This imagery of the sun captured by NASA's Solar Dynamics Observatory
from May 17-19, 2016, shows a giant dark area on the star's upper half,
known as a coronal hole. Coronal holes are low-density regions of the
sun’s atmosphere, known as the corona. Because they contain little solar
material, they have lower temperatures and thus appear much darker than
their surroundings. Coronal holes are visible in certain types of
extreme ultraviolet light, which is typically invisible to our eyes, but
is colorized here in purple for easy viewing.
These coronal holes are important to understanding the space
environment around Earth through which our technology and astronauts
travel. Coronal holes are the source of a high-speed wind of solar
particles that streams off the sun some three times faster than the
slower wind elsewhere. While it’s unclear what causes coronal holes,
they correlate to areas on the sun where magnetic fields soar up and
away, without looping back down to the surface, as they do elsewhere. Credits: NASA/SDO. Download this video (mp4 format)
Source: NASA/SDO Solar Mission