http://en.wikipedia.org/wiki/Helioseismology wrote:
<<Helioseismology is the study of the propagation of wave oscillations, particularly acoustic pressure waves, in the Sun. Unlike seismic waves on Earth, solar waves have practically no shear component (s-waves). Solar pressure waves are believed to be generated by the turbulence in the convection zone near the surface of the sun. Certain frequencies are amplified by constructive interference. In other words, the turbulence "rings" the sun like a bell. The acoustic waves are transmitted to the outer photosphere of the sun, which is where the light generated through absorption of radiant energy from nuclear fusion at the centre of the sun, leaves the surface. These oscillations are detectable on almost any time series of solar images, but are best observed by measuring the Doppler shift of photospheric absorption lines. Changes in the propagation of oscillation waves through the Sun reveal inner structures and allow astrophysicists to develop extremely detailed profiles of the interior conditions of the Sun.
Helioseismology was able to rule out the possibility that the solar neutrino problem was due to incorrect models of the interior of the Sun. Features revealed by helioseismology include that the outer convective zone and the inner radiative zone rotate at different speeds, which is thought to generate the main magnetic field of the Sun by a dynamo effect, and that the convective zone has "jet streams" of plasma (more precisely, torsional oscillations) thousands of kilometers below the surface. These jet streams form broad fronts at the equator, breaking into smaller cyclonic storms at high latitudes. Torsional oscillations are the time variation in solar differential rotation. They are alternating bands of faster and slower rotation. So far there is no generally accepted theoretical explanation for them, even though a close relation to the solar cycle is evident, as they have a period of eleven years, as was known since they were first observed in 1980. An internal jet stream moving behind schedule may explain the delayed start to the solar cycle in 2009.
Helioseismology can also be used to image the far side of the Sun from the Earth, including sunspots. In simple terms, sunspots absorb helioseismic waves. This sunspot absorption causes a seismic deficit that can be imaged at the antipode of the sunspot. To facilitate spaceweather forecasting, seismic images of the central portion of the solar far side have been produced nearly continuously since late 2000 by analysing data from the SOHO spacecraft, and since 2001 the entire far side has been imaged with this data.
Keep in mind that despite the name, helioseismology is the study of solar waves and not solar seismic activity - there is no such thing. The name is derived from the similar practice of studying terrestrial seismic waves to determine the composition of the Earth's interior. The science can be compared to asteroseismology, which considers the propagation of sound waves in stars.
Solar oscillation modes are essentially divided up into three categories, based on the restoring force that drives them: acoustic, gravity, and surface-gravity wave modes.
* p-mode or acoustic waves have pressure as their restoring force, hence the name "p-mode". Their dynamics are determined by the variation of the speed of sound inside the sun. P-mode oscillations have frequencies > 1 mHz and are very strong in the 2-4 mHz range, where they are often referred to as "5-minute oscillations". (Note: 5 minutes per cycle is 1/300 cycles per second = 3.33 mHz.) P-modes at the solar surface have amplitudes of hundreds of kilometers and are readily detectable with Doppler imaging or sensitive spectral line intensity imaging. Thousands of p-modes of high and intermediate degree l (see below for the wavenumber degree l) have been detected by the Michelson Doppler Imager (MDI) instrument aboard the SOHO spacecraft, with those of degree l below 200 clearly separated and higher degree modes ridged together. About 10 p-modes below 1.5mHz have been detected by the GOLF instrument aboard the SOHO spacecraft.
* g-mode or gravity waves are density waves which have gravity (negative buoyancy of displaced material) as their restoring force, hence the name "g-mode". The g-mode oscillations are low frequency waves (0-0.4 mHz). They are confined to the interior of the sun below the convection zone (which extends from 0.7-1.0 solar radius), and are practically inobservable at the surface. The restoring force is caused by adiabatic expansion: in the deep interior of the Sun, the temperature gradient is weak, and a small packet of gas that moves (for example) upward will be cooler and denser than the surrounding gas, and will therefore be pulled back to its original position; this restoring force drives g-modes. In the solar convection zone, the temperature gradient is slightly greater than the adiabatic lapse rate, so that there is an anti-restoring force (that drives convection) and g-modes cannot propagate. The g modes are evanescent through the entire convection zone, and are thought to have residual amplitudes of only millimeters at the photosphere, though more prominent as temperature perturbations.
* f-mode or surface gravity waves are also gravity waves, but occur at or near the photosphere, where the temperature gradient again drops below the adiabatic lapse rate. Some f-modes of moderate and high degree, between l = 117 and l = 300, (see below for the wavenumber degree l) have been observed by MDI.
The data from time-series of solar spectra shows all the oscillations overlapping. Thousands of modes have been detected. The simplest modes to analyse are the radial ones; however most solar modes are non-radial. A nonradial mode is characterized by three wavenumbers: the spherical-harmonic degree l and azimuthal order m which determine the behaviour of the mode over the surface of the star and the radial order n which reflects the properties in the radial direction. Note that if the Sun were spherically symmetric, the azimuthal order would exhibit degeneracy; however the rotation of the Sun (along with other perturbations), which leads to an equatorial bulge, lifts this degeneracy.
The age of the sun can be inferred with helioseismic studies. This is because the propagation of acoustic waves deep within the sun depends on the composition of the sun, in particular the relative abundance of helium and hydrogen in the core. Since the sun has been fusing hydrogen into helium throughout its lifetime, the present day abundance of helium in the core can be used to infer the age of the sun, using numerical models of stellar evolution applied to the Sun (Standard solar model). This method provides verification of the age of the solar system gathered from the radiometric dating of meteorites.>>
Echoes from the Depths of a Red Giant Star
