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helioseismology(heel-ee-oh-sÿz-mol -ŏ-jee) The study of the Sun's interior by the observation of solar oscillations. First detected in 1960 by measuring the Doppler shifts of Fraunhofer lines at several points on the Sun's disk away from active regions, the oscillations are a rhythmic rise and fall of regions of the solar photosphere (and lower chromosphere) several thousand kilometers in diameter. They have periods of about 5 minutes, persist at any one point for less than half an hour, and attain a maximum velocity of about 0.5 km s–1. The oscillations are thought to be produced by the outward propagation of low-frequency sound waves, generated by turbulence in the convective zone, and by analyzing their global occurrence over an extended period of time it is possible to build up a picture of the Sun's interior.
Instruments used in the observation of solar oscillations include grating spectrographs, narrow-band interference filter (Lyot filter, magneto-optical filter, or Fabry–Perot interferometer) imagers, and Michelson interferometers – notably the Fourier tachometer, which has been in operation since 1985 and simultaneously monitors more than 60 000 points on the Sun's disk. A number of collaborative programs have been established, for example the Global Oscillations Network Group (GONG). These are complemented by continuous monitoring by the helioseismology instrument on ESA's spacecraft, the Solar Heliospheric Observatory (SOHO).