RARE #BLUE #AURORAS CAUSED BY RESONANT SCATTERING: The severe geomagnetic storm of Sept. 16-17 produced something that even longtime #aurora watchers rarely see: Blue Auroras. "I photographed them at dawn on Sept. 17th," says Alan Dyer, who sends this picture from Gleichen, #Alberta:

Auroras are usually green and red. Blue is very unusual. It takes a strong geomagnetic storm to produce them -- plus one extra ingredient. "High altitude sunlight was hitting the blue rays," explains Dyer.

The process is called "resonant scattering." At the top of the aurora zone, ionized nitrogen molecules (N2+) naturally produce blue light. The blue glow is usually too faint to see. However, when these ions get hit by morning sunlight, they capture and re-emit photons from the sun, amplifying the blue color

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