The Stellar Dynamo - Scientific American Digital

cise photometric observations of some
of the Wilson stars began at the Lowell
and Sacramento Peak observatories.
Since 1992 those of us at the Smithsonian
Astrophysical Observatory and at
Tennessee State University have used automated
telescopes to observe some of
these stars. Nearly all the older stars, like
the sun, are brightest near the peak of
the activity cycle. Some stars vary as little
as our sun does—only 0.1 percent
over the last 11-year cycle—but other
sunlike stars have varied by as much as
0.6 percent in a cycle. Thus, the sun’s
current changes might be a poor indicator
of the full range of fluctuations of
which it is capable.
Over the decades, researchers have
inferred the evolutionary history of a
sunlike star from the collection of stellar
records. A young star has a relatively
rapid rotation period of several days and
high, irregular levels of surface magnetism.
Changes in brightness of several
percent accompany the magnetic variations.
The young star is, however, darkest
during the peak of magnetic activity,
presumably because the dark spots are
so large that they, not the plages, dominate.
As the sunlike star ages, it rotates
more slowly, and the magnetic activity
decreases. Maunder minima appear in
these “older” stars; furthermore, radiance
now peaks at sunspot maximum,
with fluctuations of 1 percent or less
over a cycle.
Influencing Earth
THE STAR-SPOT RESULTS point to a
change in brightness of at least 0.4 percent
between the cyclic phase and the
Maunder minimum phase. This value
corresponds to a decrease in the sun’s
net energy input of one watt per square
meter at the top of Earth’s atmosphere.
Simulations performed at the Laboratory
of Dynamic Meteorology in Paris and
elsewhere suggest that such a reduction,
occurring over several decades, is capable
of cooling Earth’s average temperature
by 1 to 2 degrees C—enough to explain
the observed cooling during the
Maunder minimum.
But greenhouse gases generated by
human activity may be warming our
planet, by trapping heat that would otherwise
radiate into space. This warming
is equivalent to Earth’s receiving radiation
of two watts per square meter at the
surface. The sun has apparently delivered
to Earth no more or less than 0.5 to 1.0
watt per square meter over the past few
centuries. Therefore, if direct heating is
the only way in which the sun affects
Earth’s climate and is presumed to act
the same as the enhanced greenhouse effect,
the added greenhouse gases should
already be dominating the climate, washing
out any correlation with the sun’s
varying activity.
The sun’s energy reaches Earth as radiation
and particles and varies over
many frequencies and periods. Yet the
link between climate and solar magnetic
activity seems rather persistent. The
length of the sunspot cycle, for example,
correlates closely with global temperatures
over the past 240 years. Minima in
solar magnetism, as traced by radiocarbon
dating in tree rings and beryllium 10
in ice cores, coincide with roughly 1,500-
year intervals of cooler climate, seen
in environmental changes going back
10,000 years. In addition, the sunspot
cycle correlates with stratospheric wind
patterns, for reasons not yet well understood.
All these pieces of evidence induce
UNRAVELING THE INFLUENCES OF THE SUN
provides vital information on the role
OUR STAR PLAYS IN CLIMATE CHANGE.
the records show that the stars are in a
dead calm, suggesting a phase similar to
our sun’s Maunder minimum. This finding
implies that sunlike stars spend a
quarter of their lives in a lull—consistent
with radiocarbon results.
We may have captured one star, HD
3651, in transition between the cyclic
and Maunder minimum phases. HD
3651’s cycles have weakened and
lengthened dramatically (from 12 to 15
years) as its surface activity has rapidly
dropped to very low levels. Sunlike stars
such as HD 3651, observed over a few
decades, offer us “snapshots” of the
range of variability that our sun—and
we—might experience over a timescale
of centuries.
The brightness of these sunlike stars
can also be compared with their magnetic
activity. In 1984 thorough and pre-
MORE TO EXPLORE
some scientists, including us, to argue
that the sun must be influencing Earth
by powerful indirect routes as well as the
obvious ones.
Variations in the sun’s ultraviolet radiation,
for example, may be changing
the ozone content of our upper atmosphere,
as well as its dynamics. Recent
simulations also indicate that winds in
the lower stratosphere can convey variations
in solar radiance down to the troposphere,
where they interact more directly
with the weather system. Such
matters are now the subject of vigorous
debate. Unraveling the ways in which the
sun warms Earth provides vital information
concerning the role played by humankind—and
the role played by the
sun—in the process of climatic change.
The Variable Sun. Peter V. Foukal in Scientific American, Vol. 262, No. 2, pages 34–41;
February 1990.
The Paradox of the Sun’s Hot Corona. Bhola N. Dwivedi and Kenneth J. H. Phillips in
Scientific American, Vol. 284, No. 6, pages 40–47; June 2001.
The Maunder Minimum and the Variable Sun-Earth Connection. Willie Wei-Hock Soon and
Steven H. Yaskell. World Scientific Publishing, 2003.
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