Auroral Corona

Caption NASA, da "NASA - Picture of the Day" del giorno 28 Luglio 2009:"Few Auroras show this level of detail. Above, a standard digital camera captured a particularly active and colorful Auroral Corona that occurred last week above Alberta, Canada. With a shape reminiscent of a flower, the spectacular Aurora had an unusually high degree of detail. The vivid green and purple auroral colors are caused by high Atmospheric Oxygen and Hydrogen reacting to a burst of incoming electrons. Many photogenic Auroras have been triggered from a Solar Wind Stream that recently passed the Earth. The Auroras were unexpected because the initiating Sun has been unusually quiet lately".