158 PART 2^ |^ THE STARS
hubcap. Rising and sinking gas currents twist the fi eld into rope-
like tubes, which tend to fl oat upward. Th e model predicts that
sunspot pairs occur where these magnetic tubes burst through
the sun’s surface (■ Figure 8-14).
Sunspots tend to occur in groups or pairs, and the magnetic
fi eld around the pair resembles that around a bar magnet with
one end magnetic north and the other end magnetic south, just
as you would expect if a magnetic tube emerged through one
sunspot in a pair and reentered through the other. At any one
time, sunspot pairs south of the sun’s equator have reversed
polarity compared with those north of the sun’s equator.
■ Figure 8-15 illustrates this by showing sunspot pairs south of
the sun’s equator moving with magnetic south poles leading, and
sunspots north of the sun’s equator moving with magnetic north
poles leading. At the end of an 11-year sunspot cycle, the new
spots appear with reversed magnetic polarity.
Th e Babcock model accounts for the reversal of the sun’s
magnetic fi eld from cycle to cycle. As the magnetic fi eld becomes
tangled, adjacent regions of the sun are dominated by magnetic
fi elds that point in diff erent directions. After about 11 years of
tangling, the fi eld becomes so complex that adjacent regions of
the sun begin changing their magnetic fi eld to agree with neigh-
boring regions. Th e entire fi eld quickly rearranges itself into a
simpler pattern, and diff erential rotation begins winding it up to
start a new cycle. But the newly organized fi eld is reversed, and
the next sunspot cycle begins with magnetic north replaced by
magnetic south. Consequently, the complete magnetic cycle is 22
years long, and the sunspot cycle is 11 years long.
Th is magnetic cycle also explains the Maunder butterfl y
diagram. As a sunspot cycle begins, the twisted tubes of magnetic
force fi rst begin to fl oat upward and produce sunspot pairs atThe Sun’s Magnetic Cycle
Th e sun’s magnetic fi eld is powered by the energy fl owing out-
ward through the moving currents of gas. Th e gas is highly ion-
ized, so it is a very good conductor of electricity. When an
electrical conductor rotates and is stirred by convection, it can
convert some of the energy fl owing outward as convection into a
magnetic fi eld. Th is process is called the dynamo eff ect, and it is
understood also to operate in Earth’s core and produce Earth’s
magnetic fi eld. Helioseismologists have found evidence that the
dynamo eff ect generates the sun’s magnetic fi eld at the bottom of
the convection zone deep under the photosphere.
Th e sun’s magnetic fi eld cannot be as stable as Earth’s. Th e
sun does not rotate as a rigid body. It is a gas from its outermost
layers down to its center, so some parts of the sun can rotate
faster than other parts. Th e equatorial region of the photosphere
rotates faster than do regions at higher latitudes (■ Figure 8-13a).
At the equator, the photosphere rotates once every 25 days, but
at latitude 45° one rotation takes 27.8 days. Helioseismology can
map the rotation throughout the interior (Figure 8-13b) and has
found that diff erent levels in the sun rotate with diff erent peri-
ods. Th is phenomenon is called diff erential rotation, and it is
clearly linked with the magnetic cycle.
Although the magnetic cycle is not fully understood, the
Babcock model (named for its inventor) explains the magnetic
cycle as a progressive tangling of the solar magnetic fi eld. Because
the electrons in an ionized gas are free to move, the gas is a very
good conductor of electricity, so any magnetic fi eld in the gas is
“frozen” into it. If the gas moves, the magnetic fi eld must move
with it. Diff erential rotation drags the magnetic fi eld along and
wraps it around the sun like a long string caught on a turning
EquatorN PoleabS Pole■ Figure 8-13
(a) In general, the photosphere of the
sun rotates faster at the equator than
at higher latitudes. If you started fi ve
sunspots in a row, they would not stay
lined up as the sun rotates. (b) Detailed
analysis of the sun’s rotation from helio-
seismology reveals regions of slow rota-
tion (blue) and rapid rotation (red).
Such studies show that the interior of
the sun rotates differentially and that
currents similar to the trade winds in
Earth’s atmosphere fl ow through the sun.
(NASA/SOI)