SOHO Sees Through the Sun
Like the unanticipated arrival of hurricanes before the advent of weather satellites, a group of previously hidden solar storm regions can rotate suddenly into view as the Sun turns, blazing away with explosive eruptions. The new technique, which uses the Michelson Doppler Imager (MDI) instrument on SOHO, gives a warning by creating a window to the far side of the Sun.
Right: An active region on the side of the Sun facing away from the Earth (the far side) causes sound waves, represented by blue arcs, that travel through the interior, bounce once off the surface, and reach the side facing the Earth (the near side). The waves generate ripples on the near side surface and are reflected back toward the active region. An active region reveals itself because it possesses very strong magnetic fields that speed up the sound waves. Waves that pass through an active region have a round trip travel time about twelve seconds shorter than the average of 6 hours. The difference becomes evident when sound waves shuttling back and forth get out of step with one another. LInks toCredit: NASA and the European Space Agency
"We've known for ten years that in theory we could make the Sun transparent all the way to the far side," according to Dr. Charles Lindsey of Solar Physics Research Corp., Tucson, AZ, and Dr. Douglas Braun of NorthWest Research Associates, Boulder, CO, authors of a paper describing the research to be published in the journal Science on March 10. "But we needed observations of exceptional quality. In the end we got them, from MDI on SOHO."
Ripples on the Sun's surface used to image the interior are caused by sound waves reverberating through the Sun. Analysis of solar sound waves is the science of helioseismology, and it opened the Sun's gaseous interior to investigation in much the same way as seismologists learned to explore the Earth's rocky interior with earthquake waves.
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The MDI instrument is the most elaborate of three helioseismic instruments on SOHO. It measures rhythmic motions at a million points across the Sun's visible surface.
Left: TBS three-panel image, showing absolute magnetic field strength of the same feature (upper left) one-half solar rotation before, and (upper right) one-half solar rotation after the (below, center) holographically imaged farside region. Links to. Credit: NASA and the European Space Agency
The technique of helioseismic holography used by Lindsey and Braun examines a wide ring of sound waves that emanate from a small region on the far side and reach the near side by rebounding internally from the solar surface. An active region reveals itself because it possesses very strong magnetic fields that speed up the sound waves. Waves that pass through an active region have around-trip travel time about 12 seconds shorter than the average of six hours. The difference becomes evident when sound waves shuttling back and forth get out of step with one another.
MDI data for March 28-29, 1998, revealed on the far side a sunspot group that was not plainly visible on the near side until 10 days later. Observations for 24 hours were more than sufficient to detect the sunspots, which means that routine monitoring is a realistic possibility.
SOHO is a cooperative project between the European Space Agency (ESA) and NASA. The spacecraft was built in Europe for ESA and equipped with instruments by teams of scientists in Europe and the USA. The far-side helioseismology research was funded by the National Science Foundation and NASA.Web Links
Additional Graphics -- including animation of a farside active region with coronal mass ejection.