Bright Spots and Plages
What Are Faculae?
The surface of the sun is constantly changing. Two of the most prominent features that are clearly visible are sunspots (dark looking areas) and faculae (singular facula), which are bright spots on the sun that occur around areas with sunspot activities[1]. Their name comes from the Latin word for “little torches”[2]. Even though faculae occur across the sun but they are often masked by the bright centre of the sun and only become visible at the edges, where the photospheric background is dimmer[3]. Faculae are caused by local changes in the sun’s magnetic field: they are essentially magnetic flux tubes with strong magnetic fields (around 0.1 tesla[4]) that are brighter than its surroundings[5].
Both sunspots and faculae are the result of magnetic variations and both are closely related. Sunspots always have an associated facula but not all faculae have sunspots[6] and faculae tend to appear prior to sunspots and remain visible for several days or weeks after the sunspots disappear[7]. Faculae are a prominent feature of solar cycle. Temperature vary within faculae but on the whole, they emit more heat than the calmer surrounding areas[8] (they are hotter by about 300˚C), which is why during peak sunspot activity (which is also when there are the most faculae), the sun is slightly brighter (by about 0.1%)[9]: The bright faculae not just counter sunspots but actually increase the total energy output of the sun[10]! These variations in solar energy output impact earth’s climate. The “Little Ice Age” has been at least partially attributed to periods of unusually low sunspot activity[11].
Underlying Mechanisms
Faculae are magnetic flux tubes or bundles—local areas where the magnetic field is much stronger than in its surrounding areas[1]. The exact reasons why this leads to these areas being brighter and hotter is still being explored. Some theories suggest that faculae are essentially holes in the photosphere, where the density of gas is dramatically reduced (pushed away) by the strong magnetic field, and as a result, the hotter underlying layers can radiate through with less obstruction, making faculae brighter and hotter[12],[13].
Plages
Faculae are usually observed in the Sun’s photosphere within the visible light spectrum. When the same bright spots are observed in the Sun’s higher chromosphere (in the CaK and Hα spectrum), they are referred to as plages[14]. Plages are larger and brighter than faculae[15] but that is simply an observational difference. Both are part of the same underlying phenomenon and the terms are often used interhcangeably[16],[17].
History
Faculae were first discovered by the English astronomer Sir Jospeh Norman Lockyer (1836-1920) [18] and named by French astronomer Henri-Alexandre Deslandres (1853-1948)[19], who referred to them as bright spots on the solar surface (“les plages brillantes de la surface solaire”); this mention resulted in some later authors to refer to them as plages (often wrongly inferred to as coming from the French term for “beach”, where in the context, “plage” in Deslandres’ writing simply means area or region[20]). Deslandres himself advocated to use the term “facular flames”[21] (facula means “small torch” in Latin), which since has been shortened to facula (singular) or faculae (plural). Today, the term plage is generally used for bright spots in the chromosphere while faculae is used for the same spots in the photosphere.
Summary
Faculae are bright spots on the Sun’s surface that are caused by strong local magnetic variations (they are essentially magnetic flux tubes). They are related to the dark sunspots and are an important part of the solar cycle. They result in increased solar energy output and thus, impact the earth’s weather and climate.
[1] Berger, T. E., Title, A. M., Tarbell, T. D., van der Voort, L. R., Löfdahl, M. G., & Scharmer, G. B. (2007). What are ‘Faculae’? New Solar Physics with Solar-B Mission, 369, 103-112.
[2] Berger, T. E., Title, A. M., Tarbell, T. D., van der Voort, L. R., Löfdahl, M. G., & Scharmer, G. B. (2007). What are ‘Faculae’? New Solar Physics with Solar-B Mission, 369, 103-112.
[3] Encyclopaedia Britannica. (2020). Facula. Encyclopaedia Britannica. URL: https://www.britannica.com/science/facula
[4] Ridpath, I. (2012). Faculae. In A dictionary of astronomy (2nd ed.). Oxford, UK: Oxford University Press.
[5] Ortiz, A., Solanki, S. K., Domingo, V., Fligge, M., & Sanahuja, B. (2002). On the intensity contrast of solar photospheric faculae and network elements. Astronomy & Astrophysics, 388(3), 1036-1047. https://doi.org/10.1051/0004-6361:20020500
[6] Encyclopaedia Britannica. (2020). Facula. Encyclopaedia Britannica. URL: https://www.britannica.com/science/facula
[7] Ridpath, I. (2012). Faculae. In A dictionary of astronomy (2nd ed.). Oxford, UK: Oxford University Press.
[8] Solov’ev, A. A., & Kirichek, E. A. (2019). Structure of solar faculae. Monthly Notices of the Royal Astronomical Society, 482(4), 5290-5301. https://doi.org/10.1093/mnras/sty3050
[9] Hathaway, D. H. (2014). Photospheric features. Solar Physics: Marshall Space Flight Center. Huntsville, AL: NASA (Marshall Space Flight Center). URL: https://solarscience.msfc.nasa.gov/feature1.shtml
[10] National Weather Service. (n.d.). The sun and sunspots. National Weather Service. Siux Falls, SD: National Weather Service. URL: https://www.weather.gov/fsd/sunspots
[11] Jackson, S. T. (2016). Little ice age. Encyclopaedia Britannica. URL: https://www.britannica.com/science/Little-Ice-Age
[12] Berger, T. E., Title, A. M., Tarbell, T. D., van der Voort, L. R., Löfdahl, M. G., & Scharmer, G. B. (2007). What are ‘Faculae’? New Solar Physics with Solar-B Mission, 369, 103-112.
[13] Scharmer, G., & Kiselman, D. (2008). Solar faculae explained. Institue for Solar Physics: University of Stockholm. URL: https://www.isf.astro.su.se/highlights/faculae-explained/
[14] Ortiz, A., Solanki, S. K., Domingo, V., Fligge, M., & Sanahuja, B. (2002). On the intensity contrast of solar photospheric faculae and network elements. Astronomy & Astrophysics, 388(3), 1036-1047. https://doi.org/10.1051/0004-6361:20020500
[15] Stanford Solar Center. (2008). Glossary. Stanford Solar Center. URL: http://solar-center.stanford.edu/gloss.html#PENUMBRA
[16] Kahil, F., Riethmüller, T. L., & Solanki, S. K. (2019). Intensity contrast of solar plage as a function of magnetic flux at high spatial resolution. Astronomy & Astrophysics, 621, A78. https://doi.org/10.1051/0004-6361/201833722
[17] Solov’ev, A. A., & Kirichek, E. A. (2019). Structure of solar faculae. Monthly Notices of the Royal Astronomical Society, 482(4), 5290-5301. https://doi.org/10.1093/mnras/sty3050
[18] Encyclopaedia Britannica. (2022). Henri-Alexandre Deslandres. Encyclopaedia Britannica. URL: https://www.britannica.com/biography/Henri-Alexandre-Deslandres
[19] Encyclopaedia Britannica. (2022). Sir Joseph Norman Lockyer. Encyclopaedia Britannica. URL: https://www.britannica.com/biography/Joseph-Norman-Lockyer
[20] Pietrow, A. G. M. (2022). Physical properties of chromospheric features: Plage, peacock jest, and calibrating it all [Doctoral thesis]. URL: https://www.diva-portal.org/smash/get/diva2:1651858/FULLTEXT01.pdf
[21] Pietrow, A. G. M. (2022). Physical properties of chromospheric features: Plage, peacock jest, and calibrating it all [Doctoral thesis]. URL: https://www.diva-portal.org/smash/get/diva2:1651858/FULLTEXT01.pdf