What causes the aurora?

A stream of charged particles from the sun called the solar wind is constantly impacting Earth’s magnetosphere, dragging it into a distorted shape with a long tail on the nightside of the Earth. In this tail – the magnetotail – energy from the interaction with the solar wind builds up and is eventually released again in a process called reconnection. This process accelerates charged particles, which may then plunge down along magnetic field lines until they collide with atmospheric particles, causing these to emit the light we see as the aurora. This is all visualized in the video below.

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Credit: NASA/Goddard Space Flight Center Conceptual Image Lab