From the notes on Youtube
I’ve never seen the Northern lights in person. It’s one of the things I would love to do before I die. The pictures and videos are so beautiful, they must be magnificent in person. Usually they’re only visible in the northern latitudes, too far north to see from anywhere I ever lived. Dad was supposed to have seen them once from somewhere in North Texas, but I never have. Occasionally, they are much brighter than other times.
Northern lights is a common name for the Aurora Borealis (Polar Aurorae) in the northern hemisphere. They are matched by the Aurora Australis in the southern hemisphere. They are caused primarily by particles from the sun (the solar wind) being funneled by earth’s magnetic field into the north and south polar areas. The charged particles, mostly electrons and protons, enter the atmosphere from above. Magnetospheric plasma interacting with the upper atmosphere also contributes its part.
Electrons and protons travelling at very high speeds interact with oxygen and nitrogen atoms in the upper atmosphere. This ionizes the molecules, causes them to emit light, and produces the brilliant colors. The protons can also gain an electron from the atmosphere, become hydrogen, and then produce emissions of a different color.
At the highest altitudes, excited atomic oxygen emits light at a wavelength of 630.0 nm (red). At lower altitudes the more frequent collisions suppress the red mode and emit more light at 557.7 nm (green). High concentration of atomic oxygen and higher eye sensitivity for green make green auroras the most common. The excited molecular nitrogen plays its role here as well, as it can transfer energy by collision to an oxygen atom, which then radiates it away at the green wavelength. (Red and green can also mix together to produce pink or yellow hues.)
Other shades of red as well as orange may be seen on rare occasions; yellow-green is moderately common. As red, green, and blue are the primary additive colours, in theory practically any colour might be possible; but the ones mentioned here are most common.
Blue, from atomic oxygen is uncommon; ionized molecular nitrogen emits light at several wavelengths in both red and blue areas of the spectrum, with 428 nm (blue) being dominant. Blue and purple emissions, typically at the lower edges of the “curtains”, show up at the highest levels of solar activity.
Ultraviolet light from aurorae (within the optical window but invisible to most people) has been observed. Ultraviolet aurorae have also been seen on Mars. Infrared light, in wavelengths that are visible to some people, also occur.