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Conveyor
belt
followed by a slower flow during the second half, creates a weak polar magnetic
field. This generates an unusually weak
and prolonged solar minimum, Nandy
says, like the most recent one. From January 2004 until March 2010, there were
780 days with no sunspots. During a similar period in a typical solar minimum, the
sun is spot-free for about 300 days.
“The Nandy article is an excellent
study of the effect of the meridional flow
on what we see at the surface over a solar
cycle,” says Pesnell.
But Hathaway says that his obser-
vations indicate that the speed of the
meridional flow was exactly oppo-
site to that required by Nandy and his
colleagues. Another solar physicist,
Yi-Ming Wang of the Naval Research
Laboratory in Washington, D.C., says
he is baffled by the
apparent contradic-
tion between the
model and Hatha-
way’s observations.
The speed of conveyor
belt–like loops of hot
plasma that circu-
late from the sun’s
equator to its poles
helps determine the
strength of the subse-
quent solar cycle.
strength of the subse-
quent solar cycle.
Back Story | SUNSPOTS, 1750–2010
300
Sunspot number
100
200
1 2 3 4 5 6 7 9 8 10 11 0
Year
1750 1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880
300
Sunspot number
100
200
The sun’s activity waxes and wanes on a cycle that averages roughly 11 years,
though cycles as short as nine years and as long as 14 years have been
observed. Chinese astronomers were already tracking the sun’s activity using
sunspots more than 2,000 years ago; the modern record of solar output starts
in 1755, with cycle 1, and runs through cycle 24, which began in late 2008.
Generated by intense magnetic ;elds, sunspots have proven one reliable
indicator of the sun’s overall output and its production of solar storms.
12 13 14 15 16 17 19 18 20 21 22 23 0
Year
1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
field along with it, and a fast flow
leads to a weaker field at the poles than a
slow flow would. Because the polar fields
are thought to be the seeds for the next
solar cycle, a weak polar field will cause
the next cycle to be weak also.
“It is possible that the current sun-
spot cycle, cycle 24, was seeded with
magnetic fields from cycle 23, or even an
earlier cycle, cycle 22,” says Matt Penn
of the National Solar Observatory in
Tucson. “This seed field may have been
weaker than normal, somehow, and it
may be producing a weaker solar activ-
ity cycle now.”
For now, the exact role that the
meridional flow plays in the solar cycle
remains a matter of debate. But the new
research “demonstrates how the inner
working of the sun, and variations in the
plasma flow deep within our parent star,
can control its magnetic and energetic
output, which in turn, determines the
environment in space and affects cli-
mate on Earth,” says Nandy.
A weaker solar cycle is accompanied
by a slightly dimmer sun, which reduces
the average temperature on Earth, says
Judith Lean of the Naval Research Laboratory. She notes, however, that the
sun’s brightness did not hit an all-time
low during the past solar minimum,
even though the sun was unusually
quiet.
Nonetheless, the decline in solar
brightness from 2002 to 2008 as solar
activity dwindled probably offset warming on Earth that would otherwise have
occurred due to greenhouse gases over
that period. “As solar activity now
increases,” she says, “we can expect this
mitigating effect to stop.” s
