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Sept.
30, 2008: Astronomers who count sunspots have announced
that 2008 is now the "blankest year" of the Space
Age.
As
of Sept. 27, 2008, the sun had been blank, i.e.,
had no visible sunspots, on 200 days of the year. To find
a year with more blank suns, you have to go back to 1954,
three years before the launch of Sputnik, when the sun was
blank 241 times.
"Sunspot
counts are at a 50-year low," says solar physicist David
Hathaway of the NASA Marshall Space Flight Center. "We're
experiencing a deep minimum of the solar cycle."
A spotless day
looks like this:
![A SOHO image of the sun taken Sept. 27, 2008.](images/blankyear/20080927_1600_mdi_igr_strip.gif)
The
image, taken by the Solar and Heliospheric Observatory (SOHO)
on Sept. 27, 2008, shows a solar disk completely unmarked
by sunspots. For comparison, a SOHO image taken seven years
earlier on Sept. 27, 2001, is peppered with colossal sunspots,
all crackling with solar flares: image.
The difference is the phase of the 11-year solar cycle. 2001
was a year of solar maximum, with lots of sunspots, solar
flares and geomagnetic storms. 2008 is at the cycle's opposite
extreme, solar minimum, a quiet time on the sun.
And
it is a very quiet time. If solar activity continues
as low as it has been, 2008 could rack up a whopping 290 spotless
days by the end of December, making it a century-level year
in terms of spotlessness.
Hathaway
cautions that this development may sound more exciting than
it actually is: "While the solar minimum of 2008 is shaping
up to be the deepest of the Space Age, it is still unremarkable
compared to the long and deep solar minima of the late 19th
and early 20th centuries." Those earlier minima routinely
racked up 200 to 300 spotless days per year.
![see caption](images/blankyear/50years2_strip.gif)
Above:
A histogram showing the blankest years of the last half-century.
The vertical axis is a count of spotless days in each year.
The bar for 2008, which was updated on Sept. 27th, is still
growing. [Larger images: 50
years, 100 years]
Some
solar physicists are welcoming the lull.
"This
gives us a chance to study the sun without the complications
of sunspots," says Dean Pesnell of the Goddard Space
Flight Center. "Right now we have the best instrumentation
in history looking at the sun. There is a whole fleet of spacecraft
devoted to solar physics--SOHO, Hinode, ACE, STEREO and others.
We're bound to learn new things during this long solar minimum."
As
an example he offers helioseismology: "By monitoring
the sun's vibrating surface, helioseismologists can probe
the stellar interior in much the same way geologists use earthquakes
to probe inside Earth. With sunspots out of the way, we gain
a better view of the sun's subsurface winds and inner magnetic
dynamo."
"There
is also the matter of solar irradiance," adds Pesnell.
"Researchers are now seeing the dimmest sun in their
records. The change is small, just a fraction of a percent,
but significant. Questions about effects on climate are natural
if the sun continues to dim."
Pesnell
is NASA's project scientist for the Solar Dynamics Observatory
(SDO), a new spacecraft equipped to study both solar irradiance
and helioseismic waves. Construction of SDO is complete, he
says, and it has passed pre-launch vibration and thermal testing.
"We are ready to launch! Solar minimum is a great time
to go."
Coinciding
with the string of blank suns is a 50-year record low in solar
wind pressure, a recent discovery of the Ulysses spacecraft.
(See the Science@NASA story Solar
Wind Loses Pressure.) The pressure drop began years
before the current minimum, so it is unclear how the two phenomena
are connected, if at all. This is another mystery for SDO
and the others.
Who
knew the blank sun could be so interesting? More to come...
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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