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September
3, 2009: The sun is in the pits of the deepest solar
minimum in nearly a century. Weeks and sometimes whole months
go by without even a single tiny sunspot. The quiet has dragged
out for more than two years, prompting some observers to wonder,
are sunspots disappearing?
"Personally,
I'm betting that sunspots are coming back," says researcher
Matt Penn of the National Solar Observatory (NSO) in Tucson,
Arizona. But, he allows, "there is some evidence that
they won't."
Penn's
colleague Bill Livingston of the NSO has been measuring the
magnetic fields of sunspots for the past 17 years, and he
has found a remarkable trend. Sunspot magnetism is on the
decline:
Above:
Sunspot magnetic fields measured by Livingston and Penn from
1992 - Feb. 2009 using an infrared Zeeman splitting technique.
[more]
"Sunspot
magnetic fields are dropping by about 50 gauss per year,"
says Penn. "If we extrapolate this trend into the future,
sunspots could completely vanish around the year 2015."
This
disappearing act is possible because sunspots are made of
magnetism. The "firmament" of a sunspot is not matter
but rather a strong magnetic field that appears dark because
it blocks the upflow of heat from the sun's interior. If Earth
lost its magnetic field, the solid planet would remain intact,
but if a sunspot loses its magnetism, it ceases to exist.
"According
to our measurements, sunspots seem to form only if the magnetic
field is stronger than about 1500 gauss," says Livingston.
"If the current trend continues, we'll hit that threshold
in the near future, and solar
magnetic fields would become too weak to form sunspots."
"This
work has caused a sensation in the field of solar physics,"
comments NASA sunspot expert David Hathaway, who is not directly
involved in the research. "It's controversial stuff."
The
controversy is not about the data. "We know Livingston
and Penn are excellent observers," says Hathaway. "The
trend that they have discovered appears to be real."
The part colleagues have trouble believing is the extrapolation.
Hathaway notes
that most of their data were taken after the maximum
of Solar Cycle 23 (2000-2002) when sunspot activity naturally
began to decline. "The drop in magnetic fields could
be a normal aspect of the solar cycle and not a sign that
sunspots are permanently vanishing."
Penn
himself wonders about these points. "Our technique is
relatively new and the data stretches back in time only 17
years. We could be observing a temporary downturn that will
reverse itself."
 The
technique they're using was pioneered by Livingston at the
McMath-Pierce solar telescope near Tucson. He looks at a spectral
line emitted by iron atoms in the sun's atmosphere. Sunspot
magnetic fields cause the line to split in two—an effect called
"Zeeman splitting" after Dutch physicist Pieter
Zeeman who discovered the phenomenon in the 19th century.
The size of the split reveals the intensity of the magnetism.
Right:
Zeeman splitting of spectral lines from a strongly-magnetized
sunspot. [more]
Astronomers
have been measuring sunspot magnetic fields in this general
way for nearly a century, but Livingston added a twist. While
most researchers measure the splitting of spectral lines in
the visible part of the sun's spectrum, Livingston decided
to try an infra-red spectral line. Infrared lines are much
more sensitive to the Zeeman effect and provide more accurate
answers. Also, he dedicated himself to measuring a large number
of sunspots—more than 900 between 1998 and 2005 alone. The
combination of accuracy and numbers revealed the downturn.
If
sunspots do go away, it wouldn't be the first time. In the
17th century, the sun plunged into a 70-year period of spotlessness
known as the Maunder Minimum that still baffles scientists.
The sunspot drought began in 1645 and lasted until 1715; during
that time, some of the best astronomers in history (e.g.,
Cassini) monitored the sun and failed to count more than a
few dozen sunspots per year, compared to the usual thousands.
"Whether
[the current downturn] is an omen of long-term sunspot decline,
analogous to the Maunder Minimum, remains to be seen,"
Livingston and Penn caution in a recent issue of EOS. "Other
indications of solar activity suggest that sunspots must return
in earnest within the next year."
Whatever
happens, notes Hathaway, "the sun is behaving in an interesting
way and I believe we're about to learn something new."
Author: Dr.
Tony Phillips | Credit: Science@NASA
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