What makes the northern lights so pretty?
This stunning image of the Aurora Borealis was taken in the past week by Bjørn Jørgensen (click through to see the photographer’s page). So stunning in fact, I’ve seen it pass through my dash four times as it gets reblogged around. We’ve all heard that increased solar activity and solar flares aimed at the Earth can make the aurorae brighter, but what’s actually going on?
When the Sun has a coronal mass ejection, the event is so violent that large amounts of hot plasma (an electron/proton high energy soup) is hurled towards us. Now, this plasma is always being puffed off the Sun in small amounts as the solar wind, but a solar flare acts as a big boost to this. Lucky for us that the Earth’s magnetic field can protect us from the solar wind by redirecting this plasma around us and to the poles.
But this is where the fun starts. The electrons and protons in the plasma interact with molecules in the atmosphere in one of two ways: either by knocking off an electron and “ionising” the molecule, making it charged, or by “exciting” an electron in that molecule. Both of these methods put the molecule in a higher energy state, and nothing in the Universe likes being in a higher energy state, it’s unstable. After a while the molecule will either neutralise by finding another electron, or the electron in the molecule spontaneously de-excites back to how it was (“the ground state”). In both cases, a photon (a packet of light) is released with the lost energy, a bit like sweating to cool down. These are specific interactions at specific energies though, and so the photons all have similar energies, which means the light all has similar colours rather than being a rainbow mess.
- When oxygen spontaneously de-excites, green or brown-ish light is given off, depending on how excited the oxygen molecule was
- When nitrogen de-excites, it gives off red light
- When nitrogen neutralises, it gives off blue light
So in this picture, there’s a lot of spontaneously de-exciting oxygen molecules! You get colour gradients because the brown-oxygen emission takes a long time to happen and sometimes the molecule collides with another, stopping this brown emission happening. So the brown-emission is more likely to happen when there’s not as many particles about i.e. at high altitude. And obviously, the stronger the solar the wind, the more of these collisions and excitations and emissions happen, so the brighter the aurorae!