At right, is a map projection of calculated magnetic field strengths on the Earth-facing solar hemisphere with yellow and red indicating high magnetic fields characteristic of solar active regions. At left is a similar map of the solar hemisphere opposite planet Earth, which shows the large active region AR9393 as the 27-day solar rotation carried it across the far side . The largest sunspot group in a decade, AR9393 was easily seen as it tracked across the Sun's Earth-facing hemisphere in late March. Known as helioseismology , analyzing solar sound waves is like using seismological records of earthquakes to probe the interior of the Earth. On the Sun, sound waves are produced by turbulent convection cells seen on the surface as dynamic solar granules .
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