The Solar System

CHAPTER 8 | THE SUN 157

Historical records show that there were very few sunspots

from about 1645 to 1715, a phenomenon known as the

Maunder minimum. This coincides with a period called the “little

ice age,” a period of unusually cool weather in Europe and North

America from about 1500 to about 1850, as shown in the graph at

left. Other such periods of cooler climate are known. The evidence

suggests that there is a link between solar activity and the amount

of solar energy Earth receives. This link has been confirmed by

measurements made by spacecraft above Earth’s atmosphere.

Far-UV image

Visual

M. Seeds

NASA/TRACE

J. Harvey/NSO and HAO/NCAR

AURA/NOAO/NSF

Simultaneous images

1650 1700 1800

0

50

100

150

200

250

300

350

1750 1850 1900 1950 2000

Year

Number of sunspots

4

3

3a Images of the sun above show that sunspots contain magnetic fields a few thousand times stronger than Earth’s. The strong fields are believed to inhibit gas

motion below the photosphere; consequently, convection is reduced below the sunspot,

and the surface there is cooler. Heat prevented from emerging through the sunspot is

deflected and emerges around the sunspot, which can be detected in ultraviolet and

infrared images.

Ultraviolet filtergram

Simultaneous images

Magnetic image

Astronomers can measure magnetic

fields on the sun using the Zeeman effect

as shown below. When an atom is in a magnetic

field, the electron orbits are altered, and the

atom is able to absorb a number of different

wavelength photons even though it was

originally limited to a single wavelength. In the

spectrum, you see single lines split into multiple

components, with the separation between the

components proportional to the strength of the

magnetic field.

Magnetic fields can reveal themselves

by their shape. For example, iron

filings sprinkled over a bar magnet

reveal an arched shape.

The complexity of an active

region becomes visible at

short wavelengths.

Visual-wavelength image

Visual

Slit allows light

from sunspot to

enter spectrograph.

Sunspot

groups

Magnetic fields around

sunspot groups

Spectral line split

by Zeeman effect

Winter severity in

London and Paris

Warm

Cold

Maunder minimum

few spots colder

winters Warmer

winters

SOHO/EIT, ESA and NASA

Far -UV imageFar -UV image

(^5) nonvisible wave- Observations at
lengths reveal that the
chromosphere and corona
above sunspots are
violently disturbed in
what astronomers
callactive regions.
Spectrographic
observations show
that active regions
contain powerful
magnetic fields.
Arched structures
above an active
region are
evidence of gas
trapped in magnetic
fields.