                        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.

                                 2021 May 30

                             Aurora over Clouds
                 Image Credit & Copyright: Daniele Boffelli

   Explanation: Auroras usually occur high above the clouds. The auroral
   glow is created when fast-moving particles ejected from the Sun impact
   the Earth's magnetosphere, from which charged particles spiral along
   the Earth's magnetic field to strike atoms and molecules high in the
   Earth's atmosphere. An oxygen atom, for example, will glow in the green
   light commonly emitted by an aurora after being energized by such a
   collision. The lowest part of an aurora will typically occur about 100
   kilometers up, while most clouds exist only below about 10 kilometers.
   The relative heights of clouds and auroras are shown clearly in the
   featured picture in 2015 from Dyrholaey, Iceland. There, a determined
   astrophotographer withstood high winds and initially overcast skies in
   an attempt to capture aurora over a picturesque lighthouse, only to
   take, by chance, the featured picture including elongated lenticular
   clouds, along the way.

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                      Tomorrow's picture: that’s a moon
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