Sensing What Cannot be Seen (September 2, 2004)
Hi-res TIF image (582K).
Using a technique developed in the past few years, scientists have been generating images of the far side of the Sun which are computed from sound travel time analysis. These are based on observations by SOHO's Michelson Doppler Imager (MDI). The images are whole-Sun maps of magnetic activity that let scientists "see" sunspots on the far side of the Sun many days before they rotate into view. A few days ago the far side imaging showed that a sunspot region would be appearing around the edge of the Sun on Aug. 31 or Sept. 1. And it did. This is the third time around for this same active region: it was observed as a sunspot group two months ago, and again one month ago. The spot has become quite diminished compared to its size when we last saw it. From the far side images, it appears that a new, large region has now appeared on the northern hemisphere, and it should come around the limb in a few days.
The far side images are maps of wave speed variations with locations of faster wave speed shown darker. These darker regions indicate locations where there is an accumulation of magnetic field on the far surface. The far side images can only be computed out to 45 degrees from the far side disk center as (un)seen from Earth. 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. This technique is called "helioseismic holography." In essence, we are seeing through the Sun. This has proven to be a useful tool in predicting when new sunspots will be appearing on the Earth-side of the Sun.
SOHO began its Weekly Pick some time after sending a weekly image or video clip to the American Museum of Natural History (Rose Center) in New York City. There, the SOHO Weekly Pick is displayed with some annotations on a large plasma display.
If your institution would also like to receive the same Weekly Pick from us for display (usually in Photoshop or QuickTime format), please send your inquiry to firstname.lastname@example.org.