Coronal holes are part of the Sun's corona and are constantly changing and reshaping because the corona is not uniform. Coronal holes are areas where the Sun's corona is darker, and colder, and has lower-density plasma than average because there is lower energy and gas levels. The sun contains magnetic fields that arch away from areas in the corona that are very thin due to the lower levels of energy and gas, which cause coronal holes to appear when they do not fall back. Thus, solar particles or solar wind escape and create a lower density and lower temperature in that area. The aurora borealis that is seen at the northern and southern poles are the result of solar wind entering the Earth’s atmosphere.
In the 1960s, the Sydney Chris Cross radio telescope took x-ray images, although it was unclear what they were. Some research also points out that coronal holes were first spotted in the early 1970s. These were found when X-ray telescopes in the Skylab mission were flown above the Earth's atmosphere to reveal the structure of the corona.
Their size and numbers vary based on the solar cycle. It is known that every 11 years coronal holes reach their maximum. During a solar maximum, the number of coronal holes actually decreases because the magnetic fields are about to reverse in the Sun’s core. New coronal holes appear at the opposite magnetic alignment. They appear and expand over the pole and continue to do so even when the sun moves to solar minimum again. Coronal holes are linked to unipolar concentrations of open magnetic field lines. During solar minimum, coronal holes are mainly found at the Sun's polar regions, but they can be located anywhere on the sun during solar maximum. There are permanent coronal holes on the north and south poles of the Sun, which is why even during solar minimum there are still coronal holes.
Coronal Holes and Solar Wind
Coronal holes generally discharge fast solar wind, which is solar material that the sun spews out into the solar system, that is about twice the speed that it normally is. The escaping solar wind is known to travel along open magnetic field lines that pass through coronal holes and do not connect back. Since, coronal holes are regions in the sun’s corona that have much lower densities and temperatures than most of the corona, these regions are then very thin, which contribute to the escape of solar wind. Since these areas are so thin, particles within the chromosphere can easily break through as magnetic field lines and allow solar wind to escape, which is usually 10^9 kg of material and is made up of electrons and nuclei of hydrogen and helium as well as other particles such as ions and more massive atoms that include silicon, sulfur, calcium, chromium, nickel, iron, and argon.
On June 18, 2013, a large coronal hole was spotted in the Sun, spanning at least 400,000 miles across (almost half of the Sun's diameter).
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