Three instruments on SOHO (MDI, VIRGO, GOLF) detect rhythmic changes on the Sun's visible face, due to sound waves reverberating through its interior.
Just as seismologists probe the Earth's interior using earthquake waves, so helioseismologists deduce from the sound waves the behaviour of the hot gas in the Sun's turbulent interior. The MDI team has detected a subsurface flow of gas from the equator towards the poles at about 80 km/h. The measurements of the speed of sound have judged that some levels in the Sun's interior are hotter, some cooler than expected.
In this MDI picture different colours represent the relative rotation speeds of different zones. Red is fast and blue is slow.
