                        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.

                               2024 January 3
    A flat landscape with a pond is imaged at night below a starfield. A
    multicolored aurora is seen in an arc across the image center. Around
   this arc is another red arc that is particularly smooth. Please see the
                 explanation for more detailed information.

                         A SAR Arc from New Zealand
      Image Credit & Copyright: Tristian McDonald; Text: Tiffany Lewis
                             (Michigan Tech U.)

   Explanation: What is that unusual red halo surrounding this aurora? It
   is a Stable Auroral Red (SAR) arc. SAR arcs are rare and have only been
   acknowledged and studied since 1954. The featured wide-angle
   photograph, capturing nearly an entire SAR arc surrounding more common
   green and red aurora, was taken earlier this month from Poolburn, New
   Zealand, during an especially energetic geomagnetic storm. Why SAR arcs
   form remains a topic of research, but is likely related to Earth's
   protective magnetic field, a field created by molten iron flowing deep
   inside the Earth. This magnetic field usually redirects incoming
   charged particles from the Sun's wind toward the Earth's poles.
   However, it also traps a ring of ions closer to the equator, where they
   can gain energy from the magnetosphere during high solar activity. The
   energetic electrons in this ion ring can collide with and excite oxygen
   higher in Earth's ionosphere than typical auroras, causing the oxygen
   to glow red. Ongoing research has uncovered evidence that a red SAR arc
   can even transform into a purple and green STEVE.

                       Tomorrow's picture: open space
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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                             & Michigan Tech. U.

