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An aurora is electromagnetic radiation (EMR), notably visible light, produced by interactions between a planet's atmosphere and a stellar wind. Earth's atmosphere is protected from the solar wind by the Van Allen belts, and auroras are normally confined to regions they protect least, near the poles, but solar activity increasing the wind can overcome the blocking effect, resulting in auroras further from the poles. When the solar wind (electrons, protons and heavier ions) strikes the upper atmosphere, collisions excite and ionize molecules in the atmosphere, and EMR is produced by this process as well as the subsequent electron relaxation and recombination. Earth auroras are typically triggered by incoming electrons, but proton auroras can occur during a solar particle event.
Auroras have also been observed around some other solar system planets and a substantial aurora is thought to have been detected at an extra-solar planet.
Aurora borealis (northern lights) is an aurora in the Earth's northern hemisphere and aurora australis (southern lights), the southern hemisphere.