 source Science@NASA
It's official: Solar minimum has
arrived. Sunspots have all but vanished. Solar flares are
nonexistent. The sun is utterly quiet.
Like
the quiet before a storm.
This
week researchers announced that a storm is coming--the
most intense solar maximum in fifty years. The prediction
comes from a team led by Mausumi Dikpati of the National Center
for Atmospheric Research (NCAR). "The next sunspot cycle
will be 30% to 50% stronger than the previous one," she
says. If correct, the years ahead could produce a burst of
solar activity second only to the historic Solar Max of 1958.
That
was a solar maximum. The Space Age was just beginning: Sputnik
was launched in Oct. 1957 and Explorer 1 (the first US satellite)
in Jan. 1958. In 1958 you couldn't tell that a solar storm
was underway by looking at the bars on your cell phone; cell
phones didn't exist. Even so, people knew something big was
happening when Northern Lights were sighted three times in
Mexico. A similar maximum now would be noticed by its
effect on cell phones, GPS, weather satellites and many other
modern technologies.
Dikpati's
prediction is unprecedented. In nearly-two centuries since
the 11-year sunspot cycle was discovered, scientists have
struggled to predict the size of future maxima—and failed.
Solar maxima can be intense, as in 1958, or barely detectable,
as in 1805, obeying no obvious pattern.
The
key to the mystery, Dikpati realized years ago, is a conveyor
belt on the sun.
We
have something similar here on Earth—the Great Ocean Conveyor
Belt, popularized in the sci-fi movie The Day After Tomorrow.
It is a network of currents that carry water and heat from
ocean to ocean--see the diagram below. In the movie, the Conveyor
Belt stopped and threw the world's weather into chaos.
 Earth's "Great Ocean Conveyor Belt." [More]
The
sun's conveyor belt is a current, not of water, but of electrically-conducting
gas. It flows in a loop from the sun's equator to the poles
and back again. Just as the Great Ocean Conveyor Belt controls
weather on Earth, this solar conveyor belt controls weather
on the sun. Specifically, it controls the sunspot cycle.
Solar
physicist David Hathaway of the National Space Science &
Technology Center (NSSTC) explains: "First, remember
what sunspots are--tangled knots of magnetism generated by
the sun's inner dynamo. A typical sunspot exists for just
a few weeks. Then it decays, leaving behind a 'corpse' of
weak magnetic fields."
Enter
the conveyor belt.
"The
top of the conveyor belt skims the surface of the sun, sweeping
up the magnetic fields of old, dead sunspots. The 'corpses'
are dragged down at the poles to a depth of 200,000 km where
the sun's magnetic dynamo can amplify them. Once the corpses
(magnetic knots) are reincarnated (amplified), they become
buoyant and float back to the surface." Presto—new sunspots!
 The sun's "great conveyor belt." [Larger
image]
All
this happens with massive slowness. "It takes about 40
years for the belt to complete one loop," says Hathaway.
The speed varies "anywhere from a 50-year pace (slow)
to a 30-year pace (fast)."
When
the belt is turning "fast," it means that lots of
magnetic fields are being swept up, and that a future sunspot
cycle is going to be intense. This is a basis for forecasting:
"The belt was turning fast in 1986-1996," says Hathaway.
"Old magnetic fields swept up then should re-appear as
big sunspots in 2010-2011."
Like
most experts in the field, Hathaway has confidence in the
conveyor belt model and agrees with Dikpati that the next
solar maximum should be a doozy. But he disagrees with one
point. Dikpati's forecast puts Solar Max at 2012. Hathaway
believes it will arrive sooner, in 2010 or 2011.
"History
shows that big sunspot cycles 'ramp up' faster than small
ones," he says. "I expect to see the first sunspots
of the next cycle appear in late 2006 or 2007—and Solar Max
to be underway by 2010 or 2011."
Who's
right? Time will tell. Either way, a storm is coming.
source Science@NASA
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