Coronal hole

Coronal holes are part of the Sun's corona and are constantly changing and reshaping because the corona is not uniform.^[1] 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,^[1] 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.^[2] The aurora borealis that is seen at the northern and southern poles are the result of solar wind entering the Earth’s atmosphere.^[1]

First Discoveries

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.^[2][3] 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.

Solar Cycle

Their size and population vary based on the solar cycle. It is known that every 11 years coronal holes reach their maximum.^[3] 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.^[4] 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.^[1]

Coronal Hole Magnetic Field Lines-image shows what happens when the field lines do not fall back, and instead allow solar wind to escape

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.^[3] 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.^[1]

Recent Discoveries

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).^[3]

Images

The solar disk with a coronal hole.

This visualization shows a coronal hole over the course of 24 hours, sampled about once per minute.

References

1. Freedman, Roger A., and William J. Kaufmann III. "Our Star, the Sun." Universe. 8th ed. New York: W.H. Freeman, 2008. 419-420. Print.
2. What is a Coronal Hole? ips.gov.au
3. "Massive Coronal Hole on the Sun". NASA. NASA. Retrieved 31 October 2014.
4. Fox, Karen. "Large Coronal Hole Near the Sun’s North Pole". NASA. NASA. Retrieved 31 October 2014.

Further reading

1. Gombosi, Tamas (1998). Physics of the Space Environment. New York: Cambridge University Press. ISBN 0-521-59264-X.
2. Jiang, Y., Chen, H., Shen, Y., Yang, L., & Li, K. (2007, January). Hα dimming associated with the eruption of a coronal sigmoid in the quiet Sun. Solar Physics, 240(1), 77-87.