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Sunspot record
reveals Sun's past
Solar
history may have links with Earth's climate
The
past 70 years has seen the longest and most intense period of sunspot
activity for 8,000 years, according to scientists who have reconstructed a
record of the Sun's last 11,000 years.
Solar physicist Sami Solanki from the Max Planck Institute in
Katlenburg-Lindau, Germany, and his colleagues found in November 2003 that
the Sun is more active now than at any time in the past 1,000 years1.
This, along with several record-breaking solar storms that occurred at
around the same time, has triggered intense debate about why the Sun is
now so active.
However, Solanki's latest data reveals that before 8,000 years
ago, the Sun went through several short periods where it was just as
active as it is today. This suggests that the current high is part of the
Sun's normal activity and that it will probably calm down again, returning
to normal levels within the next few decades, he says.
Sunspots are temporary dark patches on the Sun, caused when
its magnetic field 'pinches' at the surface. The magnetic field is
generated by hot, ionised gas that swirls around inside the star, acting
like a dynamo.
"But we don't really know how the Sun's dynamo
works," Solanki says. The new record will help solar physicists to
model how that dynamo changes over thousands of years.
Scientists already have a good observational record of
sunspots that dates back to the early seventeenth century. This shows that
the Sun's activity varies on cycles that last roughly 11 and 88 years,
although scientists are not yet sure why these cycles exist.
David Hathaway, a solar physicist from NASA's Marshall Space
Flight Centre in Huntsville, Alabama, recently predicted that the Sun's
current spotty outbreak was on the wane and should reach a minimum by
2006, although this will still be significantly more active that in
previous centuries.
Ringing in the changes
Solanki's team reconstructed their record by following an
intricate scientific breadcrumb trail. Sunspots have extremely turbulent
magnetic fields, which send extra bursts of charged particles hurtling
towards Earth, and can potentially disrupt radio communications and damage
satellites.
This boost to the solar wind also deflects more of the cosmic
rays that normally bombard the Earth's atmosphere, so there are fewer
cosmic rays to trigger nuclear reactions in the upper atmosphere that
produce a heavy form of carbon called carbon-14.
When trees absorb this as carbon dioxide, they indirectly
carry a historical record of the Sun's surface that can be dated by
measuring the variation of carbon-14 in their rings. More sunspots means
less carbon-14 in the tree's ring for that year.
"The models reproduce the observed record of sunspots
extremely well, from almost no sunspots during the seventeenth century to
the current high levels," comments Paula Reimer, a palaeoecologist
from Queen's University Belfast in Northern Ireland.
Cold snap
The period between about 1650 and 1700, when astronomers saw
virtually no sunspots, is known as the Maunder Minimum. It
coincided with the coldest part of a chilly period on Earth called the
Little Ice Age, and some scientists have speculated that this provides
evidence that the Sun's activity significantly affects our climate.
"There's a really interesting debate about this at the
moment," says Mike Hapgood, a space scientist from the Rutherford
Appleton Laboratory, Didcot, UK. The sunspot record, published this week
in Nature2,
could help to settle the issue, he says.
Solanki stresses that without fully understanding the chain of
cause and effect, we cannot be sure exactly how the changing Sun impacts
on our climate. Greenhouse gases, generated by burning fossil fuels and
volcanic eruptions, are probably far more important in controlling the
Earth's temperature, he says.
Mark
Peplow
Source:
www.Nature.com
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