What is "helioseismology"

n. The study of the propagation of pressure waves in the Sun

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.

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.

Despite the name, helioseismology is the study of solar waves and not solar seismic activity. 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 studies the propagation of sound waves in stars.