Focus 72 Stereo

NASA's Solar-TErrestrial RElations Observatory, abbreviated STEREO, mission, scheduled for launch in 2006, will send two identical satellites into solar orbit, one ahead of the Earth in its orbit and one behind, "tteir coronagraphs and other instruments will give astronomers the first three-dimensional view of coronal mass ejections. Previous observations from a coronagraph aboard a single spacecraft provided an edge on, two-dimensional view.

"tte instruments aboard the STEREO mission will advance our understanding of the origin of coronal mass ejections, their evolution in the interplanetary medium, and their coupling with the Earth's magnetosphere. "tte mission will also investigate how coronal mass ejections produce energetic particles in space and magnetic storms on Earth.

Scientists are also now using sound waves to see right through the Sun to its hidden, normally invisible, backside, describing active regions on the far side of the Sun many days before they rotate onto the side facing Earth, tte new technique uses observations of the Sun's oscillations to create a sort of mathematical lens that focuses to different depths, and it is called helioseismic holography because of its similarity to laser holography. A wide ring of sound waves is examined, which emanates from a region on the side of the Sun facing away from the Earth, the far side, and reaches the near side that faces the Earth (Fig. 7.21).

When a large solar active region is present on the backside of the Sun, its intense magnetic fields compress the gases there, making them slightly lower and denser than the surrounding material. So sound waves emerging from an active region on the far side suffer a phase shift, expressed as a change in travel time. A sound wave that would ordinarily take about 6 hours to travel from the near side to the far side of the Sun and back again takes approximately 12 seconds less when it bounces off the compressed active region on the far side. When nearside photosphere oscillations are examined; they can detect the quick return of these sound waves.

Ms remarkable result, first announced by American solar astronomers Douglas Braun (1961-) and Charles Lindsey (1947- ) in 2000, became routine within a few years, with daily far side images obtained from instruments on SOHO and shown on the web. Solar astronomers are using this technique to monitor the structure and evolution of large regions of magnetic activity as they cross the far side of the Sun, thereby revealing the regions that are growing in magnetic complexity or strength and seem primed for explosion. Since the solar equator rotates with a period of 27 days, when viewed from the Earth, this can give at least seven day's extra warning of possible bad weather in space before the active region swings into view and pummels the Earth.

FIG. 7.21 Looking through the Sun The arcing trajectories of sound waves from the far side of the Sun are reflected internally once before reaching the front side, where they are observed with the Michelson Doppler Imager, abbreviated MDI, aboard the SOlar and Helio-spheric Observatory, or SOHO for short. Sound waves returning from a solar active region on the hidden back side of the Sun have a round-trip travel time about twelve seconds shorter than the average of six hours. This timing difference permits scientists to detect potentially threatening active regions on the far side of the Sun before the Sun's rotation brings them around to the front side that faces the Earth. (Courtesy of the SOHO MDI/SOI consortium. SOHO is a project of international cooperation between ESA and NASA.)

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