Have you ever seen your night sky glows with beautiful colours of lights when you visit far north or far south of the Earth? (e. g Norway, Iceland, New Zealand) Those lights are known as Northern lights or Southern lights! A more scientific name of it is called Aurora Borealis (north) or Australis.
Science of Aurora
The lights are caused by the collision of charged particles with the air molecules in the Earth’s atmosphere. The charged particles escape from the surface of the sun and is carried towards Earth through the solar wind. Fun fact! Solar wind is blowing at Earth constantly! Luckily for us, the Earth’s magnetic field (magnetosphere) shields us from most of the solar wind, but some charged particles might penetrate the magnetosphere. From Faraday’s law of induction, a charge particle moving in a magnetic field will generate a force that is perpendicular to both the direction of current and the magnetic field. (See Fleming left hand rule) When the charged particles penetrate the magnetosphere, it is subjected to Earth’s magnetic field and the resulting motion of the charged particles is a helical motion (spiraling around magnetic field, see below animation)
Once the charged particles enter the atmosphere of Earth, they collide with the surrounding air molecules and loses energy. During the collision, the charged particles transfer it’s energy to the molecules (excitation). The excited molecules will then release that extra energy as light as it falls back to ground state. The lights are what we see in the sky as Aurora.
What determines the colour of the Aurora?
The colours of the lights depend on the type of molecules. At high altitude (ionosphere), oxygen atom is the most abundant, followed by nitrogen molecules. Looking at the emission spectrum of these 2 particles, the green aurora that we commonly see is due to the collision with low altitude oxygen atoms, while high altitude oxygen atoms gives red aurora. Other colours like purplish-blue or red aurora are caused by the collision with nitrogen molecules.
Why we can only see the red aurora at higher altitude?
This is because at low altitude, excited oxygen atom may collide with a nearby nitrogen molecule and loses its energy before emitting that red light.