I'm a long haul pilot for British Airways, and I've seen the Aurora Borealis quite a lot of times. Every time I've seen them they're Always green in colour, though it can vary. What causes it, and why do you see it on some nights but not others?
The Aurora Borealis occurs because the Earth has a magnetic field. The Sun is pumping out a million mile per hour maelstrom of charged particles and ions called the cosmic wind, and when this hits our magnetic field the two interact. This is because charged particles are deflected by a magnetic field. When this happens energy is released and this stimulates molecules in the atmosphere to get excited. The dominant molecules in our atmosphere are oxygen and nitrogen, and when excited, oxygen emits light which is mainly green. Sometimes, particles in the solar wind have much higher energy than average, and these can make the oxygen more excited, and you get other colours.