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Why Do Sunspots Appear Dark

by Lyndon Langley
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Why Do Sunspots Appear Dark

Why Do Sunspots Appear Dark

The sun is a huge ball of plasma, made mostly of hydrogen and helium nuclei with some heavier stuff thrown in for good measure. The core of this ball is hot enough to ignite nuclear fusion reactions that create an intense heat source. But even though the core is extremely hot — millions of degrees Kelvin — it isn’t actually all that dense. It has a density only slightly higher than what you’d find at sea level on Earth. That means there must be lots of space between the particles. And as those particles move around, they bump into each other. This leads to friction, which generates energy. In fact, about 90 percent of the total mass of the sun is held together by gravity, so the same basic process goes on down below its surface. Hot plasma gets stuck here, forming a layer called the convection zone.
On top of this zone is another layer called the radiative zone. Here, the temperature drops significantly, but not nearly enough to stop the generation of free electrons through collisions. As these electrons accumulate, they start generating their own electromagnetic fields, and eventually become ionized. These ions then collide with neutral atoms, causing them to lose or gain electrons. They also generate new photons through various processes, including spontaneous emission and stimulated emission. When this happens, part of the kinetic energy (energy due to motion) of the emitted photon turns into light waves. Photons generated within the radiative zone travel outward toward the solar surface. Most of them never reach the surface, however; instead, they get absorbed by molecules, particularly oxygen and silicon dioxide, leading to chemical changes. Some of these molecules release their stored energy as infrared radiation.
Now we’re getting close to where our story begins. On the surface of the sun, where sunlight reaches us, things are pretty different. The sunlight hitting the sun’s surface consists of high-energy photons. Each one carries more energy than any previous ones. Because of this, the surface of the sun can be described as being hotter than the interior parts. Even though the surface is several million kelvin, it still doesn’t have sufficient energy to excite electrons directly. Instead, the photons striking the surface cause the lower levels of the sun’s atomic structure to rise up and fill in the missing energy states. Once the excited electron falls back down to its original state, it gives off some of its excess energy as ultraviolet light.
This UV light gradually heats the atmosphere above the convection zone. When the upper layers of the sun are sufficiently heated, they begin to expand, pushing aside gas clouds near the poles. These clouds are composed primarily of iron-nickel alloy particles suspended in a plasma medium. The magnetic pressure from these expanding clouds causes small loops in the magnetic field lines to reverse direction, creating areas of weak magnetic flux and thus places where the ambient polarity will change. Loops of strong magnetic field will resist such reversals.
In addition to the magnetic effects, there are two other factors affecting the development of sunspots. First, as mentioned earlier, UV light increases the temperature of the upper sunlit layer. Second, when the temperature rises, the material becomes less dense, allowing convective motions to occur. Convection brings cool plasma upward out of the sun’s outer layers. If the plasma rising were allowed to keep going forever without collision, it would simply turn into stars. To prevent this, the rising plasma collides with itself and with the plasma already present in the upper layers. As a result of these interactions, the plasma becomes denser and darker.
Because the matter is becoming denser, it takes longer for UV light to penetrate to the deeper layers. Eventually, however, the sun’s opacity decreases sufficiently to allow this light to pass downward. At the right depth, the light can trigger further physical events similar to those that occurred at the surface. The most important difference is that now the light stimulates the formation of negatively charged particles, rather than exciting the electrons directly. Negatively charged particles, known as chromospheric evaporation fronts, carry positive charges to the next step in the sequence: the corona.
As the front moves upward, it generates electric currents. These currents are carried away through the coronal loop structures. The resulting electrical potentials pull additional negative particles from the chromosphere into the corona. This process creates a region of low density and high temperatures known as a flare. Flares may last anywhere from minutes to hours. During this time, the flare releases tremendous amounts of radiant energy in X-ray form. A typical flare contains ten billion times the amount of power released by the largest atom bomb explosion ever detonated.
After the flare subsides, it leaves behind an area of reversed magnetic field. Within this reversed region, the magnetic field is weaker than normal. If a current flows along the magnetic field lines, it will produce a magnetoionic environment conducive to sunspot formation. New sunspots develop within existing sunspots, or sometimes completely new ones do.
New sunspots may grow rapidly if they happen to lie over preexisting spots of opposite polarity. Such pairs of spots are known as pores, and they are usually located near the equator. Pores are very common, appearing in roughly half of all sunspots.
If a sunspot grows large enough, it will block the bright disk of the sun, leaving a dark spot. This type of sunspot is known as a sunspot group. There are three types of sunspot groups: bipolar, complex, and simple. Bipolar sunspot groups contain both sunspots of the same sign and sunspots of opposite signs. Complex sunspot groups contain sunspots of mixed polarity, while simple sunspot groups consist entirely of sunspots of either positive or negative polarity.
One of the most interesting aspects of sunspots is that they often show up in clusters. Typically, sunspots tend to cluster together in groups of five or six. Often they’ll migrate across the entire face of the sun, taking months or years to complete the journey. Groups of sunspots tend to align themselves with the orientation of the underlying magnetic field. For example, many sunspot groups follow a pattern of alternating hemispheres, meaning they alternate between sunspots of one sign and those of the opposite sign. Other sunspot patterns include chains, rows, zigzags, and chevrons. Occasionally, sunspots will merge, split apart, or disappear altogether. While sunspots rarely persist long after they’ve been formed, they can live for weeks or even years.
For a detailed look at how sunspots form and evolve, check out How Sunspots Work.

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