Greenlandic Inuit believed the aurora borealis, commonly known as the northern lights, were the dancing spirits of children who died during birth. There is also the equally impressive display in the Southern Hemisphere, known as the aurora australis or the southern lights, which is often visible from Tasmania. There have even been reports further north near Canberra of sightings of it.
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The journey of the aurora begins at the sun's core, where the temperature is over 14 million degrees. The immense heat strips electrons away from atoms, creating a soup of freely floating particles known as a plasma. Over time, the plasma swirls to the surface of the sun, just as hot water rises to the surface when heating a pot of water on a stove. Strong magnetic fields (much stronger than the ones produced by the magnets on your fridge) inside the sun eject this plasma from the surface, some of which hurls towards Earth at speeds of up to 1 million kilometres per hour. This is what we call the solar wind and it takes up to three days to reach Earth.
Solar wind brings along with it ionising radiation, which can affect unshielded astronauts and even cause GPS systems to be off by tens of metres. Yet solar winds occur regularly, so why can we enjoy ice blocks outside and not be trapped inside radiation-proof houses here on Earth?
The answer is that the Earth has a remarkable defence to the solar wind - the magnetosphere. The magnetosphere is a result of molten iron constantly flowing around in the Earth's outer core, creating a magnetic field that wraps around Earth and behaves like a magnet with north and south poles. This the same reason your compass works!
When the solar wind reaches Earth's magnetosphere, most of it is simply deflected away. However, some particles travel along the magnetic field lines to the magnetic poles, penetrating the magnetosphere and entering the atmosphere.
After they collide, some energy goes into nitrogen and oxygen atoms in our atmosphere, giving them an energy boost that excites their electrons, just as electricity passes through neon in a sign, lighting it up. When the electrons fall back down from their excited state, they release streams of photons, which we then perceive as, you guessed it, light. The light forms "auroral ovals" in the Northern and Southern Hemispheres.
The aurorae are most intense and frequent when there are stronger solar winds known as coronal mass ejections. The colour of the lights depends on the colour that is unique to the atom and the location where the sun's particles interact with atmospheric atoms. The green colour, for instance, comes from oxygen. The dancing movement of the aurora traces the Earth's magnetic field lines and is caused by the variation in the solar wind.
Light from sun in the day outshines the aurora, and at night it can be hidden due to moonlight and/or cloud cover.
Hopefully one day you are lucky enough to witness the aurora. But the word I'd suggest using to describe the experience wouldn't be awe-inspired or mesmerised, but rather "star-struck".
- Aman Chopra is a student at the Australian National University.
