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October
27, 2009: Every 11 years, the sun undergoes a furious
upheaval. Dark sunspots burst forth from beneath the sun's
surface. Explosions as powerful as a billion atomic bombs
spark intense flares of high-energy radiation. Clouds of gas
big enough to swallow planets break away from the sun and
billow into space. It's a flamboyant display of stellar power.
So
why can't we see any of it?
 Almost
none of the drama of Solar Maximum is visible to the human
eye. Look at the sun in the noontime sky and—ho-hum—it's the
same old bland ball of bright light.
"The
problem is, human eyes are tuned to the wrong wavelength,"
explains Tom Woods, a solar physicist at the University of
Colorado in Boulder. "If you want to get a good look
at solar activity, you need to look in the EUV."
Right:
The active sun photographed at EUV wavelengths by
the Solar and Heliospheric Observatory in the year 2000. [more]
EUV
is short for "extreme ultraviolet," a high-energy
form of ultraviolet radiation with wavelengths between 1 and
120 nanometers. EUV photons are much more energetic and dangerous
than the ordinary UV rays that cause sunburns. Fortunately
for humans, Earth's atmosphere blocks solar EUV; otherwise
a day at the beach could be fatal.
When
the sun is active, intense solar EUV emissions can rise and
fall by factors of thousands in just a matter of minutes.
These surges heat Earth's upper atmosphere, puffing it up
and increasing the drag on satellites. EUV photons also break
apart atoms and molecules, creating a layer of ions in the
upper atmosphere that can severely disturb radio signals.
To
monitor these energetic photons, NASA is going to launch a
sensor named "EVE," short for EUV Variability Experiment,
onboard the Solar Dynamics Observatory as early as this winter.
"EVE
gives us the highest time resolution (10 sec) and the highest
spectral resolution (< 0.1 nm) that we've ever had for
measuring the sun, and we'll have it 24/7," says Woods,
the lead scientist for EVE. "This is a huge improvement
over past missions."
 Right:
The Extreme Ultraviolet Variability Experiment (EVE) with
its primary sensors labeled. [more]
Although
EVE is designed to study solar activity, its first order of
business is to study solar inactivity. SDO is going
to launch during the deepest solar minimum in almost 100 years.
Sunspots, flares and CMEs are at low ebb. That's okay with
Woods. He considers solar minimum just as interesting as solar
maximum.
"Solar
minimum is a quiet time when we can establish a baseline for
evaluating long-term trends," he explains. "All
stars are variable at some level, and the sun is no exception.
We want to compare the sun's brightness now to its brightness
during previous minima and ask ourselves, is the sun getting
brighter or dimmer?"
Lately,
the answer seems to be dimmer. Measurements by a variety of
spacecraft indicate a 12-year lessening of the sun's "irradiance"
by about 0.02% at visible wavelengths and 6% at EUV wavelengths.
These results, which compare the solar minimum of 2008-09
to the previous minimum of 1996, are still very preliminary.
EVE will improve confidence in the trend by pinning down the
EUV spectrum with unprecedented accuracy.
Above:
Space-age measurements of the total solar irradiance or "TSI".
TSI is the sun's brightness summed across all the wavelengths
of the electromagnetic spectrum--visible light and EUV included.
TSI goes up and down with the 11 year solar cycle. Credit:
C. Fröhlich.
The
sun's intrinsic variability and its potential for future changes
are not fully understood—hence the need for EVE. "The
EUV portion of the sun's spectrum is what changes most during
a solar cycle," says Woods, "and that is the part
of the spectrum we will be observing."
Woods
gazes out his office window at the Colorado sun. It looks
the same as usual. EVE, he knows, will have a different story
to tell.
Author: Dr.
Tony Phillips | Credit: Science@NASA
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