Chapter 15--Our Sun - Geological Sciences

e quator
N N N
magnetic
field line
differing
rotation
rates
time
time
S S S
Figure 15.22 The Sun rotates more quickly at its equator than it does near its poles. Because gas circles
the Sun faster at the equator, it drags the Sun’s north-south magnetic field lines into a more twisted configuration.
The magnetic field lines linking pairs of sunspots, depicted here as green and black blobs, trace out
the directions of these stretched and distorted field lines.
The magnetic field associated with the solar wind constantly
interacts with Earth’s magnetic field. Occasionally
these fields interconnect. When that happens, large amounts
of energy are released from the magnetic field into the
charged particles near the interconnection zone. Many
of these energized particles then flow down Earth’s magnetic
field lines toward the poles (Figure 15.23a). Collisions
between the charged particles and atoms in Earth’s upper
atmosphere cause electrons in the atoms to jump to higher
energy levels [Section 4.4].These excited atoms subsequently
emit visible-light photons as they drop to lower energy levels,
creating the shimmering light of auroras (Figure 15.23b).
Because coronal mass ejections are particularly energetic, the
auroras they stimulate can be especially spectacular.
Particles streaming from the Sun after the occurrence
of solar flares, coronal mass ejections, or other major solar
storms can also have practical impacts on society. For example,
these particles can hamper radio communications,
disrupt electrical power delivery, and damage the electronic
components in orbiting satellites. During a particularly
powerful magnetic storm on the Sun in March 1989, the
U.S. Air Force temporarily lost track of over 2,000 satellites,
SPECIAL TOPIC
Long-Term Change in Solar Activity
Figure 15.21 shows that the sunspot cycle varies in length and
intensity, and it sometimes seems to disappear altogether. With
these facts as background, many scientists are searching for longerterm
patterns in solar activity. Unfortunately, the search for longerterm
variations is difficult because telescopic observations of
sunspots cover a period of only about 400 years. Some naked-eye
observations of sunspots recorded by Chinese astronomers go
back almost 2,000 years, but these records are sparse, and nakedeye
observations may not be very reliable. We can also guess at
past solar activity from descriptions of solar eclipses recorded
around the world: When the Sun is more active, the corona tends
to have longer and brighter “streamers” visible to the naked eye.
Another way to gauge past solar activity is to study the amount
of carbon-14 in tree rings. High-energy cosmic rays [Section 19.2]
coming from beyond our own solar system produce radioactive
carbon-14 in Earth’s atmosphere. During periods of high solar
activity, the solar wind tends to grow stronger, shielding Earth
from some of these cosmic rays. Thus, production of carbon-14
drops when the Sun is more active. All the while, trees steadily
breathe in atmospheric carbon, in the form of carbon dioxide,
and incorporate it year by year into each ring. We can therefore
estimate the level of solar activity in any given year by measuring
the level of carbon-14 in the corresponding ring. No clear evidence
has yet been found of longer-term cycles of solar activity,
but the search goes on.
Theoretical models predict a very long term trend of lessening
solar activity. According to our theory of solar system formation,
the Sun must have rotated much faster when it was young
[Section 9.3].Because a combination of convection and rotation
generates solar activity, a faster rotation rate should have meant
much more activity. Observations of other stars that are similar
to the Sun but rotate faster confirm that these stars are much
more active. We find evidence for many more “starspots” on these
stars than on the Sun, and their relatively bright ultraviolet and
X-ray emissions suggest that they have brighter chromospheres
and coronas—just as we would expect if they are more active
than the Sun.
chapter 15 • Our Star 515