Helioseismology is the study of the internal structure and dynamics of the Sun using its oscillations. The Sun is a star and, like all stars, it consists of a plasma – a hot ionized gas – held together by its own gravity. The pressure and temperature in the interior of the Sun are so high that nuclear fusion can take place, producing energy that makes the Sun shine.
The Sun’s convective zone is the outermost layer where convection currents circulate hot plasma up from the deep interior to cooler layers nearer the surface. These convection currents create turbulence which gives rise to sound waves that propagate through the Sun. Helioseismologists use these sound waves to probe conditions inside the Sun and map out its internal structure, much like how seismologists use earthquakes to study Earth’s interior.
Solar oscillations were first detected in 1960 by physicists Robert Leighton, Bruce Noyes and George Wetherill using Doppler shift measurements of spectral lines formed in iron atoms. Doppler shift is a change in wavelength caused by motion; if an object is moving towards you, wavelengths are compressed (shifted towards blue) while if an object is moving away from you, wavelengths are stretched (shifted towards red). By measuring Doppler shifts in iron spectral lines coming from different parts of the solar surface at different times, Leighton et al. were able to construct a map of acoustic wave propagation through the solar interior – essentially a “soundmap” of conditions inside our nearest star.
Since then, helioseismology has evolved into a highly sophisticated field with numerous applications. Helioseismic data have been used to measure properties such as rotation rate, diameter and age of stars other than our own Sun; they have also been used to test theories about dark matter and investigate possible explanations for why our Universe appears to be expanding at an accelerating rate. In addition to providing valuable insights into stellar evolution and cosmology, helioseismology also plays an important role in forecasting space weather events such as solar flares which can impact Earth-based technologies such as satellites and power grids