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The Northern Lights, also called the aurora borealis, is an aurora that is only seen in far northern latitudes. This phenomenon is commonly visible between 65 and 72 degrees north, but can be seen in more temperate climates in the event of a magnetic storm.
An aurora is a naturally occurring light show that is particularly easy to see close to the poles. They are caused by the collision of charged particles against the Earth’s magnetic field. An aurora occurs 24 hours a day, but is usually observed at night. Auroras typically occurs in the ionosphere.
Auroras result from the emission of photons into the Earth’s atmosphere, above 80 km, from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to their ground state. These atoms are ionized by the collision of solar wind particles being funneled down, and accelerated along, the Earth’s magnetic field lines: oxygen emits green or brownish-red light while nitrogen emits blue or red. Blue if the atom gains an electron and red if it is returning to its ground state.
Oxygen can take three quarters of a second to emit green light and up to two minutes to emit red. Collisions with other molecules will absorb the excitation energy and prevent the emission of light. The very top of the atmosphere has a higher percentage of oxygen and it is so thin that collisions are rare, which allows enough time for oxygen to emit red, so there is a color differential with altitude: at high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common of all aurora colors. Behind it is pink, a mixture of light green and red, followed by pure red, yellow, then pure blue. Now that you know how the Northern Lights are formed, the mystery may be gone, but the wonder at their beauty will never evaporate.
We have written many articles about the Northern Lights for Universe Today. Here’s an article about what’s beyond the power and beauty of Northern lights, and here are some pictures of Northern lights.
We’ve also recorded an episode of Astronomy Cast all about Auroras. Listen here, Episode 163: Auroras.