GREENBELT, Md. (NASA) - Behold five and a half years worth of full-sun observations from XRT. A dramatic illustration of the solar cycle, this movie begins about one year before the first reversed-polarity sunspot ushered in the current cycle on January 8, 2008. The solar cycle is a periodic variation in the Sun's activity that is caused by the gradual 'tangling' and eventual reversal of its magnetic field.
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