Chapter 15--Our Sun - Geological Sciences

a This close-up view of the Sun’s surface
(right) shows two large sunspots and several
smaller ones. Both of the big sunspots are
roughly as large as Earth.
Figure 15.15 Sunspots are regions of intense
magnetic activity.
VIS
tight magnetic field lines suppress convection within each
sunspot and prevent surrounding plasma from sliding sideways
into the sunspot. With hot plasma unable to enter the
region, the sunspot plasma becomes cooler than that of the
rest of the photosphere (Figure 15.17a).
The magnetic field lines connecting two sunspots often
soar high above the photosphere, through the chromosphere,
and into the corona (Figure 15.17b). These vaulted loops
of magnetic field sometimes appear as solar prominences,
in which the field traps gas that may glow for days or even
weeks. Some prominences rise to heights of more than
100,000 kilometers above the Sun’s surface (Figure 15.18).
The most dramatic events on the solar surface are
solar flares,which emit bursts of X rays and fast-moving
charged particles into space (Figure 15.19). Flares generally
occur in the vicinity of sunspots, leading us to suspect that
they occur when the magnetic field lines become so twisted
and knotted that they can no longer bear the tension. The
magnetic field lines suddenly snap like tangled elastic bands
twisted beyond their limits, releasing a huge amount of
energy. This energy heats the nearby plasma to 100 million
K over the next few minutes to few hours, generating X rays
and accelerating some of the charged particles to nearly the
speed of light.
The Chromosphere and Corona
The high temperatures of the chromosphere and corona
perplexed scientists for decades. After all, temperatures
gradually decline as we move outward from the core to the
VIS
b Spectra of sunspots can be
used to measure the strength
of their magnetic fields. This
image shows the spectrum
of a sunspot and its surroundings.
The sunspot region
shows up as dark horizontal
bands because it is darker
than the rest of the solar surface
in its vicinity. The vertical
bands are absorption lines
that are present both inside
and outside the sunspots.
The influence of strong magnetic
fields within the sunspot
region splits a single absorption
line into three parts.
Measuring the separation
between these lines tells us
the strength of the magnetic
field within the sunspot.
top of the photosphere. Why should this decline suddenly
reverse? Some aspects of this atmospheric heating remain
a mystery today, but we have at least a general explanation:
The Sun’s strong magnetic fields carry energy upward from
the churning solar surface to the chromosphere and corona.
More specifically, the rising and falling of gas in the
convection zone probably shakes magnetic field lines beneath
the solar surface. This shaking generates waves along the
magnetic field lines that carry energy upward to the solar
510 part V • Stellar Alchemy