By Sabine Stanley, Ph.D., Johns Hopkins University
Nuclear fusion creates energy in the deep core of the Sun. But this energy has to travel through several layers of the Sun before it reaches the solar surface. What are these different layers, and how do they impact the equilibrium temperatures of the planets?
Radiative Zone of the Sun
When a photon, a little bundle of light energy, is created in the core, it begins traveling away from the center. Right above the core is the radiative zone, which extends from about 25% to 70% of the Sun’s radius. Temperature decreases with radius: dropping from 14 million Kelvin in the core to two million Kelvin at the top of the radiative zone.
Photons carry their energy from the core outward through the radiative zone. But they do so in a somewhat sluggish manner. The density is very high here, so the photons experience lots of collisions.
When a photon collides with a hydrogen or helium ion, it gets absorbed. But then it gets emitted again. It travels a bit, and then gets absorbed by another ion, and then emitted again, and so on. The time for a single photon to make it from the core to the top of the radiative zone can take 10,000 years or even hundreds of thousands of years!
Convection Zone of the Sun
Above the radiative zone is the convection zone, which spans the outer 30% of the Sun’s interior. The convection zone begins when the temperatures are low enough that radiation isn’t a dominant form of heat transfer. In the convection zone, energy is transported by the movement of the plasma or the hydrogen and the helium ions. Plasma churns vigorously, transporting heat energy to the surface of the Sun through convection.
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Photosphere or the Surface of the Sun
There’s no real outer edge of the Sun, so we define the ‘surface’ as the radius where the Sun becomes optically opaque. That’s where we can’t see any further. This surface is called the photosphere. The convection cells from the interior emerge at the photosphere as little bumps or bubbles in the surface known as solar granules. Any layers of the Sun above the photosphere are considered as part of the Sun’s atmosphere.
Studying the Interior of the Sun: Seismology
In order to study the interior of the Sun, scientists use a technique similar to the one used to study the Earth’s interior: seismology. The Sun’s surface oscillates at a range of frequencies and wavelengths. These oscillations come from sound waves that are traveling throughout the Sun. Unlike earthquakes we have on Earth, seismic events on the Sun are initiated by the convective motions occurring in the convection zone of the Sun.
The convection constantly disrupts the other layers of the Sun, just like disturbing the surface of the water in a pond will cause waves to travel throughout the pond. The amplitude and pattern of these oscillations on the Sun’s surface can be used to determine the structure of the Sun’s interior.
Rotation of the Radiative Zone
Using seismic observations, the joint NASA and European Space Agency mission named ‘SOHO’ determined that the deeper radiative zone is rotating like a rigid body, all at the same rotation rate.
But in the convection zone, the rotation rate depends on latitude. The equatorial regions rotate almost twice as fast as the polar regions. This happens all the way from the surface of the Sun down to the convection or radiation zone boundary. But rotation has to transition at this boundary between the rigid rotation of the deep interior to the latitude-based rotation above.
That transition from rigid to latitude-based rotation produces an important shear zone in the Sun called the tachocline. This is where the differentiated flows from above are quickly decelerated to reach the rigid rotation profile deeper inside.
Scientists don’t fully understand why the Sun’s convection zone rotates in a differential manner, but planets often have differential rotation of their atmosphere, which is sometimes caused by latitude-based variations in temperature.
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The Reason behind the Sun’s Differential Rotation
One possibility is that the Sun’s differential rotation is related to latitude-based temperature variations near the tachocline. Vigorous motions in the convection zone and tachocline happen to have a side effect: they create a dynamo in the Sun that produces a magnetic field. Magnetic fields get stored and sheared in the tachocline until they are strong enough to rise up through the convection zone and emerge at the surface.
At the surface, the solar magnetic field consists of two parts. First, there is a global, large- scale, dipolar field, similar to the dipolar magnetic field generated by the Earth’s dynamo. The second part of the solar magnetic field is related to sunspots. Sunspots appear as dark blotches on the surface of the Sun. They are caused by intense magnetic fields that reduce the convection in these regions, thereby letting less heat out and making the Sun look darker there.
