Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2003 December 17
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A Proton Aurora
Credit: IMAGE Satellite, NASA

Explanation: What are auroras made out of? Most auroras are caused by the solar wind exciting electrons that are funneled down the Earth's magnetic field. These electrons strike air molecules, liberating other electrons that glow when re-acquired. Sometimes, however, auroras composed mostly of heavier protons impact the Earth, causing a more energetic display with strong ultraviolet emission. A proton aurora captured by the IMAGE satellite in ultraviolet is shown above ringing the north magnetic pole of planet Earth. Most electrons and protons never reach the Earth to cause auroras because they are completely deflected away at a great distance by the Earth's magnetic field. The bright spot in the auroral ring indicates a particularly deep crack in the Earth's magnetic field where protons were able to flow along a temporarily connected region between the Sun and the Earth, relatively undeflected, until they impacted the Earth's ionosphere.

Tomorrow's picture: Express to Mars

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