Apr 26, 2001

The Transparent Sun




Giant sunspot 9393 is making its second transit across the face of the Sun after spending two weeks in hiding. Its unusual reappearance came as no surprise to scientists who tracked the behemoth by peering right through our star.


Marshall Space Flight Center


(requires RealPlayer)

April 27, 2001 -- 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.


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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. [more]

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.




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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."


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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. [more information]

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.

Above: Click on the image for a 1.8 Mb Quicktime movie of trapped solar sound waves. [more]

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.

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, 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!





Looking Up on Sun-Earth Days


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April 27th and 28th are Sun-Earth Days, a national celebration of the Sun, the space around Earth, and how all of it affects life on our planet. A great Sun-Earth Day activity is sunspot watching. Active region 9393 is still a whopper spanning 20 Earth diameters from end to end -- and it's located near the center of the solar disk.


Left: On April 25, 2001, sunspot 9393 crossed the center of the Sun for the second time in four weeks. This white-light image of the Sun is courtesy of the Solar and Heliospheric Observatory.

This active region is so big that astronomers call it a "naked-eye" sunspot. But don't try looking at AR9393 without protection. Staring directly at the Sun --even through clouds-- can lead to blindness. Instead, use a safe solar projector or filter. The sunspot should remain visible until May 1st, when it will vanish (for the second time) over the Sun's western limb.

Web Links

MDI Solar Farside Imaging -- from the Michelson Doppler Imager team at Stanford

SWAN Solar Farside Imaging -- from SOHO and the French space agency, Centre National d'Etudes Spatiales.

Surface Waves and Helioseismology -- discover the basics of helioseismology and watch a movie of seimic waves on the Sun. From the NASA/Marshall Space Flight Center.

It's SOHO's 5th Anniversary -- learn more about SOHO's astounding accomplishments. It's a satellite no space weather forecaster could live without!

Sunspots in History -- find out how AR9393 compares to other giant sunspots.

AR9393 Triggers a Geomagnetic Storm -- see photos of auroras from all over the world captured when a coronal mass ejection from AR9393 hit Earth.


SOHO spies the far side of the Sun -- learn more about the SWAN technique in this 1999 Science@NASA article


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