The Solar and Heliospheric Observatory's Michelson-Doppler Imager (MDI) can monitor sunspots on the far side of the Sun using a technique called "helioseismic holography." Typical holographic maps look like this:

The false colors represent condensations of magnetic flux -- that is, sunspots. This holographic map captured April 12, 2001, shows the giant sunspot AR9393 on the back side of the Sun a full week before it emerged into direct view over the Sun's eastern limb. MDI holographic images reveal the Earth-facing side of the Sun 70 degrees from the disk center, and the far side of the Sun 50 degrees from disk center.

How does helioseismology work?

The Sun is a hummimg ball of sound waves launched by turbulent convective motions in our star's outer layers. "The waves we monitor [using MDI] have a period of about 5 minutes," says Phil Scherrer of Stanford University, principal investigator for the MDI instrument. "That's roughly the turn-over time of the California-sized bubbles that appear as granulation of the photosphere." Solar granulation is what excites the Sun's internal 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. (Click on the image, left, for a 1.8 Mb Quicktime movie of trapped solar sound waves.) 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 transit times of sound waves bouncing from one side of the Sun to the other, variations that the MDI can detect and transform to reveal magnetic condensations (i.e, sunspots) on the hidden side of the Sun. Called "helioseismic holography," this technique can produce actual images of the far side of our star.

For more information please visit the MDI Farside Imaging home page at Stanford. See also "Acoustic Imaging the Backside of the Sun", a collection of press releases and multimedia resources.