How do the Northern Lights form?

Answer 1

Although I agree with what both of the first two contributors shared with you, I don't know if their answers were the best for your question.

Since the sun appears to be a massive furnace of nuclear fusion, where your typical hydrogen atom, one proton and one electron, get bombarded together and come out as one helium atom, two protons and two electrons, but, with the difference being that on earth helium atoms usually also contain two neutrons, whereas on the sun, they don't. On the sun, the most common isotope of helium consists of two protons and two electrons but only one neutron.

So there are theoretically a lot of subatomic particles that didn't quite get smashed into the more stable Helium isotopes that are floating around in the Sun's atmosphere, which are therefore less dense than your typical hydrogen molecule, so when the sun burps them out, they react to the sun's gravitation much less than the sun's hydrogen and helium, and shoot out above the sun's surface for literally millions, and sometimes hundreds of millions of miles, before the Sun's gravity can reattract them back down to it.

That's what's called a solar flare, which often shoot out into space as far away as the orbit of Mercury, and even more rarely reach out past the Earth, but have been recorded to reach Mars, and theoretically could reach any of the planets in our solar system, before the sun's gravitation eventually pulls it back towards the sun.

These subatomic particles are of course invisible to the naked eye, so they really aren't flares like we often use as road and marine signals, but, that's what they're called.

The way that they form into the Northern Lights, or the Arora Borealis, is that as the earth passes through a flare, they collide with the gas molecules in the upper atmosphere where they transfer some of their energy to those molecules, and as the electrons of those molecules change their state, they flouresce, or give off light, like in a flourescent light, but not usually as bright, which is why they can only be seen in the night sky, and not in daylight, although theoretically the same phenonemenon occurs millions of times more often or intensely on the sunward side of the earth.

The reason that they can be seen on the dark side of the earth at all, is that the Earth's magnetic field tends to cause their trajectory to bend, like a magnet will bend the trajectory of a steel ball bearing, pulling them into the atmosphere even on the dark side of the Earth.

Similarly, the Aurora Australius can be seen on those same nights in the Southern Hemisphere, but are not called the "northern lights" for hopefully obvious reasons.

Answer 2

What Gene above said is correct. When there's an unusually active solar event (a flare, say), and the number of high-energy particles from the sun increases dramatically, the northern (and southern) lights can be seen much farther from the magnetic pole; A few years ago, they were seen as low as Kentucky & North Carolina during a particular solar mass ejection.

Answer 3

They are cause mostly by electrons emitted from the sun which follow the earth's magnetic field to the pole and interacts with gases in the high atmosphere. The electrons excite the gasses which re-radiate the energy as light.