Here's why:

* Auroras are caused by charged particles from the sun interacting with Earth's atmosphere. These particles excite atoms in the atmosphere, causing them to emit light.

* The color of the aurora depends on the type of atom being excited and the energy level of the excitation.

* Green auroras are caused by excited oxygen atoms. Oxygen atoms emit green light when they are excited to a high energy level.

* Red auroras are also caused by excited oxygen atoms, but at a lower energy level. This lower energy level results in a red color.

Therefore, since green auroras are caused by oxygen atoms excited to a higher energy level than red auroras, they give off higher energy light.