At sunrise or sunset, when the sun is just below the horizon, this effect can be seen if the correct atmospherical conditions are met. What you see here is simply the earth's shadow being cast in the sky. One could say that it's the border between day (above) and night (below).

Since this phenomenon is seen all the way from side to side on the horizon, it's difficult to take a picture of the whole effect, this being only a small part. The horizon is faintly seen here and is levelled with the picture. The shadow, however, is leaning. Try to imagine it as a part of the whole bow displayed as the shadow of a spherical object, the planet earth.

The beginning of the shadow is seen here on the left, where the dark area rises above the horizon. To the right the typical atmospheric red shift at dawn or dusk is seen as well.

This is a cool pic with the full moon just above the earth shadow..