What are coronal holes?

<ul style=";text-align:left;direction:ltr"><li style=";text-align:left;direction:ltr">Coronal holes are regions of open magnetic fields that appear as dark regions in the solar corona (the outer part of the sun&#39;s atmosphere).</li><li style=";text-align:left;direction:ltr"> Coronal holes appear darker in optical and X-ray images and have lower temperatures and densities than the surrounding parts of the halo.</li><li style=";text-align:left;direction:ltr"> Some coronal holes are so large that they occupy nearly a quarter of the sun&#39;s surface.</li><li style=";text-align:left;direction:ltr"> Coronal holes were first spotted by NASA&#39;s Skylab in the early 1970s, but scientists are still not entirely sure what causes them to form.</li><li style=";text-align:left;direction:ltr"> Coronal holes can appear at any time of the solar cycle (the cycle that the sun&#39;s magnetic field goes through about every 11 years) but are most common during the declining phase of the cycle.</li><li style=";text-align:left;direction:ltr"> The current solar cycle (No. 25) began in 2019 and is expected to continue until approximately 2030.</li></ul>