NASA's Space Place
Dark Bits on the Bright Sun
Alex H. Kasprak
National Aeronautics and Space Administration
The sun is a giant ball of burning gas 93 million miles away. It burns so brightly it warms us here on Earth and gives us energy. Because it’s so hot and bright, you might be surprised to learn that sometimes the sun has dark regions in its atmosphere.
We can’t see these dark patches with our own eyes, of course. And don’t try—it will hurt your eyes! We see these dimmer areas with spacecraft like the Solar Dynamics Observatory. This satellite orbits the Earth and watches the sun from afar, taking pictures as it goes. On March 16, it found two big holes in the outer atmosphere, or corona, of the sun.
One coronal hole is located near the southern pole of the sun. It is one of the largest coronal holes scientists have seen in decades. It covers about eight percent of the surface of the sun. That might not sound like much, but for comparison, eight percent of your body surface is the front of one whole leg. The other coronal hole is long and skinny. It covers less than one percent of the sun, but that’s still almost four billion square miles. That’s the same area as 20 earths!
These coronal holes are colder and less dense than the rest of the sun’s atmosphere. Because the magnetic fields in those regions are open to space, coronal holes let energy and particles escape from the sun and go out into the solar system. This blast of charged particles is called solar wind.
The solar wind takes two or three days to reach us on earth, and when it does, it can cause auroras. Those are the bright bands of light that you can see in the sky, usually near the north pole.
Learn more about weather on the sun and how it can affect us here on Earth! http://spaceplace.nasa.gov/solar-cycles.
This image from NASA’s Solar Dynamics Observatory on March 16, 2015, shows two dark spots, called coronal holes. The lower coronal hole was one of the biggest observed in decades. Image Credit: NASA/SDO