How does the Aurora Borealis actually work?

Guest Blogger: Alex Diaz

I have been looking to the sky since I was a very young boy. When I first saw the Northern Lights or, Aurora Borealis, they seemingly appeared within the blink of an eye. Nothing but green surrounded me and I remember wondering how does such a dazzling display of lights come to fill the night sky? Well, Auroras are actually collisions between electrically charged particles from the Sun that enter earth’s atmosphere at its magnetic poles. In the northern hemisphere they are known as “Aurora Borealis” and “Aurora Australis” in the southern hemisphere.

These Auroral displays come in many colours with the most often seen colour being a striking green. It occasionally shows off its many colours ranging from red to pink, blue to purple, and dark to light. The reason for these colours is that our atmosphere is made up of many different molecular compounds like Oxygen and Nitrogen. How exactly does this work though? When the charged particles that come from the Sun hit the atoms and molecules of the Earth’s atmosphere, they excite those atoms, giving off light. Since we have different atoms in our atmosphere, we get different colours of the spectrum when they are excited!

So what atoms give us what colours exactly? Well depending on the density of the charged particles coming from the Sun, Oxygen is always the first atom to get excited, it gives off a yellow-green to green colour about 100km high in the atmosphere. Oxygen compounds at higher altitudes (320km above us) gives us red auroras. The reason for the red colour in the upper atmosphere is because Oxygen is unusual in terms of its return to ground state: it can take three quarters of a second to emit green light and up to two minutes to emit red. Collisions with other atoms or molecules absorb the excitation energy and prevent emission. Because the highest atmosphere has a higher percentage of oxygen and is thinly distributed, such collisions are rare enough to allow time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen, and eventually even green light emissions are prevented the closer we get to the ground. This is why there is a color differential with altitude; at high altitudes oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common color as mentioned. Then comes pink, a mixture of light green and red, followed by pure red, then yellow (a mixture of red and green), and finally, pure blue.

Nitrogen molecules are energized in 2 different ways. Neutral Nitrogen is what we call excited Nitrogen, which simply means that it is excited in the same way as Oxygen. It absorbs energy from high-energy electrons and releases the energy as red-purple light. But if high-energy electrons strike Nitrogen molecules some of the Nitrogen’s own electrons are knocked loose, it becomes ionized. Once it regains its lost electrons though, it returns to its normal energetic state, the extra energy is released as a bluish light.

Think of Neon lights that you would see businesses using as signs in our modern world. They have electricity running through them that excites the Neon gas giving off a brilliant red-orange colour. Neon lights are the same idea as Auroras, only much smaller on the scale in terms of energy and size. Just imagine if our atmosphere were made of Neon gas and Sodium gas. We would see red-orange and yellow auroras! Now that you’re an expert on Auroras, you probably want to know how you can see them. Areas that are not subject to ‘light pollution’ are the best places to watch for the lights. Areas in the north, in smaller communities, tend to be the best. As for the ideal time, local midnight is your greatest bet for catching them but remember that they come in interval bursts so you usually have to be up for several hours to get a good glimpse of an entire “show”. But of course as with anything astronomical, patience is your friend for observing this beautiful silent symphony of the night and believe me, it’s well worth it.