12.4 Solar Magnetic Activity
The surface waves described above are but one of many kinds of disturbances in the Sun's outer regions.
A wide class of spectacular phenomena on the Sun are caused by the interaction of its magnetic field
with the hot gas that makes up the Sun. This magnetic activity is also of interest because it affects the
Earth, where it triggers auroral displays and affects our climate.
Solar Magnetic Fields
The Sun has an overall magnetic field similar to the Earth's, with a magnetic north pole and south pole.
Like the Earth, the Sun's magnetic field is thought to arise from the rotation of electrically conductive
material deep in its interior, although this is hot ionized gas in the Sun instead of the molten iron in the
Earth. At the surface the average magnetic field strength on the Sun is only about twice as strong as on
the Earth's surface, but the effects are far more dramatic. To understand why, we need to consider how
magnetic fields interact with electrically charged particles.
The electromagnetic force, as its name implies, is a force not just between electrically charged particles,
but with magnetic fields as well. For example, when electrons and other charged particles encounter a
magnetic field, they spiral around the field lines, as illustrated in figure 12.15. The stronger the magnetic
field is, the tighter electrically charged particles spiral around the lines, although they can move freely
parallel to the field lines. The particles are essentially “frozen” to travel along a magnetic field line when
the field is strong enough. On the Earth, the magnetic field deflects energetic charged particles in the
outskirts of our atmosphere along the field lines toward the magnetic poles, where they create auroras.
Charged particles in the Sun experience similar deflections, but the Sun is entirely made of charged
particles, so the interactions shape motions throughout the Sun.
Figure 12.15
Charged particles are tied to magnetic field lines—spiraling around a line as they move along it. If forces
move the gas containing the particles, it can drag the field with it, and vice versa.
By Newton's law of action and reaction, charged particles in the Sun not only feel a force from the
magnetic field, they exert a force that “freezes” the magnetic field in the gas. Thus, if external forces
push the gas to the side (fig. 12.15), the magnetic field is dragged along with the gas. This interaction
between magnetic fields and the gas causes a variety of “storms” on the Sun that are unlike anything we
see on the Earth.
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Sunspots, Prominences, and Flares
One of the most readily seen effects of the Sun's magnetic field is a sunspot. This is a large, dark­
appearing region (fig. 12.16) that may have a size of a few hundred up to many thousands of kilometers
across. Sunspots last from a few days to over a month. They are darker than the surrounding gas because
they are cooler—about 4500 K as opposed to 6000 K of the normal photosphere. Spattering a few drops
of water onto a hot electric stove burner will show a similar effect. Each drop momentarily cools the
burner, making a dark spot. If they were seen in isolation, sunspots would look bright; they look dark
only in contrast to the brighter surrounding regions.
Figure 12.16
Visible­light photograph of a large group of sunspots. The darker areas are cooler gas, but they are still
bright—they appear dark in the image just in contrast to the surrounding hotter regions of the
photosphere.
What makes sunspots grow cooler? One clue is that they are regions where the Sun's magnetic field is
concentrated to more than a thousand times its normal strength. (See Extending Our Reach: “Detecting
Magnetic Fields: The Zeeman Effect” for how astronomers can measure the strength of a sunspot's
magnetic field.) The strong magnetic field blocks the normal convection beneath the Sun's surface,
preventing hot gas from rising in the region of the sunspot, as illustrated in figure 12.17. As a result, the
surface cools and darkens.
Figure 12.17
Diagram of the connection between magnetic fields and sunspots. In the region of a sunspot the magnetic
field is strong and emerges through the photosphere. The ionized gas moves along the field lines, which
block the upward flow of gas carrying heat to the surface.
Left to themselves, magnetic field lines push away from each other. This is like the repulsion you have
felt between a pair of magnets oriented in the same direction. So the final piece of the puzzle is
understanding how magnetic fields grow so concentrated in sunspots. The tight connection between the
magnetic fields and the gas provides the answer. A solar storm is a little like a low­pressure storm system
on Earth. It draws surrounding gas in toward it, and because the magnetic field is frozen into the gas, the
magnetic field is dragged inward and grows more concentrated. This in turn cools the region, intensifying
the storm.
