The Quiet Sun

Part I

We divide the discussion of the Sun into two parts: the Sun without activity on the surface (the Quiet Sun), and the solar activity, mostly in the form of sunspots (the Active Sun).

The solar composition is important to understand what the universe is made of and also to see what the proportions of life sustaining atoms might be in the primitive solar system. The bulk of the Sun is made of hydrogen. Most of what is not hydrogen is helium. Because of this only three groups of elements as needed by astronomers: Hydrogen, Helium, and Other (called metals). Of the atoms that make up life (C, H, O, N) - these are relatively abundant in the Sun.

The Sun is a ball of gas bigger than the orbit of the Moon - about one million kilometers in diameter. The structure of the Sun is:

Interior

• Core energy generation region
• Radiative Zone radiative transfer of energy
• Convective Zone convective transfer of energy

Atmosphere

• Photosphere visual surface
• Chromosphere "colored sphere"
• Corona "crown"

Throughout most of the 20^th century a question that has lingered in astronomy is the power source of the Sun. We now know how much energy the Sun is producing. If this energy has been pouring out of the Sun since the beginning, we can calculate the energy reserves that the Sun must have. Knowing the age of the solar system (at least 4.7 billion years) permits astronomers to judge whether any particular energy source is viable.

One of the first proposals was that the Sun shone by a chemical fire. But the entire Sun would be consumed in a mere 3000 years by this process. We can eliminate fire as an energy source.

Next attention turned toward gravitational potential energy. Models show that stars form from highly distended gas and dust clouds that begin to collapse. Maybe the Sun is still in the process of collapsing, releasing energy and light as it does. This idea can be tested by comparing the duration of historical total solar eclipses. Our reliable time scale is not long enough for a definitive conclusion, but this process can power the Sun for a few hundred million years before the Sun would have shrunk to a point. Also a much larger Sun in the past than today seems to be prohibited by the climatic record of the Earth. A larger Sun would produce a warmer environment in the early history of our planet.

The beginning of particle physics produced new ideas about the energy source of the Sun. Hydrogen fusion is the process of combining hydrogen nuclei into helium nuclei. The fundamental forces of nature in order of strength are:

• Strong Nuclear Binds the particles of the nucleus together
• Electromagnetic Responsible for all chemical reactions
• Weak Nuclear Mediates radioactivity
• Gravitational

If two protons (hydrogen nuclei) approach each other slowly, they will repel each other. But if they are moving rapidly, we can get them into the realm of the strong nuclear force before they have a chance to repel. Two protons bound together in a nucleus is helium. Now moving fast is the same as the concept of temperature, so fusion can only happen at high temperature. The threshold temperature for hydrogen fusion is about ten million degrees. High pressure is also necessary so that the particles have a chance of finding one another. Such extreme conditions are only found in the core of the Sun, so only the core can contribute in the energy production.

Part II

The net reaction that is believed to produce the Sun's energy is

4 H ® He + e⁺ + n + energy ,

where H is the proton (hydrogen nucleus), He is the helium nucleus, e⁺ is the positron, and n is the neutrino. Each step in the reaction sequence has been verified in the particle accelerators. Now the mass of four protons is very slightly more than the mass of one helium nucleus, and the other particles listed are insufficient to make up the difference. The missing mass appears as the energy by the famous equation

E = mc²

Here E is energy, m is mass, and c is the speed of light. A little mass can produce a lot of energy. The process of hydrogen fusion can power a star like the Sun for approximately 10 billion years.

Now that we have found an energy source that will work, the next question to ask is does this actually occur in the Sun. Is there something in the reaction can actually be observed coming from the Sun? The energy comes out in the form of photons, but photons love to interact with matter. It is estimated that the photons being produced in the core of the Sun now will be emitted on the surface of the Sun in 10 million years. The positron will quickly annihilate itself with a nearby electron. The neutrino, however, is a very illusive particle. Originally, this particle was "invented" to solve some problems encountered in balancing the nuclear reactions. It was given the properties of zero mass and an almost absolute inability to interact with matter. After the neutrino was discovered (some 22 years after its prediction), it had precisely these properties. It is the illusive nature that makes it a prime candidate to test the hydrogen fusion sequence believed to power the Sun. Once produced in the Sun's core, it streams out of the Sun, interacting with nothing. Eight minutes later, traveling at the speed of light, it arrives at Earth. If we could somehow capture the neutrino, we could study the nuclear reactions taking place right now.

