Below is an official NASA caption for an image made about one month before the image on this Wikimedia Commons page:

(Source: https://sdo.gsfc.nasa.gov/gallery/main/item/580 )

The most dominant feature on the Sun the past week has been quite a large, dark coronal hole at the bottom of the Sun (Nov. 30 - Dec. 4, 2014). Coronal hole are areas where the Sun's magnetic field is open ended and where high-speed solar wind streams into space. The area appears darker there because there is less material being imaged in this wavelength of extreme ultraviolet. At its widest point, the hole extends more than 50 times the size of Earth. The video covers almost five days of activity. Credit: Solar Dynamics Observatory, NASA.

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