The Sun oscillates. Think of a drum. A drum vibrates when it is hit, which excites its resonant modes. Which modes are excited depends on the skin head and where it is hit. The Sun is similar. It has a resonant cavity in which acoustic waves are trapped, like an organ pipe. Resonance occurs when an integral number of half wavelengths fit into the cavity between the turning points where the waves are reflected or refracted. In the Sun acoustic waves are trapped by reflections near the surface where the temperature drops rapidly and the wavelength of the waves becomes larger than the scale height of the density stratification. The waves are trapped in the solar interior where the increasing temperature and sound speed refracts that waves back up toward the surface.
The Sun’s resonant modes are excited by turbulence in the convection zone. In realistic solar convection zone simulations, acoustic oscillations are also naturally excited. Such simulations have been used to study wave propagation properties, resonant mode excitation, effects on mode frequencies, and as a known data set to test, validate and refine local helioseismic inversion methods. Local helioseismology is a powerful tool for probing the subsurface layers of the Sun. For a review of its methods and results see Gizon and Birch (2005) and Zhao (2008).
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