We get Southern and Northern lights at the same time – it’s just where the magnetic field lines on the Earth allow charged particles from the Sun to get down low enough to cause the atmosphere to glow.
It is possible for the North and South pole to switch though (but that’s not what causes the Aurorae). There’s evidence to show this has happened a lot in the past and in fact we’re overdue for a magnetic reversal!
Ah, well the Southern Lights are just the same as the Northern Lights; they are both aurorae. Just one is in the south and one in the north, so if the poles did switch then it would take the same time for the lights to move too. Then the aurorae above Britain and the Arctic would be the Southern Lights and those above Antarctica the Northern Lights.
The aurorae can last a long time, generally as long as the sun is pumping out charged particles. Sometimes they go very faint, and we can’t see them without special cameras, but I’m not really sure if they stop entirely.
The poles switch rapidly and randomly: about 200 million years ago, the poles didn’t switch for over 40 million years, but more recently they have switched about every 150,000 years. I was reading an article recently which suggested that poles switch less when all the continents join up and the last time that happened was 200 millions years ago too. So its complicated!
We get northern and southern lights all the time. The reason you only see one or the other is because you can’t be in two places at the same time! They only appear at poles because of some complicated interactions between the atmosphere, the solar wind, and the magnetic field, which is at its strongest at the poles.
Because the magnetic field is made by the spinning liquid iron inside the earth’s core, sometimes the spin gets messed up (like dropping a hulahoop!) and the magnetic field switches direction, so compasses would point towards the south! This happens randomly every 100,000 to a million years, and we know it happens because it is recorded in the rocks made in underwater volcanoes. Pretty cool stuff!