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                           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   
        __________________________________________________________________   
      
          Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)   
               NASA Official: Phillip Newman Specific rights apply.   
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                         A service of: ASD at NASA / GSFC,   
                              NASA Science Activation   
                                & Michigan Tech. U.   
      
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