spaceplasma: Helioseismology: seismology of the...

Helioseismology: seismology of the Sun
The Sun oscillates and vibrates at many frequencies, like an ocean surface or …like a bell. Certain frequencies are amplified by constructive interference(wave propagation) and the turbulence “rings” the sun like a bell. Unfortunately, sound does not carry through the vacuum between the Sun and the earth, so we have to “listen“ to the oscillations by looking at the motions of material on the surface of the Sun. With the right instruments, scientists can "hear” this ringing or pulsations from the Sun. To do this, they use an instrument called a Michelson Doppler Imager (MDI), mounted on the SOHO spacecraft and the Helioseismic and Magnetic Imager (HMI), one of the three instruments that make up the Solar Dynamics Observatory (SDO).
Although direct study of its interior is impossible —mostly because the Sun is nearly opaque to electromagnetic energy, insights into the conditions within the Sun may be gained by observing oscillating waves, rhythmic inward and outward motions of its visible surface. These oscillations on the surface are due to sound waves generated and trapped inside the sun. Sound waves are produced by pressure fluctuations in the turbulent convective motions of the sun’s interior. These trapped sound waves set the sun vibrating in millions of different patterns or modes. Using this acoustic energy, we can “see into the Sun”, just as geologists use seismic waves to study the structure of the Earth, the discipline of helioseismology makes use of acoustic pressure waves (infrasound) traversing the Sun’s interior. These oscillations are seen as volumes of gas called granules near the Sun’s surface that rise and fall with a particular frequency. It is like seeing the rolling motions of convection cells on the surface of boiling water. This happens very close to the surface where the flow of energy that started in the nuclear reactions in the core reaches the surface and suddenly escapes. The sound from the convection is then trapped and filtered inside the sun to produce the solar music.
Helioseismologists can use the properties of these waves to determine the temperature, density, composition, and motion of the interior of the sun. The spectral lines emitted from gas moving upwards will be slightly Doppler-shifted to the blue; spectral lines from gas moving downwards will be slightly Doppler-shifted to the red. In this way the rolling motions of convection near the Sun’s surface can be mapped out. There are three types of oscillations. Pressure modes (p-modes) are sound waves trapped in the temperature gradient (like an echo bouncing around inside a cavern). Fundamental modes (f-modes) or surface gravity waves are caused by gravitational interactions with the sun’s surface and resemble ocean waves. Gravity modes (g-modes) are not completely understood, but they are believed to be the result of buoyancy effects. All the known pressure and fundamental modes (some 10 million) have oscillation periods of less than 18 minutes, and most are around 5 minutes. The gravity modes are not known conclusively to exist, but they are predicted to have periods of 40 minutes or longer (160-min)
Further readings:
Imaging sunspots using helioseismic methods
Helioseismology and the Sun’s interior
Global Oscillation Network Group
Helioseismic and Magnetic Imager
Magnetohydrodynamical (MHD) simulations of solar convection