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                        Astronomy Picture of the Day

   [1]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 April 18
   [2]Rainbow airglow over the Azores. See Explanation. Moving the cursor
     over the image will bring up an annotated version. Clicking on the
        image will bring up the highest resolution version available.

                       Rainbow Airglow over the Azores
        Image Credit & Copyright: [3]Miguel Claro ([4]TWAN); Rollover
                         Annotation: [5]Judy Schmidt

   Explanation: Why would the sky glow like a giant repeating rainbow?
   [6]Airglow. Now air glows all of the time, but it is usually hard to
   see. A disturbance however -- like an approaching storm -- may cause
   noticeable rippling in the [7]Earth's atmosphere. These [8]gravity
   waves are [9]oscillations in air analogous to those created when a
   [10]rock is thrown in calm water. The long-duration exposure nearly
   along the vertical walls of [11]airglow likely made the undulating
   structure particularly visible. OK, but where do the colors originate?
   The deep red glow likely originates from [12]OH molecules about
   87-kilometers high, excited by [13]ultraviolet light from the Sun. The
   orange and green [14]airglow is likely caused by [15]sodium and
   [16]oxygen atoms slightly higher up. The [17]featured image was
   captured during a [18]climb up [19]Mount Pico in the [20]Azores of
   [21]Portugal. Ground lights originate from the island of [22]Faial in
   the [23]Atlantic Ocean. A spectacular sky is visible through this
   banded airglow, with the central band of our [24]Milky Way Galaxy
   running up the image center, and M31, the [25]Andromeda Galaxy, visible
   near the top left.

              Explore Your Universe: [26]Random APOD Generator
                Tomorrow's picture: infrared galactic center
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    Authors & editors: [38]Robert Nemiroff ([39]MTU) & [40]Jerry Bonnell
                                 ([41]UMCP)
          NASA Official: Phillip Newman [42]Specific rights apply.
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                A service of: [44]ASD at [45]NASA / [46]GSFC
                           & [47]Michigan Tech. U.

References

   1. https://apod.nasa.gov/apod/archivepix.html
   2. https://apod.nasa.gov/apod/image/2104/GravityWaves_Claro_1486.jpg
   3. https://www.miguelclaro.com/wp/biography/
   4. https://www.twanight.org/
   5. https://geckzilla.com/
   6. https://www.atoptics.co.uk/highsky/airglow2.htm
   7. http://www.nasa.gov/mission_pages/sunearth/science/atmosphere-layers2.html
   8. https://en.wikipedia.org/wiki/Gravity_wave
   9. http://www.atoptics.co.uk/highsky/hgrav.htm
  10. https://www.youtube.com/watch?v=T9QwiBFN9gI
  11. https://apod.nasa.gov/apod/ap150904.html
  12. https://en.wikipedia.org/wiki/Hydroxyl
  13. https://science.nasa.gov/ems/10_ultravioletwaves
  14. http://www.pnas.org/content/112/49/E6728.abstract
  15. https://periodic.lanl.gov/11.shtml
  16. https://periodic.lanl.gov/8.shtml
  17. http://www.miguelclaro.com/wp/?portfolio=rainbow-bands-of-airglow-in-gravity-waves-above-pico-island
  18. https://www.youtube.com/watch?v=-GlioVTgjuI
  19. https://en.wikipedia.org/wiki/Mount_Pico
  20. https://en.wikipedia.org/wiki/Azores
  21. https://en.wikipedia.org/wiki/Portugal
  22. https://youtu.be/jPg7KFv2a-s
  23. https://en.wikipedia.org/wiki/Atlantic_Ocean
  24. https://apod.nasa.gov/apod/ap080713.html
  25. https://apod.nasa.gov/apod/ap150830.html
  26. http://apod.nasa.gov/apod/random_apod.html
  27. https://apod.nasa.gov/apod/ap210417.html
  28. https://apod.nasa.gov/apod/archivepix.html
  29. https://apod.nasa.gov/apod/lib/apsubmit2015.html
  30. https://apod.nasa.gov/apod/lib/aptree.html
  31. https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
  32. https://apod.nasa.gov/apod/calendar/allyears.html
  33. https://apod.nasa.gov/apod.rss
  34. https://apod.nasa.gov/apod/lib/edlinks.html
  35. https://apod.nasa.gov/apod/lib/about_apod.html
  36. http://asterisk.apod.com/discuss_apod.php?date=210418
  37. https://apod.nasa.gov/apod/ap210419.html
  38. http://www.phy.mtu.edu/faculty/Nemiroff.html
  39. http://www.phy.mtu.edu/
  40. https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html
  41. http://www.astro.umd.edu/
  42. https://apod.nasa.gov/apod/lib/about_apod.html#srapply
  43. https://www.nasa.gov/about/highlights/HP_Privacy.html
  44. https://astrophysics.gsfc.nasa.gov/
  45. https://www.nasa.gov/
  46. https://www.nasa.gov/centers/goddard/
  47. http://www.mtu.edu/