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:
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.
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.
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. 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?
EXTRA:
here's the chart for the last 100 years...
From Science @ NASA
