Answer:
Gaseous particles coming from the sun entering atmosphere through the poles, where Earth's magnetic field is the weakest.
The colors of the aurora borealis depends of the gas that is in the particles entering the atmosphere: oxygen will produce a pale yellowish-green, while nitrogen will produce blue or purplish-red, for example.
It's a phenomenon that can be seen near both poles... but of course it's much more accessible to watch in the Northern hemisphere.
