The Aurora Borealis and Aurora Australis are truly spectacular displays of one of the many effects the Sun has upon the Earth. Whilst the science behind them is incredible, it is easy to forget the details when you observe them, as they really are a beautiful sight.
Their existence is due to solar wind'; relentless streams of charged particles that are emitted by the Sun in all directions. The Earth is protected from these particles by the presence of its magnetic field. This magnetosphere' effectively slows the supersonic particles to subsonic speeds rather abruptly, and particles are deflected away from the Earth when they reach the magnetopause'; a boundary of the magnetic field where the outward force is balanced by the inward pressure created by the solar wind.
Whilst the magnetosphere is effective at keeping the majority of solar particles out of harms way, some manage to leak through and become trapped in regions above the Earths surface called Van Allen belts'. These are doughnut shaped rings, the inner of which is at 2000km altitude and is 3000km thick, and the outer Van Allen belt is at an altitude of 16000km, and 6000km thick. Protons emitted by the Sun are trapped by the inner Van Allen belt, whilst the outer belt contains mostly electrons.
The magnetosphere however, can only hold a certain amount of protons and electrons in the Van Allen belts before a leakage occurs. The particles cascade through to the Earths upper atmosphere (the thermosphere) where the phenomenal displays of the aurorae can be seen. The awe-inspiring light show is due to the high speed particles released from the Van Allen belts colliding with the atoms of the Earths thermosphere and exciting them to higher energy levels. As the atoms return to their ground states (their original energy levels), they emit visible light. This is very similar to the way that neon light is produced.
The intensity of the aurorae is dependant on solar activity. If the Sun emits a particularly violent burst of particles known as a Coronal Mass Ejection (CME) the volume of particles cascading through to the thermosphere will be greater, resulting in a brighter aurora that stretches past its regular latitudes above Canada and Australia.
Due to their beauty, it can be easy to ignore or forget that the causes of such a phenomena are so incredible dangerous, and that if it were not for the Earths natural defences like the magnetic field, we would not be so lucky as to see a spectacular light show, and the resultant effects would be much more dramatic