The Solar System

CHAPTER 8 | THE SUN 159

magnetic force arch up through the surface closer to the equator.

As a result, the later sunspot pairs in a cycle appear closer to the

equator.

Notice the power of a scientifi c model. Th e Babcock model

may in fact be incorrect in some details, but it provides a frame-

work around which to organize descriptions of complex solar

activity. Even though the model of the sky in Chapter 2 and the

model of the atom in Chapter 7 are only partially correct, they

served as organizing themes to guide your thinking. Similarly,

although the precise details of the solar magnetic cycle are not

yet understood, the Babcock model gives you a general picture

of the behavior of the sun’s magnetic fi eld (How Do We

Know? 8-2).

Spots and Magnetic Cycles

of Other Stars

Th e sun seems to be a representative star, so you should expect

other stars to have similar cycles of starspots and magnetic fi elds.

Th is is a diffi cult topic, because, except for the sun, the stars are

so far away that no surface detail is visible. Some stars, however,

vary in brightness in ways that suggest they are mottled by dark

spots. As these stars rotate, their total brightnesses change

slightly, depending on the number of spots on the side facing

Earth. High-precision spectroscopic analysis has even allowed

astronomers to map the locations of spots on the surfaces of

Magnetic

field line

Bipolar

sunspot

pair

The Solar Magnetic Cycle

Sun

For simplicity, a

single line of the

solar magnetic

field is shown.

Differential rotation

drags the equatorial

part of the magnetic

field ahead.

As the sun rotates, the

magnetic field is

eventually dragged all

the way around.

Differential rotation

wraps the sun in many

turns of its magnetic

field.

Where loops of tangled

magnetic field rise

through the surface,

sunspots occur.

■ Figure 8-14

The Babcock model of the solar magnetic cycle explains the sunspot cycle
as a consequence of the sun’s differential rotation gradually winding up the
magnetic fi eld near the base of the sun’s outer, convective layer.

higher latitudes on the sun. Consequently the fi rst sunspots in a
cycle appear farther north and south of the equator. Later in the
cycle, when the fi eld is more tightly wound, the tubes of

SN

S

S

N

N

N S

Rotation

Leading spot is

magnetic north.

Leading spot is

magnetic south.

■ Figure 8-15

In sunspot groups, here simplifi ed into pairs of major spots, the leading

spot and the trailing spot have opposite magnetic polarity. Spot pairs in

the southern hemisphere have reversed polarity from those in the northern

hemisphere.