Prominences are another manifestation of the intense magnetic disturbances in the low­density, virtually
transparent hot gases above the Sun's visible surface. Prominences are huge plumes of glowing gas that
jut from the lower chromosphere into the corona. Figure 12.18 shows an example. You can get some
sense of their immensity from the pale blue dot in the image, which shows the size of the Earth.
Prominences form where the Sun's magnetic field reduces heat flow to a region. They are cooler than the
gas around them, which means, according to the perfect gas law, that the pressure inside is less than the
pressure outside. Thus, the hot external gas “bottles up” the cooler gas of the prominence. Under
favorable conditions, this cooling gas, trapped in its magnetic prison, may glow for weeks.
Figure 12.18
Image of a solar prominence made from the SOHO satellite.
Q. Approximately how high does the prominence reach above the Sun's surface? (Use the size of the
Earth or Sun for comparison.)
answer
ANIMATION
Prominences on the Sun's limb
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Time­lapse movies show gas streaming through prominences, sometimes rising into the corona,
sometimes raining down onto the photosphere. The flow is channeled by and supported by the magnetic
field, which often arcs between sunspots (fig. 12.17). Thus, prominences also are related to sunspots.
Prominences on the Suns Limb
Sunspots also give birth to solar flares, brief but bright eruptions of hot gas in the chromosphere (fig.
12.20A). Over a few minutes or hours, gas near a sunspot may dramatically brighten. Such eruptions,
though violent, are so localized they hardly affect the total light output of the Sun at all. Generally, you
need a telescope to see their visible light, though they can increase the Sun's radio and X­ray emission by
factors of a thousand in a few seconds.
Figure 12.20
(A) A solar flare—the bright spot on the right side of the image. This image, made in high­energy
ultraviolet light with SOHO in 2003, is one of the strongest ever recorded. (B) SOHO visible­light image
of a coronal mass ejection event with an image of the Sun from about the same time superimposed over
the “occulting disk” used to block the light of the photosphere.
Extending our reach
DETECTING MAGNETIC FIELDS: THE ZEEMAN EFFECT
Astronomers can detect magnetic fields in sunspots and other astronomical bodies by the Zeeman effect,
a physical process in which the magnetic field splits some of the spectrum lines of the gas into two, three,
or more components. The splitting occurs because the magnetic field alters the atom's electron orbits,
which in turn alter the wavelength of its emitted light.
Figure 12.19A shows the Zeeman effect splitting spectrum lines in a sunspot. The line is single outside
the spot but triple within, and by mapping the splitting across the Sun's face, astronomers can map the
Sun's magnetic field, creating a magnetogram, as seen in figure 12.19B. The colors in a magnetogram
show the strength and polarity of the magnetic field (recall that magnetic fields have a north or south
polarity indicating their direction). Notice that the field is strong around spots and weak elsewhere.
Figure 12.19
(A) Spectrum of the Zeeman effect in a sunspot. Notice that the line is split over the spot where the
magnetic field is strong but that the line is unsplit outside the spot where the field is weak or absent. (B)
Magnetogram of the Sun. Yellow indicates regions with north polarity, and blue indicates regions with
south polarity. Notice that the polarity pattern of spot pairs is reversed between the top and bottom
hemispheres of the Sun. That is, in the upper hemisphere, blue tends to be on the left and yellow on the
right, and the opposite In the lower hemisphere.
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Flares are poorly understood, but magnetic fields appear to play an important role. One theory suggests
that the field near a spot gets twisted by gas motions, a bit like winding up a rubber­band­powered toy.
But such twisting can go only so far before the rubber band breaks. So, too, the magnetic field can be
twisted only so far before it suddenly re­adjusts, whipping the gas in its vicinity into a new configuration.
The sudden motion heats the gas, and it expands explosively. Large solar flares are sometimes followed
by coronal mass ejections. These ejections are enormous bubbles of hot gas and trapped magnetic fields
that burst from the corona out into space as shown in Figure 12.20B. If directed toward the Earth, the gas
can reach the Earth in a few days, producing auroral events. Such a burst created the spectacular auroral
displays seen in March 1989, shown in figure 12.21.
Figure 12.21
Photograph of the great aurora of March 1989, which knocked out the power grid in parts of Canada.
