helioseismographs

Helioseismographs are instruments designed to measure oscillations on the Sun's surface, enabling the study of the solar interior through helioseismology. They record tiny fluctuations in the Sun's surface velocity or brightness that are driven by acoustic waves within the solar interior.

Most helioseographs measure line-of-sight Doppler shifts of photospheric spectral lines to track vertical motions, while some instruments monitor brightness variations with high precision. Modern devices use spectrometers or Fabry-Pérot interferometers, and in space even Sun-as-a-star photometry supplements velocity measurements.

Global helioseismology analyzes disk-integrated data to identify a large set of acoustic modes characterized by radial order n and angular degree l, yielding the solar sound-speed and density profiles. Local helioseismology uses spatially resolved data to map subsurface flows and structures via time-distance analysis, ring-diagram analysis, and related methods.

The field emerged in the 1960s with detections of solar oscillations; since then space missions and networks have provided long, continuous observations. Notable instruments include the MDI instrument on the SOHO spacecraft and the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory, as well as ground-based networks such as the Global Oscillation Network Group (GONG) and the Birmingham Solar-Oscillations Network (BiSON).

Results from helioseographs constrain models of the Sun's internal rotation, sound speed, and density, and track changes with the solar cycle. They have helped locate the tachocline, test theories of solar evolution, and improve estimates of solar age and composition.

Challenges include instrumental and environmental noise, especially for ground-based measurements, as well as mode leakage and data calibration requirements for accurate inferences.