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What Is the Exterior Structure of the Sun?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Finding the boundary between the interior and exterior of a gas-plasma object is challenging. Oftentimes, we look for behavior of regions to help us delimitate between what is inside and outside an object. For the Sun, the exterior is defined as the first thing we actually see, and that is the photosphere.

Photosphere

This is the visible surface of the Sun and is very thin compared to the rest of the object. It extends down to 500 kilometers between it and the next layer and has an average temperature of about 5800 Kelvin. The reason why the photosphere is the visible layer and nothing below is has to do with the density of the gas/plasma present. Conditions are so crowded that light cannot escape without being altered, meaning the photosphere is the first chance light from below gets to escape unscathed. That being said, the photosphere is about 3400 times denser than the air on Earth, or about 4000 kg/m3 (Seeds 120-1, Sharp, Hathaway “The photosphere”).

Granules

Granules

Granules

When examined, the photosphere displays a granulation pattern that appears small but in really each spot is about the size of Texas (about 1000 kilometers across)! Their lifespan is a mere 10 to 20 minutes before convection causes the granule to be reabsorbed and fall below. The reason we can see the granules is due to the 100-degree cooler edges giving off less light than the warmer areas. That’s the amazing part: The edges are giving off tons of light but because its less than the warmer areas it simply appears to be darker. The edges are the material that moved from the hot center then radially outward. This is the convection cycle demonstrated, with the center moving up and the edges is downward, both at roughly 0.4 km/s. (Seeds 121, Hathaway “Photospheric features”).

The chromosphere, with its spectrum displayed.

The chromosphere, with its spectrum displayed.

Chromosphere

Above the photosphere lies the chromosphere, which is about 13,000 km in height and about 1000 times fainter than the photosphere. The only way to actually spot it is when a total solar eclipse occurs, and it is pink in color due to the red, blue, and violet hydrogen lines being emitted as hydrogen is fused. Looking at these lines, we can estimate the density of the chromosphere at about 0.000001% that of Earth’s air, or about 10 mg/m^3. Another useful filter for studying the chromosphere is ionized calcium, which emits in the violet part of the EM spectrum. This can be used to further resolve details and processes happening here (Seeds 122, Sharp, Hathaway “The Chromosphere”).

Just because the chromosphere is nearly transparent doesn’t mean it isn’t impacting the light exiting the Sun. Certain wavelengths of light are being absorbed by excited material. By using a filtergram, we can look more closely at the light, which is being absorbed in the lower regions, then reemitted at the higher altitudes, and gain certain details about the chromosphere that would otherwise be missing. For example, filtergrams of the H-alpha line reveal great details on spicules, which are jets of plasma and gas that go outward into the chromosphere and last 5-15 minutes a piece. They push into the higher regions of the chromosphere and heat up there, becoming brighter in the process (Seeds 122-3, Sharp).

As one moves up in the atmosphere, the level of ionization increases due to increasing temperatures. This ends up being a good tool to find temperatures at different altitudes around the photosphere boundary, yielding temperatures of about 4500 K there. And at the corona boundary, we can get to nearly a million K! (Seeds 122).

The corona

The corona

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Solar Corona

Above the chromosphere lies the solar corona, which is visible during a solar eclipse but even then, remains very faint. Hence why during non-eclipse portions of the year we have a coronagraph to block out the main disc of the Sun to be able to study the corona. Sometimes, we have seen the corona extend as much as 20 solar radii away from the surface! Though these observations, it has been shown via spectroscopy that the outer corona can have the same spectrum as the main body of the Sun, despite those aforementioned absorptions of parts of the spectrum. This seems to be because of dust reflecting light (Seeds 123).

But other times it is seen that the corona has no absorption lines at all. In this case, photons leaving the Sun encounter free electrons, causing scattering of the light. In the 1 million Kelvin environment present here, this creates huge Doppler shifts in the spectrum in random places. Taking these actions and stacking them causes a smearing out effect until it appears as though we have a continuous spectrum. Using that ionized gas relation to temperature, we find that the lower corona is about 500,000 Kelvin (which using spectroscopy leads to a density of about 1 million atoms per cubic centimeter) and the upper corona to be in excess of 2 million Kelvin (leading to a density of about 1-10 atoms per cubic centimeter!) (Ibid).

How is it that something so far removed from the surface of the Sun can be so hot? The answer likely lies in magnetic fields. In the photosphere, many magnetic field loops are present and extend outward into the chromosphere and the corona. Up here, with density decreasing, changes to the loops are easier to implement, meaning movement is greater which raises temperatures to such extremes. Not only twisted field lines but twisted waves themselves can convey heat via Alfven waves. Another possible source of heat may come from nanoflares, which are constantly going off across the sun and possible as a collective whole are releasing energy to the corona. An even stranger source of heat may come from super-tornadoes, which are a direct result of hot gas interacting with magnetic fields. At any given moment, about 11,000 of these are occurring and can pull heat up to the corona (Seeds 123, Sharp, “Alfven”).

But the ultimate source of heat may lie in Alfven waves, or the movement of purely magnetic waves through the stars. While we know field lines do extend outward of the sun, these waves are a result of fields themselves being twisted in the suns atmosphere due to the movement of material creating electric – and therefore magnetic – fields, from convective motions and even pressure waves. They can travel all across the Sun have been spotted in the corona and the chromosphere. Sometimes these can themselves transfer energy or force smaller magnetic fields to tangle and release energy in the form of 1000s of mini explosions (“Alfven,” Fleck “Hinode”, “Five”).

Works Cited

“Alfven Waves – Our Sun is Doing the Magnetic Twist.” Science20.com. ION Publications LLC, 18 Mar. 2009. Web. 17 Feb. 2022.

Fleck, Bernhard. “Hinode: new insights on the origin of solar wind.” Esa.int. ESA, 12 Jul. 2007. Web. 28 Feb. 2022.

Hathaway, David H. “Photospheric Features.” Solarscience.msfc.nasa.gov. NASA, 11 Aug. 2014. Web. 17 Feb. 2022.

---. “The Chromosphere.” Solarscience.msfc.nasa.gov. NASA, 11 Aug. 2014. Web. 17 Feb. 2022.

---. “The Photosphere.” Solarscience.msfc.nasa.gov. NASA, 11 Aug. 2014. Web. 17 Feb. 2022.

Seeds, Michael A. Horizons. Tenth Edition, Thomson Brooks/Cole, Belmont, CA. 2008. Print. 120-3.

Sharp, Tim. “The sun’s atmosphere: Photosphere, chromosphere, and corona.” Space.com. Future US, Inc., 01 Nov. 2017. Web. 17 Feb. 2022.

© 2022 Leonard Kelley

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