Helioseismology is the study of ‘sunquakes’. Astronomers detect in-and-out motion of the Sun’s surface by measuring the dark absorption lines in the Sun’s spectrum (Figure 1). If a patch of the Sun’s surface is moving towards us, the lines are Doppler shifted to a higher frequency (shorter wavelength), and if it moves inwards, away from us, the line frequencies are shifted downwards. Precise measurements reveal surface motion at speeds less than 1ms −1, with oscillation periods ranging from about 1.5 to 20 minutes.
The complex oscillations of the Sun’s surface can be expressed as a superposition of many million standing waves, each corresponding to one of the Sun’s natural resonant frequencies (Figure 2). Study of these oscillations reveals information about the density, temperature and composition of the Sun’s interior. In addition to the standing waves, tsunami-like waves are sometimes seen on the Sun’s surface, produced by violent events such as solar flares (Figure 3).
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