The 11-Year Solar Cycle of Sunspots
for 11 years. (Image: NSO/AURA
/NSF/CC BY (https://creativecommons.org
/licenses
/by/4.0/Public domain)
The magnetic fields in sunspots are about 1000 times more intense than the Sun’s global dipole field. Both the sunspots and the global magnetic field participate in the ‘solar cycle’. This cycle lasts about 11 years. At the beginning of each cycle, the dipole field has a certain polarity, and Sunspots begin to form at mid-latitudes on the Sun’s surface. These sunspots include bundles of magnetic fields that emerge from the convection zone.
As time goes on, those sunspots disappear and more and more new spots appear at latitudes closer to the equator. That continues until the sunspots start appearing close to the equator at the 11-year mark. Then, the Sun reverses the polarity of its dipole magnetic field, and the sunspots begin to appear at mid-latitudes again.
Sunspot Butterfly Diagram
If one looks at a graph of the number of sunspots at each latitude over time, there is an interesting pattern that emerges, which looks like butterfly wings. This has led people to call this the ‘sunspot butterfly diagram’. The butterfly diagram demonstrates how the locations of sunspot emergence change to lower and lower latitudes through a cycle, and then they reappear at higher latitudes again at the beginning of the next cycle.
Differences in the Number of Sunspots and Solar Activity
Along with the change in number and location of sunspots, there are also changes in the levels of solar radiation and solar flare activity during the cycle. The Sun is most active during solar maximum: when the most sunspots appear, about halfway through each cycle. But there are differences in the number of sunspots and solar activity between cycles as well.
Over the past 400 years, astronomers have been counting sunspots. The total number of sunspots observed at solar maximum usually varies between 40 to 250 or so. But something unusual happened between 1645 and 1715. Very few sunspots were seen during this time, and it’s become known as the ‘Maunder Minimum’, named for a husband and wife team who studied sunspot trends.
The Sun seemed to go through a phase of about 70 years where it was much less active. But the Sun bounced back again, returning to typical sunspot numbers and activity shortly thereafter.
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The Impact of the Sun’s Fusion on Planets
The fusion in the Sun’s core not only produces light energy that travels up through the radiative and convection zones, but it also generates the solar magnetic field. The character of the planets is fundamentally shaped by their experiences of both the heat and magnetic field from the Sun. Let’s start by discussing the heat.
Temperatures across the solar system are determined by distance from the Sun. Earth receives an average solar power—also known as irradiance or insolation—of about 1400 watts per square meter over the entire disk that faces the Sun. In comparison, Mercury receives over 9000 watts per square meter. Neptune, at the end of the solar system, receives a mere 1.5 watts per square meter.
The Equilibrium Temperature of Planets
The solar irradiance at a planet can be used to determine its equilibrium temperature. This is the temperature the surface of a planet would have if it had no greenhouse effect in its atmosphere and no other sources of heat. We also need to take into account that the planets may reflect some of the light from the Sun, so not all of that solar power is used to heat the planet.
The amount that’s reflected is determined by the albedo of the planet. For example, if a planet has a high albedo—which means that lots of sunlight is reflected off the surface rather than absorbed by the surface—then the planet equilibrium temperature will be lower. Venus is the planet with the highest albedo, mostly because of its thick, yellowish cloud layer. But there are some bright icy moons with even higher albedo.
Saturn’s moon Enceladus has the highest albedo of anybody in the solar system, reflecting almost 99% of solar light incident upon it. In terms of the other planets, after Venus, Jupiter and Saturn come in with the next highest albedos, as one might expect from their bright color. Then there are Uranus and Neptune, then Earth, Mars, and finally Mercury. Mercury’s albedo is the lowest of the planets. It’s very dark and absorbs about 90% of the solar radiation that arrives.
Common Questions about the Layers of the Sun
The main inner layers of the Sun are core, radiative zone, convection zone, and the outer layer or the surface is called the photosphere.
The core of the Sun is encircled by two other inner layers: the radiative zone and the convection zone.
The radiative zone is the largest part of the Sun. It extends from about 25% to 70% of the Sun’s radius.
