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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.
"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."
Storm areas on the Sun, called active regions, are much larger than the Earth and consist of strong magnetic fields on the Sun's surface. Active regions produce explosions, called flares, and eruptions of plasma (hot, electrically charged gas), called coronal mass ejections. The radiation and plasma from these events sweep past the Earth, sometimes affecting spacecraft, power systems and disrupting radio communications. Understanding and forecasting solar eruptions and their consequences is a relatively new science called space weather.
For more than 100 years, scientists have been aware that sunspots (groups of dark areas on the Sun's visible face) are often the scene of flares and other eruptions. Now they watch the Sun more closely than ever, because modern systems are much more vulnerable to solar disturbances than old technology. The experts can still be taken by surprise because the Sun rotates, bringing the effects of hidden active regions to bear on Earth. With a far- side preview of sunspots, nasty surprises for the space weather experts may now be avoidable.
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.
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.
Computers can interpret the motions in terms of sound waves travelling through the Sun. The waves are affected by the various layers of gas and different motions that they encounter. The MDI has already revealed many unknown features of the solar interior, including hidden jet streams circling the Sun's poles.
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 a round-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.
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