Natasha James, of St Peter's College, Palmerston North, asks :-

What causes the Northern/Southern Lights?

Dick Dowden, an upper atmospheric physicist retired from Otago University, responded.

This bright but diffuse glow in the night sky, seen when looking towards the Earth's poles on dark nights, is also called aurora.

The Sun, as well as pouring out a vast amount of light, ejects enormous numbers of nuclear particles such as electrons and protons. These take about four days to reach Earth.

As they approach the outer reaches of the Earth's atmosphere, at an altitude of about 200km, these very fast electrons and protons start hitting atoms of nitrogen and oxygen in the rarefied air. Naturally the rate of these collisions increases as the fast electrons and protons get to lower and lower altitudes where the gas molecules are closer and closer together. Eventually, at about 100 km altitude, the electrons and protons are 'spent' -- they have lost all their energy from hitting atoms so they can't go any further.

The atoms hit by the fast electrons and protons ring like bells. Atoms are too small to ring at sound frequencies, so they 'ring' at light frequencies and emit red and green light characteristic of nitrogen and oxygen atoms and molecules. (In much the same way as neon atoms emit their characteristic red light when excited by the electrical discharge in a red neon advertising sign.)