Helioseismology
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Helioseismology is the study of how pressure waves propagate in the Sun. These waves are generated by the turbulence near the surface of the sun, and certain frequencies are amplified by constructive interference. In other words, the turbulence "rings" the sun like a bell. These oscillations are detectable on almost any time series of solar images, but are best observed by measuring the doppler shift of photospheric emission lines. Changes in the propagation of pressure waves through the Sun reveal inner structures and allows 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 to generate the main magnetic field of the Sun, and that the convective zone has "jet streams" of plasma thousands of kilometers below the surface. These jet streams form broad fronts at the equator, breaking into smaller cyclonic storms at high latitudes.
Helioseismology can also be used to detect sunspots on the far side of the Sun from Earth.
Keep in mind that despite the name, Helioseismology is the study of solar pressure 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.
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Types of solar oscillations
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Solar oscillations are essentially divided up into three categories: acoustic, gravity, and surface-gravity waves.
- 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.)
- g-mode or gravity waves are essentially density waves which have gravity 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, and are practically unobservable.
- f-mode or surface gravity waves are also gravity waves, but occur at or near the solar surface.
External links
- Lecture Notes on Stellar Oscillations (http://www.obs.aau.dk/~jcd/oscilnotes/) by Jørgen Christensen-Dalsgaard
Satellite instruments
- GOLF (http://www.astro.ucla.edu/~bertello/golf.html)
- VIRGO (http://www.ias.u-psud.fr/virgo/html/virgomain.html)
- SOI/MDI (http://soi.stanford.edu)
- TRACE (http://sunland.gsfc.nasa.gov/smex/trace/)
Ground instruments
- BiSON (http://bison.ph.bham.ac.uk/)
- Mark-1
- GONG (http://helios.tuc.noao.edu/)
- HiDHN (http://physics.usc.edu/solar)