Sirius Geography

Solar Granules

Solar Granules

The solar surface is neither static nor smooth or flat but made up of constantly moving 3-dimensional convection cells that build up and die normally within 8 to 20 min[1]. These movements create bright plumes of hot rising gas called granules and darker lines between them called intergranular lanes[2]. On average, individual granules are around 1,500 km across[3] but they vary in size: The smallest currently observable granules are around 100 km across while the largest ones reach a diameter in the order of 3,000 km[4]. Individual granules may look small compared the vast size of the solar disk but they are enormous! The average diameter of 1,500 km makes most of them larger than any country in central Europe (France is about 1,000 km long and wide[5] while the UK is also around 1,000 km from Scotland to the south coast but much narrower, covering 500 km at its widest[6]). The convective speed of cells averages 1 km/s[7] but can reach supersonic speeds of over 7 km/s, causing sonic booms and other noise that triggers waves across the solar surface[8].

Close up image of granules seen on the solar surface in an area with little activity. (Image Credits: Vasco Henriques and Ainar Drews; European Solar Telescope

Mesogranules and Supergranules

An average granule is comparable in area to Greenland, Mexico, or Indonesia! But they can get even bigger than that: Structures with a diameter in the order of 5,000 km are called mesogranules, which have much stronger convection (around 60 m/s and a lifetime of about 3 hrs[9]. Supergranules refer to structures with a diameter in the order of 32,000 km, which is more than twice the diameter of our planet earth! Their lifetime is about 20 hrs and their convection is even stronger (around 400 m/s)[10].

Formation

The Sun is a giant nuclear reactor. Energy produced in the interior regions of the Sun is initially carried outwards by radiation but outside the core, the temperature gradient becomes superadiabatic, which means that if a plasma parcel or “bubble” is raised by a small amount, it will remain buoyant even as it expands and cools in the reduced pressure[11]. In simpler terms it means, once a hot bubble forms outside the core, it will keep rising towards the surface, forming a convection cell. The speed at which these cells rise is called the convection speed, which can reach supersonic levels[12].

Each granule is such a “bubble.” When the convection cell reaches the solar surface, it cools down and starts flowing back into the interior of the Sun along the granule’s edges (while the remaining hot plasma still pushes up in the centre of the cell). These edges appear darker because they are cooler[13].

History

Granulation on the solar surface was first observed by the German-British astronomer and composer William Heschel (1738-1822)[14] in 1801[15] and first photographed by the French astronomer and chemist who also discovered the chemical element helium (which the Sun produces) Pierre Jules Janssen (1824-1907)[16]  in 1878[17]. The German astrophysicist Albrecht Unsöld (1905-1995)[18] was the first to identify these granules as convection cells in 1930[19].

Left: German-British astronomer William Herschel (1738-1822) who first observed solar granules. (Image Credit: Wikimedia Commons). Centre: French astronomer and chemist Jules Janssen (1824-1907) who took the first photograph of solar granules. Right: Albrecht Unsöld (1905-1995), the German astrophysicist who first identified granules as convection cells. (Image Credit: Kiel University)

Summary

The surface of the sun is far from flat but shows 3-dimensional structures called granules, which are convection cells of hot plasma rising from the interior, cooling down and dissipating rapidly (on average within 8 to 20 min). These granules are massive and larger than most countries! Mesogranules are even larger and supergranules can  be more than twice the size of earth.

[1] Vacca, M. (2022). Sun Granules. Universities Space Research Association: EPOD. URL: https://epod.usra.edu/blog/2022/05/sun-granules.html

[2] Díaz Castillo, S. M., Asensio Ramos, A., Fischer, C. E., & Berdyugina, S. V. (2022). Towards the Identification and Classification of Solar Granulation Structures Using Semantic Segmentation. Frontiers in Astronomy and Space Sciences, 9, 896632. https://doi.org/10.3389/fspas.2022.896632

[3] Soloviev, A. A., Parfinenko, L. D., Efremov, V. I., Kirichek, E. A., & Korolkova, O. A. (2019). Structural features of sun photosphere under high spatial resolution. arXiv preprint arXiv:1911.02556.

[4] Muller, R., Hanslmeier, A., Utz, D., & Ichimoto, K. (2018). Does the solar granulation change with the activity cycle? Astronomy & Astrophysics, 616, A87. https://doi.org/10.1051/0004-6361/201732085

[5] France.fr. (2022). Geography and climate. Explore France. URL: https://uk.france.fr/en/holiday-prep/geography-and-climate

[6] Joyce, P. (2022). United Kingdom. Encyclopaedia Britannica. URL: https://www.britannica.com/place/United-Kingdom

[7] Rast, M. P. (2003). The scales of granulation, mesogranulation, and supergranulation. The Astrophysical Journal, 597(2), 1200-1210. http://dx.doi.org/10.1086/381221

[8] 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

[9] Rast, M. P. (2003). The scales of granulation, mesogranulation, and supergranulation. The Astrophysical Journal, 597(2), 1200-1210. http://dx.doi.org/10.1086/381221

[10] Rast, M. P. (2003). The scales of granulation, mesogranulation, and supergranulation. The Astrophysical Journal, 597(2), 1200-1210. http://dx.doi.org/10.1086/381221

[11] Proctor, M. R. E. (2004). Solar convection and magnetic fields. Astronomy & Geophysics, 45(4), 4-14. https://doi.org/10.1046/j.1468-4004.2003.45414.x

[12] 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

[13] Johnston, H. (2020). Convection cells the size of Texas dazzle with clarity on the Sun. Physics World. URL: https://physicsworld.com/a/convection-cells-the-size-of-texas-dazzle-with-clarity-on-the-sun/

[14] Encyclopaedia Britannica. (2022). William Herschel. Encyclopaedia Britannica. URL: https://www.britannica.com/biography/William-Herschel

[15] Rast, M. P. (2003). The scales of granulation, mesogranulation, and supergranulation. The Astrophysical Journal, 597(2), 1200-1210. http://dx.doi.org/10.1086/381221

[16] Encyclopaedia Britannica. (2022). Pierre Janssen. Encyclopaedia Britannica. URL: https://www.britannica.com/biography/Pierre-Janssen

[17] Rast, M. P. (2003). The scales of granulation, mesogranulation, and supergranulation. The Astrophysical Journal, 597(2), 1200-1210. http://dx.doi.org/10.1086/381221

[18] Baschek, B. (1996). Nachruf: Albrecht Unsöld. Mitteilungen der Astronomicshen Gesellschaft, 79, 11-15.

[19] Rast, M. P. (2003). The scales of granulation, mesogranulation, and supergranulation. The Astrophysical Journal, 597(2), 1200-1210. http://dx.doi.org/10.1086/381221