Why is the upper atmosphere brighter than the lower atmosphere in some photos from space?
The glow observed in those pictures in the upper layers of the atmosphere is the airglow from OH molecules. OH emission peaks between 75 and 105 km of elevation (Blamont and Reed, 1967) with intensities peaking at different wavelengths, the most prominent peaks in the visible spectrum are 557nm (Green), and multiple other peaks between 620 and 750 nm (red)

In 1965 the OGO-II satellite was launched with the propose to measure airglow, the following image show a typical altitude vs intensity profile:



From Blamont and Reed, (1967). The peak around 100 km is mostly due to OH airglow and the broader peak around 250 km is mostly due to atomic oxigen.

Regarding the color observed in the pictures, while it is difficult to trust them due to photo editing, they are consisten with OH airglow, in the greens, yellows and reds. This is the airglow spectrum from Broadfoot  and Kendall (1968), where I've highlighted in yellow the most important emission bands of OH in the visible range:



To get a sense of the color associated to each band here is a approximate scale (remember 1 nm = 10 Å)



From wikipedia

The fading yellow glow that goes below the brightest band can be explained as the OH glow at the same altitude band (75-105 km) but originating from the part of the atmosphere that is closer (or further) to the spacecraft from which the picture was taken. As that altitude band is a shell around the Earth and not a ring. For the same reason the peak intensity in the picture is likely a bit lower than altitude of the peak in emissions.