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Q: What are coronal holes?

The Answer

Last update 2023.03.30

Coronal holes are regions of open magnetic fields that appear as dark regions in the solar corona (the outer part of the sun's atmosphere).
Coronal holes appear darker in optical and X-ray images and have lower temperatures and densities than the surrounding parts of the halo.
Some coronal holes are so large that they occupy nearly a quarter of the sun's surface.
Coronal holes were first spotted by NASA's Skylab in the early 1970s, but scientists are still not entirely sure what causes them to form.
Coronal holes can appear at any time of the solar cycle (the cycle that the sun's magnetic field goes through about every 11 years) but are most common during the declining phase of the cycle.
The current solar cycle (No. 25) began in 2019 and is expected to continue until approximately 2030.

What are coronal holes?

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