One of my colleagues recently went to Iceland. He showed us many beautiful pictures of geysers and rivers and hills. It looked very lovely and very cold.
He also took some pictures of the northern lights, which made me very envious. I would love to see them one day. I just need to get the funds together and also learn to deal with very cold weather without spending the whole time wondering why I am not inside.
They are beautiful, and as someone with a background in astronomy, people often ask me what causes them, to which I usually manage to stumble through a vague answer. But I thought, no more! I am going to actually look up what is going on! They’re amazing and people deserve to know!
Anyway, the reason we have the northern (and southern) lights is because Earth has a magnetic field. The magnetic field lines look a lot like there is a bar magnet going through the Earth, as exhibited in my beautiful diagram below:
The Earth has a magnetic field because of a very important rule in physics: A changing electric current causes a magnetic field, and a changing magnetic field causes an electric current.
The Earth’s outer core is made up of liquid iron alloys. They all have different compositions, temperatures and pressures, which causes them to move around. The more dense, cool alloys sink and the hotter, less dense alloys rise.
The spin of the Earth causes them to move around as well. When iron is moved around like this it generates an electric current, this produces magnetic fields, which cause more electric currents! As the Earth is rotating, all of the magnetic fields are moving in roughly the same direction, these combine to create the magnetic field that surrounds the Earth.
When the Sun ejects a cloud of gas (called a coronal mass ejection, or CME) and it reaches Earth, the particles collide with its magnetic field. The particles in the cloud of gas are charged, so they travel along the magnetic field lines and end up at the poles.
So, why the lights?
As the charged particles get closer to Earth they are more likely to collide with particles from our atmosphere. If the charged particle does collide with another particle it will transfer some of its energy and slow down.
The particle from Earth that has been collided with has gained some energy and is excited. When the excited particle returns to its non-excited state it will give off the energy it was given by the charged particle in the form of light that we can see. The colour of the light will depend on how much energy it was given.
And there we have it!
This video also explains things in a pretty cool way too: