The Sounds of the Sun
The sun continuously generates sound waves. These solar sounds are recorded by observing vibrations of the sun's surface and are used by solar seismologists to study the structure and dynamics of the sun’s interior. Inside the sun huge vortexes of hot plasma produce sunspots – concentrations of strong magnetic field that are often bigger than the Earth in size. The magnetic energy of sunspots is a source of flares and solar storms that affect the Earth’s space environment and life. Observations of solar sounds help us to learn about the mechanisms of the sun’s magnetic activity, monitor the solar subsurface weather, and predict formation of sunspots. Some of these sound waves are captured inside the sun causing the whole sun to resonate, providing also information about the central energy-generating core. Samples of the solar sounds converted to the audible frequency range are provided by Dr Kosovichev, http://sun.stanford.edu/~sasha/SOUNDS/.
The solar vibrations have been observed uninterruptedly by the SOHO spacecraft and GONG network of ground-based stations for more than 10 years. However, the mechanism of the sound generation is not fully understood. The sounds are produced by small turbulent motions in granules - convection cells covering the solar surface. However, the sound-generating turbulent motions are not visible even with the largest telescope. Therefore, for understanding how the sun makes the sound waves scientists use realistic computer models. Such modeling was initiated by Prof Robert Stein (Michigan State University) and Prof Aake Nordlund (University of Copenhagen) several years ago. The computer models try to describe as accurately as possible the complicated processes of the energy transport below the solar surface. However, a key element, description of the small turbulent motions, was missing.
Similar turbulence problems arise in many areas of fluid dynamics including weather forecast and modeling of aircraft engines. A group of scientists at Stanford and NASA Ames Research Center combined their expertise in turbulence modeling and solar seismology and developed a new computer model that accurately describes generation of solar sounds. They discovered that so-called “Dynamic Smagorinsky’s Model” previously developed at the Stanford Center for Turbulence Research for engineering applications provides the best description of the solar sound spectrum among other turbulence models. This result provides important insight in the generation of solar sounds, and also opens perspectives for modeling and understanding other turbulent phenomena on the sun.
In particular, it has been known from helioseismology observations that magnetic regions around sunspots generate higher frequency sounds than the normal “quiet-sun” regions. This phenomenon was called “acoustic halos” and puzzled solar seismologists for years. The new computer model of solar turbulence reproduced the phenomenon and provided a solution to this puzzle. It is found that in the presence of magnetic field the solar granules become smaller, and turbulent motions in these granules become faster generating higher pitch sounds than in the quiet sun. The result is important for developing the helioseismic of sunspots and magnetic regions on the sun and also on other stars. Next year, NASA is preparing two space missions, Solar Dynamics Observatory and Kepler, which among other tasks will “listen” to solar and stellar sounds.