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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.   
      
                                  2023 December 5   
         An illustrations depicts a high energy cosmic ray starting an air   
         shower in the Earth's atmosphere. Below is an array of air shower   
       detectors. Please see the explanation for more detailed information.   
      
                       Energetic Particle Strikes the Earth   
            Illustration Credit: Osaka Metropolitan U./L-INSIGHT, Kyoto   
                              U./Ryuunosuke Takeshige   
      
      Explanation: It was one of the most energetic particles ever known to   
      strike the Earth -- but where did it come from? Dubbed Amaterasu after   
      the Shinto sun goddess, this particle, as do all cosmic rays that   
      strike the Earth's atmosphere, caused an air shower of electrons,   
      protons, and other elementary particles to spray down onto the Earth   
      below. In the featured illustration, a cosmic ray air shower is   
      pictured striking the Telescope Array in Utah, USA, which recorded the   
      Amaterasu event in 2021 May. Cosmic ray air showers are common enough   
      that you likely have been in a particle spray yourself, although you   
      likely wouldn't have noticed. The origin of this energetic particle,   
      likely the nucleus of an atom, remains a mystery in two ways. First, it   
      is not known how any single particle or atomic nucleus can practically   
      acquire so much energy, and second, attempts to trace the particle back   
      to where it originated did not indicate any likely potential source.   
      
         Open Science: Browse 3,200+ codes in the Astrophysics Source Code   
                                      Library   
                       Tomorrow's picture: torched by stars   
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          Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)   
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