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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.   
      
                                 2022 November 20   
       The featured image shows a dark field with a photographer lit in red   
      imaging a night sky tinged with green airglow and decorated with clouds   
           that appear collectively like a giant spiral. Please see the   
                    explanation for more detailed information.   
      
                            Airglow Ripples over Tibet   
                        Image Credit & Copyright: Jeff Dai   
      
      Explanation: Why would the sky look like a giant target? Airglow.   
      Following a giant thunderstorm over Bangladesh in late April, giant   
      circular ripples of glowing air appeared over Tibet, China, as pictured   
      here. The unusual pattern is created by atmospheric gravity waves,   
      waves of alternating air pressure that can grow with height as the air   
      thins, in this case about 90-kilometers up. Unlike auroras powered by   
      collisions with energetic charged particles and seen at high latitudes,   
      airglow is due to chemiluminescence, the production of light in a   
      chemical reaction. More typically seen near the horizon, airglow keeps   
      the night sky from ever being completely dark.   
      
                         Tomorrow's picture: butterfly sky   
        __________________________________________________________________   
      
          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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