Image Credit & Copyright: Jeff Dai
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 above. 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.
APOD NASA 01-Sep-14
Image Credit & Copyright: Phil Hart
What shines in the world at night? Just visible to the eye, a rare electric blue glow spread along the shores of Victoria Lake on January 16, 2013. Against reflections of a light near the horizon, this digitally stacked long exposure recorded the bioluminescence of noctiluca scintillans, plankton stimulated by the lapping waves. Above, the night skies of the Gippsland Lakes region, Victoria, Australia shine with a fainter greenish airglow. Oxygen atoms in the upper atmosphere, initially excited by ultraviolet sunlight, produce the more widely seen fading atmospheric chemiluminescence. Washed out by the Earth’s rotation, the faint band of the southern summer Milky Way stretches from the horizon as star trails circle the South Celestial Pole.
APOD NASA 09-Aug-14
Image Credit & Copyright: Cui Yongjiang
Long after sunset on January 25 an unusually intense red airglow floods this south-looking skyscape. The scene was recorded with a long exposure using a digital camera over Yunnan Province in southwest China. At best faintly visible to the eye, the lingering airglow is due to chemiluminescence, the production of light through chemical excitation. Originating at an altitude similar to aurora, it can found around the globe. The chemical energy is initially provided by the Sun’s extreme ultraviolet radiation On this night, despite the luminous atmosphere, the band of the Milky Way clearly stretches above the horizon with bright star Sirius near the top of the frame. Both airglow and starry sky are beautifully reflected in region’s watery Yuanyang rice terraces below.
NASA APOD 06-Feb-2014