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American Scientist, Volume 102
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Sightings
The Real Sun,
Unmasked
The Solar Dynamics Observatory produces stunning
images while investigating the origins of space storms.
Catherine Clabby
F
rom 150 million kilometers away, the Sun seems like a model of consistency.
Day after day the colossal nuclear reactor bathes Earth in steady streams
of electromagnetic energy. That radiation keeps the planet’s climate in balance and anchors our food webs. But take a closer look and it becomes clear
that our star has a hidden side that is more complicated, impulsive, and sometimes
downright dangerous.
Like all stars, the Sun is highly dynamic. Sometimes it produces powerful magnetic eruptions, including solar flares and coronal mass ejections. These occur on a
wide range of scales. In 1859, the most powerful recorded solar storm to hit Earth
sent electrical surges sparking down telegraph networks and lit up brilliant aurora
displays as far south as Cuba. Today, a geomagnetic disturbance on that scale could
cause extensive damage to satellites, GPS systems, power grids, and radio communications. Although estimates are uncertain, the cost could exceed $1 trillion according to a National Research Council report.
A network of observing stations, in space and on the ground, now watches the
Sun across a wide range of electromagnetic wavelengths to monitor its effect on
space weather and to provide advance warning when major storms are headed our
way. NASA’s Solar Dynamics Observatory (SDO) is the newest member of this network. It circles 36,000 kilometers above the ground, in a geosynchronous orbit that
keeps it roughly above northern Mexico. From that vantage point, SDO’s instruments are producing some of the most stunning images ever made of the Sun and
capturing events in real time.
“In the past images were taken every 3 or 15 minutes or half an hour. Now we’re
taking one every 12 seconds. If you see something happen, you can now go back in
the database and see what happened beforehand. That allows you to study the little
things that likely influence the big things,” says Dean Pesnell, project scientist for
SDO, which launched in 2010. The observatory also records the Sun in 13 different
wavelengths. Each wavelength reveals different details in the Sun’s surface, the photosphere, the overlying chromosphere, and into the corona, the Sun’s atmosphere.
Magnetism is key to understanding solar activity, because the Sun is composed
not of atoms but of plasma, a brew of positively charged ions and negatively
charged electrons. The motion of that electrically conductive plasma—stirred by
convection, rotation, and the force of escaping solar winds—creates twisted and
variable magnetic fields. When these fields interact, they can unleash the energy
that powers flares and mass ejections. “The Sun’s magnetic field makes all this
NASA’s Space Dynamics Observatory (SDO) captured a long filament of solar plasma
erupting from the Sun’s corona in August 2012, here in extreme ultraviolet light. With
resolution never achieved before, the SDO observes the Sun in 13 spectra every 12
seconds, recording the solar changes that can create space weather.
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The speed and direction of comets as they approach the Sun give
scientists insights into the shape
and strength of the Sun’s magnetic fields. In 2013, the European
Space Agency and NASA’s Solar
and Heliospheric Observatory
captured the motions of comet
ISON in the time-lapse image
above. At left, the SDO’s helioseismic and magnetic imager creates maps of magnetic fields on
the Sun’s surface.
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Viewing the Sun in different wavelengths allows scientists to see
a wide variety of solar materials in different locations, from the
surface of the Sun up through its atmosphere. These images were
made at the same moment when the Moon moved between SDO
telescopes and the Sun. From top to bottom they were observed
in 193-angstrom light, 131-angstrom light, and 304-angstrom light.
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happen. We want to know where it comes
from. We want to know how it gets to the
surface. We want to know how it gets destroyed,” says Pesnell. The Sun goes through
an 11-year cycle of activity, currently near its
peak, but eruptions can happen any time.
(See “Reconnecting Magnetic Fields,” September–October 2009.)
One instrument aboard SDO measures
the strength and direction of the Sun’s magnetic fields by observing and interpreting
how light travels through those fields. Another way to study fields is to study “Sungrazing” comets that interact with magnetic
fields while passing through the Sun’s corona. “Magnetic fields in the higher part of the
solar atmosphere are very difficult to measure,” says C. Alex Young, associate director
for science for NASA’s heliophysics science
division. “By observing this with SDO, we
have a new, indirect way to determine the
magnetic fields, one of the keys to understanding the causes of space weather.” SDO
is also monitoring coronal holes, temporary
openings in the corona where the solar wind
escapes at very high speeds, around 750 kilometers per second. A British team has just
reported that fast solar winds seem to increase the number of lightning storms here
on Earth. (See “The Origin of the Solar Wind,
November–December 2002.)
Although the current solar cycle is quieter
than the few that came before, the Sun is
hardly calm. In January, a huge solar flare
delayed the launch of a private rocket transporting cargo to the International Space
Station. Orbital Sciences, the maker of the
spacecraft, did not want to risk radiation
from the flare damaging the rocket’s gyroscopes or avionics. Data from SDO helped
the company make that call.
Even better views of how the Sun releases
energy are on the way. The Magnetospheric Multiscale mission, to be launched by
NASA in 2015, will study how the Earth’s
magnetic field lines break apart and reconnect, improving our understanding of space
weather and solar magnetism in general. A
far more daring mission is Solar Probe Plus,
which NASA intends to launch by 2018. It
will swing within 7 million kilometers of
the Sun’s surface, far closer than any other
spacecraft has managed.
Solar Probe Plus will sample the Sun’s
atmosphere to better understand how solar
particles are energized. It will be our world’s
first space mission to a star.
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January 2012
September 2012
March 2011
May 2013
May 2010
March 2014
These images capture increased activity on the surface of
the Sun, measured by the number of sunspots visible on
the solar disk, between 2010 and 2014. Solar cycles occur
roughly every 11 years, reaching a peak in magnetic activity during what’s called the solar maximum. The current
cycle has displayed the weakest activity ever recorded.
In 2011, SDO recorded a solar flare, the white
flash at left, on the surface of the Sun, along
with a coronal mass ejection, the darker material, in extreme ultraviolet light. Above, a sizable coronal hole, observed here in three wavelengths of ultraviolet light, rotated towards
Earth over several days in 2013. Coronal holes
release strong solar wind gusts that carry solar
particles to Earth’s protective magnetosphere
and beyond, sometimes generating aurora.
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American Scientist, Volume 102
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