Why northern lights form

Northern lights form when charged particles, mostly electrons guided by Earth's magnetic field, strike atoms and molecules high in the atmosphere and make them glow. The display sits in ovals around the magnetic poles because that is where those field lines come down into the air.

The lights are not sunlight reflected off polar ice, and they are not the atmosphere burning. The glow is excitation, the same broad family as a discharge tube, in which particles hit atmospheric gas and the gas sheds the energy as light.

The mechanism is more particular than the slogan. The solar wind loads energy into the magnetosphere, the region dominated by Earth's magnetic field, and disturbances there accelerate electrons along field lines. Those electrons dive into the upper atmosphere in the auroral zones and collide with oxygen and nitrogen, which then radiate.

Follow it one step further. Color depends on which gas is struck, at what height, and whether the atom or molecule is left in a particular state. Atomic oxygen produces the common green and, higher up, a red, while molecular nitrogen contributes blues and purples, often along the lower edge of a curtain.

The case worth keeping in mind is specific. A quiet night at high latitude may show a green arc that barely moves, and after a strong solar disturbance the oval widens so the same glow can appear much farther from the pole. The physics did not change when a rarely visited town sees it, because the oval moved over that town.

That case has a boundary. Not every magnetic disturbance produces a display you can see, because the particles must be energetic enough, the sky dark enough, and the line of sight aimed at the oval. Cloud, moonlight, and city light hide a glow that instruments may still record.