Noctilucent cloud: Why would the sky still glow after sunset?

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.
2013 August 19

Noctilucent Clouds and Aurora Over Scotland
Video Credit: Maciej Winiarczyk; Music: Jolanta Galka-Kurkowska

Explanation: Why would the sky still glow after sunset? Besides stars and the band of our Milky Way galaxy, the sky might glow because it contains either noctilucent clouds or aurora. Rare individually, both are visible in the above time lapse movie taken over Caithness, Scotland, UK taken during a single night earlier this month. First noted in 1885, many noctilucent clouds are known to correlate with atmospheric meteor trails, although details and the origins of others remain a topic of research. These meandering bright filaments of sunlight-reflecting ice crystals are the highest clouds in the Earth’s atmosphere. The above video captures not only a variety of noctilucent clouds, but also how their structure varies over minutes. Lower clouds typically appear dark or fast moving. About halfway through the video the clouds are joined by aurora. At times, low clouds, noctilucent clouds, and aurora are all visible simultaneously, each doing their own separate dance, and once — see if you can find it — even with theBig Dipper rotating across the background.

REFERENCE: http://apod.nasa.gov/apod/ap130819.html

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