Q. Aurora was the goddess of dawn in Roman mythology. Why might that name have been chosen for
this phenomenon?
answer
An aurora is a beautiful sight, but the stream of charged particles that causes them can have more serious
consequences. Communications satellites have been disabled by some major flares, and even electrical
power grids can be destabilized as the magnetic field carried by the stream of gas causes electric currents
to surge in electrical transmission wires. The 1989 solar outburst caused blackouts in several locations
across North America. An even more remarkable event occurred in 1859, when the aurora was seen even
in the tropics, and telegraph equipment was set afire by huge magnetic surges. Today, an array of
satellites monitors “space weather,” but these events can have serious consequences for Earth.
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Heating of the Chromosphere and Corona
Although the Sun's magnetic field cools sunspots and prominences, it heats the chromosphere and
corona. Other stars also have chromospheres and coronas, although we cannot detect their light
separately, so it is additionally important to understand how the Sun's atmosphere behaves to help us
interpret the light from other stars. To begin, we need first to recall that the temperature of a gas is a
measure of how fast its particles are moving. Anything that speeds atoms up increases their temperature.
An analogy may help you understand how magnetic waves can heat a gas. When you crack a whip, a
slow motion of its handle travels as a wave along the whip. As the whip tapers, the wave's energy of
motion is given to an ever smaller piece of material. With the same amount of energy and less mass to
move, the tip accelerates and eventually breaks the sound barrier. The whip's “crack” is a tiny sonic
boom.
A similar speedup occurs in the Sun's atmosphere when magnetic waves formed in the photosphere move
into the corona along the Sun's field lines (fig. 12.22). As the atmospheric gas thins, the wave energy is
imparted to an ever smaller number of atoms, making them move faster in random directions. But
“faster” in this case means hotter. Thus, the upper atmosphere heats up as the waves travel into it.
Figure 12.22
Diagram illustrating how magnetic waves (blue) heat the Sun's upper atmosphere. As the waves move
outward through the Sun's atmosphere, they grow larger, imparting ever more energy to the gas (red dots)
through which they move, accelerating and thereby heating it.
How are the magnetic waves generated? They probably start in the convection zone where rising bubbles
of gas shake the magnetic field and create magnetic waves just as shaking the loose end of a rope makes
it wiggle. Thus, the high temperature of the chromosphere and corona is another example of the
importance of the Sun's magnetic field and its convection zone. This theory is supported by observations
of other stars with active convection zones—they also have active chromospheres and coronas.
The Solar Wind
In addition to the mass it loses in the outbursts of flares, the Sun undergoes a steady, less dramatic loss of
mass. The corona's high temperature gives its atoms enough energy to escape the Sun's gravity. As these
atoms stream into space, they form the solar wind, a tenuous flow of mainly hydrogen and helium that
sweeps across the Solar System. The amount of material lost from the Sun is small: less than 1 ten­
trillionth of its mass each year. Nevertheless, the solar wind sweeps the gas boiling off the surface of a
comet into a tail that may extend tens of millions of miles from the nucleus, as we saw in chapter 11.
The solar wind arises because, unlike the rest of the Sun, the corona is not in hydrostatic equilibrium.
Recall that the temperature in the Sun's atmosphere increases with altitude, making the corona much
hotter than the photosphere. The corona's high temperature, according to the perfect gas law, creates a
pressure within it larger than we might otherwise expect for its distance above the photosphere. The
pressure is in fact sufficient to overcome the Sun's gravitational force on gas in its upper atmosphere. As
a result, it pushes that material outward into space. The expanding gas has a very low density, only a few
hundred atoms in a cubic centimeter—the volume of a thimble. For comparison, a thimbleful of the air
we breathe contains about 1019 molecules!
The gas atoms begin their outward motion slowly but accelerate with increasing distance as the Sun's
gravitational attraction on them weakens. On average, the wind speed is about 500 kilometers per second
at the Earth's orbit, but it speeds up and slows down in response to changes in the Sun's magnetic field.
From the Earth outward, the wind coasts at a relatively steady speed that carries it at least to the orbit of
Neptune. At some point, it impinges on the interstellar gas surrounding the Solar System. The two
Voyager spacecraft launched in 1977 are now 3 to 4 times farther from the Sun than Neptune, where they
are probing the solar wind and magnetic field far from the Sun. They have entered a turbulent region that
may mark the boundary between the solar wind and interstellar gas.