By 1968 a solar neutrino telescope began operations in Lead, SD. It is 100,000 gallons of cleaning fluid buried one mile underground in an abandoned gold mine. It takes advantage of the possible reaction

³⁷Cl + n ® ³⁷Ar ,

that is, the neutrino might change a radioactive chlorine atom into a radioactive argon. Cleaning fluid is mostly chlorine. The experiment is to periodically check the tank for argon atoms, which would signal that a neutrino had passed through the tank. The results to date show that only about one-third to one-half of the expected neutrinos are being captured. This deficiency has been called the Solar Neutrino Problem. Many attempts have been made to resolve the problem, but none have been completely successful.

Most physicists and astronomers are very happy today that the energy source of the Sun has been found in hydrogen fusion, although some problems do still exist.

Once the energy is produced in the core, it must get out to the surface. The energy is transferred out to the surface in two distinct ways in the Sun. Just above the core the temperature and pressures are such that the energy is transported radiatively. This region is called the radiative zone. Atoms will absorb the photons and remit them. The photons work out toward the surface in a random walk. As we move outward we reach a place where the rather slow, radiative transfer is not efficient enough and the layers begin to boil. In the convective zone energy gets transported by physically raising hot blobs of material toward the surface, carrying the energy along with them.

The radiative and convective zones of the Sun contribute in another very important way. The mass that they contain presses on the core, keeping the temperature and pressure high enough in the core to sustain the nuclear reactions. Without the overburden of the weight of these zones, the Sun could not shine.

Part III

The surface of the Sun has structure also. The visible surface (the section we see in visible light) is called the photosphere. A few simple observations will allow us to make some far reaching conclusions about the structure of the photosphere. The first of these observations is that the limb of the Sun is sharp. In a very small angle (about one arcsecond), corresponding to a very short distance in the photosphere (about 700 km), the gases of the photosphere go from opaque to transparent. This can happen if the density of these gases falls dramatically with height in the photosphere. Because of the sharp limb of the Sun and the conclusion that the density varies rapidly, we generally put the limit of how far we can see through the photospheric gases at 700 km.

The second simple observation is that the Sun shows limb darkening, the edges of the photosphere appear darker than the center of the disk. Lines of sight toward the limb do not permit us to see into the photosphere as deeply as the line of sight toward the center of the disk. It looks darker at the limb because the temperature of the region we see to is less. Temperature must decrease rapidly with height in the photosphere. In the short 700 km of the photosphere, the temperature drops almost 2000 K.

Under high magnification we see that the surface of the photosphere is composed of small cells called granulation. These cells represent a convection pattern. Hot gases rise up in the centers of the cells. The gas cools and sinks back below the surface at the edges of the cells. Recall that directly below the photosphere lies the convective zone. We may be seeing the topmost layer of convective cells.

Above the photosphere is the chromosphere (colored sphere) of the Sun. The color here is the characteristic color of the H_a line of hydrogen at 6563 Å, which is the strongest emission line in its spectrum. The chromosphere can only be observed by blocking the light of the photosphere (total solar eclipses) or through a specialized spectrograph called a spectroheliograph. The spectrograph looks only at wavelengths very close to the deepest absorption lines in the spectrum of the Sun. At these wavelengths it is as if the fog has gotten thicker, absorbing light not only in the usual way, but also extra light due to the element causing the deep absorption. We cannot penetrate as deeply into the gas layers of the Sun. In fact, we cannot penetrate beyond the chromosphere. The chromosphere is about 7000 km thick and has its own granulation pattern called supergranulation.

The chromosphere has jets of gas rising quickly upward (spicules). One of the most perplexing aspects of the chromosphere is the fact that the temperature rises as we move up in this layer. Starting at the 4150 K of the uppermost part of the photosphere, the temperature rises to over 50,000 K at the top of the chromosphere.

The upper atmosphere of the Sun is called the corona (crown). This region has no distinct end, extending farther from the Sun as we increase the exposure of the camera taking the picture. The shape of the corona depends on the level of activity on the photosphere. When there are few sunspots on the Sun, the corona is not as extensive and is quite symmetrical. During times of sunspot maximum, the corona is more extensive and looks rather like the result of sprinkling iron filings on a bar magnet. The gasses of the corona must be controlled by the magnetic field of the Sun.

The corona is an X-ray source, which must mean that the temperature is very high (millions of kelvins). The X-ray emission is not uniform. There are X-ray-bright regions and coronal holes. The solar wind comes from the holes. The solar wind is simply the outer layers of the corona boiling away into space.

Another line of reasoning leads to the conclusion that the corona is very hot. Spectral lines coming from the corona are due to FeX, FeXIV, and FeXV. The only way of ionizing the iron atoms this many times is to have a very high temperature (about 2 million K).