters very much where you have looked before. If you've been looking into the sun for a moment, the rest of the scene will appear very dark, if you've been looking into darkness only, it will appear brightter. That's not in the simulation yet.
If you do a hue comparison between your screenshot and the real photo, you'll find that while your screenshot is darker (as explained above), the hue in FG is actually a bit less brown and more reddish than in reality. So despite this being dominated by multiple scattering processes (which is computationally really tough because you'd need another integral), the color hues are actually pretty good.