| Science@NASA
In
late March, the biggest sunspot of the current solar cycle glided across the face
of our star. Covering an area equal to fourteen planet Earths, the sprawling complex
known as "AR9393" was an impressive sight.
It not only looked menacing
-- it was. Just as the behemoth was poised to vanish over the Sun's western limb
on April 2nd (carried away by the Sun's 27-day rotation), it unleashed the most
powerful solar flare ever recorded.
Although the blast was directed mostly
away from Earth, it nevertheless triggered a radiation storm around our planet
and a dazzling display of Northern Lights. Aurora watchers enjoyed the show, but
many also breathed a sign of relief when the giant spot went away. A direct blast
from AR9393 could have triggered widespread radio blackouts, disrupted satellite
communications and even collapsed power grids.
When AR9393 rotated over
the Sun's western limb on April 3rd, most Sun watchers assumed it was gone for
good. "Sunspots rarely persist for more than a single solar rotation," explains
David Hathaway, a solar physicist at the NASA Marshall Space Flight Center, "although
big ones like AR9393 tend to last longer than usual."
Indeed, the extraordinary
spot remained whole throughout its two week journey across the back side of the
Sun and it reappeared on April 19th.
Were solar researchers surprised?
Not really. They knew AR9393 was returning because they never lost sight of it.
Instruments on board the ESA/NASA Solar and Heliospheric Observatory (SOHO) had
tracked the active region by peering right through the Sun!
"We've developed
the extraordinary capability to monitor the far side of the Sun using two of SOHO's
instruments: the Solar Wind Anisotropies Experiment (SWAN) and the Michelson Doppler
Imager (MDI)," explains Bernhard Fleck, the European Space Agency's chief scientist
for the SOHO mission. "These new techniques are still a work in progress, but
already we can predict the appearance of large sunspots days before they rotate
into direct view."
How is it possible to see through an opaque ball of
gas a million miles wide?
SWAN's method --pioneered by a European team
of scientists headed by Jean Loup Bertaux, of the CNRS Service d'Aronomie in Frances--
is, perhaps, the easiest to understand.
Sunspots are like high-energy lighthouses.
Magnetic loops above sunspots are the "lightbulbs" -- they harbor superheated
gas that shines brightly at ultraviolet (UV) wavelengths. As the Sun turns, beams
of UV radiation sweep through space and illuminate the interplanetary medium,
a thin haze of gas and dust between the planets. SWAN --a telescope on board SOHO
that can map the whole sky in ultraviolet light-- can see UV "hot spots" caused
by active regions on the far side of the Sun.
The MDI team's approach to
peering through the Sun is more subtle.
The Sun is a hummimg ball of sound
waves launched by turbulent convective motions inside our star. We can see those
motions in the form of granules: thousand-km wide bubbles that rise to the Sun's
surface and then fall again. "These bubbles rising and collapsing are the source
of the Sun's acoustic noise," says Phil Scherrer of Stanford University, principal
investigator for the MDI instrument. "The sound waves we monitor have a period
of about 5 minutes -- that's roughly the turn-over time of the California-sized
granules."
Solar sound waves are mostly trapped inside our star -- they
refract away from the Sun's hot core and reflect back and forth between different
parts of the photosphere (the Sun's surface). By monitoring the Sun's vibrating
surface, helioseismologists can probe the stellar interior in much the same way
that geologists use seismic waves from earthquakes to probe the inside of our
planet.
Intense
magnetic fields around sunspots affect the propagation velocity of sound waves
bouncing around inside the Sun, variations that MDI can detect and transform to
reveal magnetic condensations --that is, sunspots. Called "helioseismic holography,"
this technique can produce actual images of the far side of our star.
MDI
and SWAN are complementary in their efforts to see through the Sun. MDI's helioseismic
holography pinpoints hidden sunspots while SWAN's data reveal how active they
are.
"When we started work with SOHO five years ago, most experts thought
it would be impossible to see right through the Sun," comments Scherrer. "Now
we do it regularly in real time. For practical purposes we've made the Sun transparent."
Scherrer
and his team are so confident in their newly developed technique, they're willing
to share their views with the general public. Beginning today anyone can access
MDI's farside images of the Sun by visiting SpaceWeather.com, the SOHO web page
(which also includes SWAN farside images), or Scherrer's MDI site at Stanford.
Armed with only a modem and a dial-up connection, you too can see right
through the Sun!
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