The Northern lights...

Something that I'm sure we've all heard of, but only some are fortunate enough to have seen. For those of you who are unsure as to what they are exactly, why they only appear near the poles, and what actually causes them, well you've come to the right place!

So let us start with what they actually are....

Here's a picture of what they look like if you haven't seen them before

The Northern lights are a consequence of energetic charged particles colliding with the Earth's atmosphere, and appear as a multi-coloured haze that shoots across the sky.

They mostly appear green, but if you've ever seen them before, then you might know that they can take on a variety of different colours. Hopefully, you're wondering why this is the case...

Well, it's because the concentration of gases in our atmosphere can change from time to time. For example, when the radiation hits oxygen atoms, in particular the electrons of an oxygen atom, it "excites" them, which is simply a scientific way (scientists have a strange use of vocabulary...) of saying that they gain energy.

However, electrons naturally want to be at lower energy levels, and hence get rid of that gained energy by ejecting it in the form of light.

The quantity of energy that is released, is dependent on the specific energy levels of the atom, which is different for every single element.

In the case of oxygen, it's light with a wavelength that corresponds to the colour green. The other colours such as pink and blue come from Nitrogen atoms that are present in the atmosphere. So that's quickly what they are, and why they can be different colours, but we've yet to discuss why they happen, which is perhaps the most important question.

If you're interested in learning more about why different atoms/electrons release different colours, then I'd recommend checking out another article here.

Remember how we said that they're "energetic charged particles"?

Well, those charged particles are coming from the Sun, specifically in the form of solar wind and solar flares.

I talk more about solar flares and wind here if you're interested, but essentially, solar flares form from eruptions on the surface of the Sun, due to the magnetic field tangling up, and cause the release of highly energetic radiation such as X-rays and UV radiation.

On the other hand, solar wind is a result of the outward expansion of plasma from the surface of the Sun, and contains energetic charged particles, such as protons and ions which travel through space at high speeds, although still only at a fraction of the speed of light. Hence, they can take and hour so to reach the earth upon erupting from the Sun, where as light only takes around 8 minutes.

When these particles come into contact with Earth's magnetic field, they get forced around the Earth, up to the poles. Since the way magnetic fields are produced (by converging at the poles), the charged particles are forced along this path, where they eventually travel across either end. See below for a picture of this happening as I know it can be a little hard to imagine from words.

Solar flares are a different ball-game, and could pose serious risk to life on Earth if a powerful enough was generated and released in our direction.

Weak solar flares can also cause the Northern Lights, although they're far less common than those produced form solar wind.

Illustration of the Earth's magnetic field that shows how it converges at the poles, and also shows how it interacts with charged particles. Image Credit: NASA

The charged particles then collide with the atmosphere at the poles (because Earth's magnetic field converges at the poles) and are absorbed by the gases in our atmosphere. To reiterate from before, the gases, specifically the electrons of those gases, then emit the energy that they absorbed as visible light radiation, which we see as the colours.

Summary

Charged particles such as protons and ions are periodically released from the surface of the Sun which we call solar wind, and when they collide with Earth's magnetic field, they're forced along the magnetic field lines towards the poles, where they're absorbed by the electrons that make up the gases in our atmosphere.

The electrons then emit this energy, as they don't like to be in excited states, in the form of visible light radiation that our eyes/brains can detect. The different colours are produced because different elements in our atmosphere release different amounts of radiation, which correspond to different colours of light.

You can read more about that last part here

They truly are a spectacular event to see for your own eyes, so I would highly suggest booking a trip sometime over to Iceland, Norway, or Sweden etc (basically any country that's close enough to the north/south pole) to see them for yourself.

I hope that you could learn something from this article, and perhaps consider checking out some of our other content here at Expansive!

Thanks for reading!