Note: The text on this page has not been updated since it was written in 1996.
The Sun is oscillating! Interference patterns from ten million resonant modes of oscillation are detectable on the surface of the Sun. The periods of these modes may range from minutes to hours, and the (horizontal) wavelength may be as short as a few thousand kilometers or as long as the radius of the Sun itself. The frequencies of solar oscillation modes are determined by the structure and dynamics of the Sun; thus, we can learn about the interior of our star by understanding what these oscillations mean.
Helioseismology, the study of solar oscillations, began as recently as 1960 with the discovery that the Sun was covered in little patches that seemed to oscillate with periods near five minutes. Although these five-minute oscillations were initially taken to be local disturbances, a decade later it was suggested that these oscillations were actually the superposition of resonant acoustic modes in the interior of the Sun.
As the theories of helioseismology were developed, it was discovered that the solar convective envelope was much deeper than was predicted from standard solar models. Also, it became possible to better constrain the interior helium abundance and thus, its effects on the neutrino flux. Other major triumphs of helioseismology have been determining the angular velocity and the adiabatic soundspeed throughout much of the interior of the Sun. Standard solar models must now compare their results against helioseismological data.