What are the Northern Lights?

Question posed by Jennie.

The northern lights, also known as aurora borealis (in the Northern Hemisphere), aurora australis (in the Southern Hemisphere), or polar aurorae, are impressive natural light displays in the sky. They are most often observed at night (because it's darker, so they're easier to see) and your chances of observing one improve the further north (in the northern hemisphere) or south (in the southern hemisphere) you are. They are usually green or red in colour, though they can be pink, blue, violet and other colours at times. They mostly take the form of a glow in the sky, but can also form 'curtains' or 'arcs', or they can change constantly as they are observed.


It all begins with the Sun

The Earth's magnetic field protects us from a constant barrage of charged particles emitted by the Sun as a result of surface temperatures that reach and exceed a million degrees centigrade. This stream of plasma is emitted in all directions, and is known as the solar wind. Most of this wind is deflected around the Earth by its magnetic field, but when the solar wind is perturbed (this may happen for a variety of reasons) some of the charged particles are caught up in it and start to follow the magnetic field lines down to Earth. The field lines converge on the magnetic poles, so this is the way that the particles are channelled- towards either the north or south pole.

When the charged solar wind particles enter the Earth's atmosphere, they begin to interact with it, and collide with atoms. These collisions excite electrons in the atoms that are hit, and these electrons take a 'quantum leap' - a jump to a different energy level - which causes some of the atom's kinetic (movement) energy to be converted into visible light.

Most of the collisions are with oxygen atoms in the ionosphere (a layer of the Earth's atmosphere), and these collisions result in the green and red light seen in most aurorae. Different atoms in the upper atmosphere will produce different colours: helium atoms give off a purple colour when their electrons are excited; nitrogen is responsible for light blue and some greenish colours; and neon can produce a rarely seen orange aurora. Different mixtures of these atoms (compounds) can produce different (and mixed) colours still. The strength and level of activity of the solar wind can also have an effect on the colours observed, as well as the intensity of light.


How can I increase my chances of seeing one?
The closer you are to one of the planet's poles, the more likely you are to see an aurora. The level of aurora activity also seems to be linked to sunspot activity, which follows an eleven year cycle. The best time to see an aurora is at (or shortly after) the peak of this cycle. Also, the geomagnetic storms that can start off aurorae appear to be most active around the time of the equinoxes (when the Earth is exactly 'side on' to the Sun).


Do they happen on other planets?

Yes. Aurorae have been observed in the atmospheres of Jupiter, Saturn and Mars, and even Venus*. The Hubble Space Telescope has also observed aurorae on some of the larger moons of the solar system, including Europa and Io.




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* This is quite interesting because Venus has a very weak planetary magnetic field, which means that Venusian aurorae are quite different to those of other planets: they are generally brighter, more variable in shape, pattern and location.