What causes the Northern Lights?

Short answer: the Northern Lights—aurora borealis—appear when energized charged particles are guided by Earth’s magnetic field into the upper atmosphere. There they collide with oxygen and nitrogen atoms and molecules, transferring energy to them. As those gases return to lower-energy states, they emit visible light.

The Sun supplies the disturbance

The Sun continuously sends out the solar wind, a stream of charged particles. Solar activity can disturb Earth’s magnetosphere and load it with energy. When that stored energy is released, electrons can be accelerated along magnetic field lines toward the polar regions, where auroras are most common.

That is why strong geomagnetic activity can make auroras brighter and sometimes visible much farther from the poles than usual. A dramatic aurora is not simply sunlight reflecting from the atmosphere; it is light emitted by atmospheric gases after particle collisions.

Why auroras are green, red, purple or blue

Color depends on which atmospheric species are excited and at what altitude. Oxygen is responsible for much of the familiar green and some red auroral light, while nitrogen contributes blue, purple and pink tones. The exact color also depends on the energy of the incoming particles and the density of the atmosphere where the collisions occur.

Why they form rings around the poles

Earth’s magnetic field funnels charged particles preferentially toward high latitudes. From space, auroras therefore tend to form oval-shaped zones around the magnetic poles rather than a single glowing patch directly above the geographic pole.