Here's why:

* Green aurora: Caused by excited oxygen atoms at a lower altitude (around 100 km). This lower altitude means the oxygen atoms are denser and more likely to collide with other particles, releasing energy in the form of green light.

* Red aurora: Caused by excited oxygen atoms at a higher altitude (around 200-300 km). At this altitude, oxygen is less dense and more likely to be excited by lower-energy particles, producing red light.

Since the green aurora is produced by oxygen atoms being excited at a lower altitude, they are subjected to more energetic collisions, resulting in higher-energy green light.