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The following article is from www.science.nasa.gov, September 3, 2009,
and updates the research done by Livingstone and Penn at the National Solar
Observatory in Arizona.
"This work has
caused a sensation in the field of solar physics."
- NASA sunspot
expert, David Hathaway
Are
Sunspots Disappearing?
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.
"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.
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."
* * * * *
The
following chart shows solar activity since 1849, with Spotless Days (SD) in
blue. Note the unusual lows (high for solar output) into the late 1980s, which
associated with the late warming trend. Additional explanation is provided
below the graph.
Smoothed
monthly Wolfnumber and smoothed monthly number of spotless days
This
graph shows the smoothed monthly Wolfnumber (Rs) and smoothed monthly number
of spotless days (SDs) since 1849. For obvious reasons, the instances of
minimum Rs and maximum SDs happen for most cycles in the same month. Exceptions
are SC14 (3 months later) and SC23 (4 months later). There is a strong
relationship between the two parameters during solar cycle minimum: SDsmax
= -1,4755 Rsmin + 27,076 (r²=0,92). The smoothed Rs and
SDs both reached a minimum in December 2008 (resp. values of 1,72 and 24,2).
This is comparable with the low activity transits in the early 20th-century,
and certainly contrasts with the previous 4 cycle transits.
Source: Spotless
Days Page
Bob Hoye
Institutional
Advisors
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