Why Do Sunspots Occur?

Why Do Sunspots Occur?
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  • 0:01 The Zeeman Effect
  • 1:56 Plasma and the Magnetic Fields
  • 2:57 The Solar Cycle
  • 4:05 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will outline for you why sunspots occur in its photosphere as we go over the Zeeman effect, polarity, and how the sunspot cycle and solar cycle are linked.

The Zeeman Effect

Sun spots on your skin are the result of the addition of something to your skin, the addition of a pigment called melanin. Actually, you already have it, it's just that there's more of it clumped together in places where the sun spots like to develop, like the back of a person's hand. Sunspots on the sun, on the other hand, are the result of the lack of something, the inhibition of the process of convection, which normally transfers heat to the sun's surface.

For our lesson, a sunspot is a relatively dark, irregularly shaped region of the photosphere with a lower temperature than the other parts of the photosphere, which is the part of the solar atmosphere that's the bright surface of the sun.

Astronomers can measure the magnetic field on the sun thanks to the Zeeman effect, the splitting of a spectral line emitted by an atom into two or more separate lines, due to the presence of a magnetic field. A spectral line is simply a clue, like a fingerprint, to a gas's chemical makeup. The larger the magnetic field, the wider the separation of the split lines.

By examining the spectra of different areas of the photosphere, astronomers can find out the strength of the magnetic field in a specific area as well as that region's polarity, which in this lesson's case is the orientation of the magnetic field (north or south), just like a magnet. Such examinations have revealed that sunspot groups have very strong magnetic fields with areas of both north and south polarity. These large magnetic fields inhibit the warming process of convection right below the sunspots.

Picture a pot of boiling water. The bubbles are the regions of hot water rising towards the cooler surface. At the same time cooler water sinks to the bottom to be heated so it can rise to the top as hot water bubbles later on. That's convection for you.

Plasma and the Magnetic Fields

The fluid that makes up the solar atmosphere and undergoes convection isn't water, however -- it's gas. Technically, the solar atmosphere contains ionized atoms and thus, it's a special kind of gas-like substance called plasma, ionized gas, made of freely moving, electrically charged ions and electrons.

Such electrically charged objects are easily deflected by the sun's magnetic fields, like two magnets with the same poles facing one another push each other away.

Recall that the magnetic fields are very strong where the sunspots are located. Thus, when hot plasma rises from the sun's interior to the surface, thanks to convection, it's just pushed away! This causes such areas to become cooler and dimmer than the rest of the photosphere which, by definition, is exactly what sunspots are.

The heat that's deflected emerges around the sunspot, and this is detectable with ultraviolet and infrared imagery.

The Solar Cycle

The sunspot cycle, a regular increase and decrease in the number of spots on the sun, is an 11-year cycle where there's an increase in the average number of sunspots to a sunspot maximum and then a decrease to a sunspot minimum before rising to a maximum again. This cycle is related to the magnetic polarities of sunspot groups.

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