helioseismic

Helioseismic analysis, commonly referred to as helioseismology, is the study of oscillations and waves in the Sun to infer its internal structure and dynamics. By analyzing tiny variations in the Sun's surface velocity and brightness, researchers deduce properties of the solar interior that cannot be measured directly.

Oscillations arise from convective motions near the surface and propagate as standing or traveling waves. The most readily observed are global p-modes (pressure modes) with periods around five minutes; f-modes; and gravity modes (g-modes), which are theoretical predictions and rarely detected. Observations rely on Doppler shifts and intensity fluctuations collected by ground-based networks such as GONG and space-based instruments on spacecraft like SOHO and the Solar Dynamics Observatory; data sets from HMI, MDI, and others provide long, continuous time series.

Global helioseismology uses the frequencies of many p-modes to infer the radial profiles of sound speed, density, and rotation. Local helioseismology uses techniques such as time-distance analysis, ring-diagram analysis, and holography to image subsurface flows and inhomogeneities over smaller regions of the solar interior.

Key results include the solar rotation profile, with differential rotation in the convection zone and near-solid-body rotation in the radiative interior; the tachocline at the base of the convection zone; inferences on solar composition and helium abundance; tests of solar models; and constraints on how the solar cycle affects internal dynamics.

History notes that observations of the Sun's five-minute oscillations began in the early 1960s, and the field matured through the late 20th century with contributions from researchers such as Duvall and Christensen-Dalsgaard, among others.