Time-Distance Helioseismology

THE application of seismology to the study of the solar interior1, 2(helioseismology) has advanced almost solely by the prediction and measurement of the Sun's frequencies of free oscillation, or normal modes. Direct measurement of the travel times and distances of individual acoustic waves—the predominant approach in terrestrial seismology3—would appear to be more difficult in view of the number and stochastic nature of solar seismic sources. Here, however, we show that it is possible to extract time-distance information from temporal cross-correlations of the intensity fluctuations on the solar surface. This approach opens the way for seismic studies of local solar phenomena, such as subsurface in homogeneities near sunspots, and should help to refine global models of the internal velocity stratification in the Sun.