Goals
After Learning completing this lesson, you should be able to answer the following questions: What are sunspots? What observational facts do we know about sunspots? How do they form? How does this explain the observational facts? What do we mean when we say that the Sun rotates differentially? What is the Zeeman Effect? What are prominences and active regions? In what layer(s) of the Sun do we observe them? What are solar flares and coronal mass ejections? How do they occur? How are the various solar phenomena related?
Sunspots
Sunspots have been observed on the surface of the Sun ever since Galileo first observed them with his telescope. After observing the Sun over hundreds of years, astronomers have discovered the following information:
Sunspots appear darker than the surrounding regions of the Sun. The position and number of sunspots change over time. Strong magnetic fields are associated with sunspots.
Sunspots
Sunspots
On short time scales, we can explain the motion of sunspots by the rotation of the Sun. Since the Sun is not a solid, it doesn't need to rotate like one. The Sun rotates differentially. Different latitudes will rotate at different rates.
The equator rotates once every 25.4 days. Higher latitudes take longer to rotate (the rotation rate at a latitude of 45 is 27.8 days).
On longer scales, the number and positions of sunspots also change. Individual spots will last anywhere from 1 to 100 days. As we look at sunspots they may appear to be totally dark, but that's not true. While sunspots are dimmer than the surrounding photosphere, they are actually quite bright. It is simply that the much brighter photosphere outshines them, so the contrast makes them appear to be darker than the photosphere. The Stefan-Boltzmann law tells us that the intensity of light is related to its temperature. The fact that sunspots are dimmer than the photosphere indicates that they are cooler. Their average temperature is only around 4500 K (compared to 5800 K for the photosphere). In addition, the average latitude of sunspots changes over time. Sunspots tend to be found at higher latitudes when there are fewer of them, and at lower latitudes when there are more of them.
Sunspot Cycles
Sunspots appear to follow an 11-year cycle in the number of sunspots that are visible. When the number of sunspots is highest (at sunspot maximum), hundreds of sunspots are visible in one year. The most recent sunspot maximum was in 2001, and the next one will occur in 2012. In between (around 2006) we experienced a sunspot minimum during which very few sunspots were observed.
The Zeeman Effect
When we observe the spectrum of a sunspot, we notice that some of the lines in the spectrum are split into more than one line. Scientists have found that this "splitting" of certain spectral lines occurs when a gas is in the presence of a strong magnetic field. This is known as the Zeeman Effect. The Sun's magnetic field is a few thousand times stronger than Earth's. In the case of sunspots, these magnetic field lines prohibit the rising gas from delivering heat to the surface, so the result is that sunspots have cooler temperatures and appear dark.
Click on each Sun to observe the Sun during either a sunspot maximum or minimum.
Sunspot Maximum (2001)
Sunspot Minimum (2006)
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The Formation of Sunspots
Putting all of this together, we can form a model which explains how sunspots are formed.
Which of the following statements is not true about the sunspots?
A) They cluster at high latitudes when solar activity is at a minimum. B) They reach maximum numbers at roughly 11-year intervals. C) They typically measure about 10,000 km across. (Larger than the radius of the Earth!) D) They appear in larger numbers at about the time of the solar maximum. E) They distributed are approximately uniformly over the Sun's surface in position and in time.
Magnetic fields stay tied to charged particles, and can be deformed as the charges move. The Sun also experiences differential rotation, so the regions near the Sun's equator rotate faster than those near the poles. The differential rotation drags the magnetic field and causes it be stretched and wrapped around the Sun. After many rotations, the field gets tangled up. Sunspots develop wherever loops of the tangled magnetic field rise through the photosphere to the surface. Every 11 years, the twisting of the magnetic field lines reaches a maximum at which point they somehow straighten out, but with one difference. They are now reversed and pointing in the opposite direction.
Correct. When there are Correct. When there are few sunspots they tend to few sunspots they tend to be found at higher be found at higher latitudes, but as sunspots latitudes, but as sunspots become more abundant, become more abundant, they are found near lower they are found near lower latitudes. latitudes.
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Video courtesy of NASA
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Prominences
Other features have been observed in various layers of the Sun. Here is an image of the upper chromosphere (observed over a narrow wavelength range centered on an ionized helium emission line). Prominences (hot ionized gas trapped in magnetic field lines and suspended in the Sun's corona) are produced when the Sun's magnetic field lines emerge from the surface and extend upwards into the corona, dragging along with it ionized particles.
Here is a TRACE (Transition Region and Coronal Explorer) image of filaments of hot gas, with temperatures of millions of degrees Kelvin, rising upwards as they follow the magnetic field lines. Prominence A scale model of Earth has been included for comparison. See the movie, which shows this in action.
Active Regions
Other active regions (regions of disturbed activity) are found in the chromosphere. The brightest areas represent the hottest features.
Image courtesy of NASA
Video courtesy of NASA
Solar Flares
When the Sun's magnetic field becomes unstable, enormous amounts of energy can be released in minutes in the form of solar flares. A solar flare occurs when stored energy is released in an explosive event, sending a stream of photons and charged particles out into space. The stream of particles is called a coronal mass ejection or CME. View the movie for an animation of the formation of sunspots and subsequent release of energy in the form of a solar flare.
Coronal Mass Ejections
A Coronal Mass Ejection (CME) is a giant magnetic "bubble" of ionized gas that is ejected by the Sun when a flare occurs. It travels through interplanetary space and can take 3-5 days to move from the Sun to Earth.
A series of images taken by SOHO (Solar and Heliospheric Observatory) in 1998 demonstrate this feature, which you can see if you follow this link: http://sohowww.nascom.nasa.gov/gallery/Movies/C3may98/C3may98sm.mpg
The Sun (for size comparison), blocked out by a filter.
Video courtesy of NASA
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Here are four images of the Sun, taken simultaneously at different wavelengths and temperatures. Notice the coincidence of the locations of sunspots, active regions, and prominences. All of the features which we have discussed are related to the Sun's magnetic field activity, and so are related to each other as well.
Optical image of Sun
H image of chromosphere
Images courtesy of NASA
Magnetogram image of magnetic field strength
Extreme UV/X-ray image of corona
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