| 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.
 Above:
This SOHO white light animation shows AR9393's first transit across
the Sun between March 27th and April 2nd. Just after the end of the movie the
spot unleashed an X20-class solar flare -- the most powerful 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.
 | | Above:
The Solar Wind Anisotropies (SWAN) experiment on SOHO can spot UV hot
spots in the interplanetary medium caused by sunspots 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."
 | | Above:
Click on the image for a 1.8 Mb Quicktime movie of trapped solar sound
waves. |
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.
 Above:
This MDI image shows AR9393 transiting the far side of the Sun on April
12, 2001. In this false color image, yellows and reds indicate magnetic condensations
that affect the propagation speed of sound waves in the solar interior. Another
MDI image from March 30, 2001, shows AR9393 on the near side of the Sun.
"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! |
