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Although direct study of its interior is impossible, insights into the conditions – temperature, composition, and motions of gas – within the Sun may be gained by observing oscillating waves, rhythmic inward and outward motions of its visible surface. The study of these solar oscillations is called helioseismology. In many ways, it resembles the study of seismic waves generated by earthquakes to learn about the Earth's interior.

The complex pattern of periodic throbbing motions appears on the surface due to acoustic (sound) waves that are trapped inside the Sun. Although they cannot be observed with the naked eye, the tiny motions can be detected as subtle shifts in the wavelength of the spectral absorption lines. Light from a region that is rising displays a shortening in wavelength which causes a small blue shift. Sinking columns are slightly redshifted. Images showing the overall pattern of these spectral shifts are known as Dopplergrams.

The movements are also visible as minuscule variations in the Sun's light output. Based on shifts in specific spectral lines, the helioseismic images reveal millions of vertical, gaseous motions generated by sound waves all over the photosphere. The most intense of these are low‐frequency waves that oscillate on a time scale of about 5 minutes, coinciding with velocities of 0.5 km/s. However, the overall pattern is extremely complex – the result of millions of oscillations, both large and small, that simultaneously resonate with periods ranging from a few minutes to one hour. Motions as slow as a few millimeters per second have been detected, but they may also be remarkably long‐lived, persisting for up to one year.

Although observations of the Sun from any single location on Earth (except the poles) are generally limited in duration due to the planet's rotation, a worldwide network of observatories known as the Global Oscillation Network Group (GONG) has been set up to enable 24‐hour helioseismic studies. Meanwhile, spacecraft such as SOHO have an uninterrupted view of the Sun from orbit.

Figure 2.13 A computer representation of one of nearly ten million modes of sound wave oscillations of the Sun, showing receding regions in red and approaching regions in blue. By measuring the frequencies of many such modes and using theoretical models – a technique known as helioseismology – it is possible to infer a great deal about the Sun's internal structure and dynamics. Such oscillations are measured by the ground‐based Global Oscillation Network Group (GONG), as well as orbiting observatories such as SOHO.

(National Solar Observatory/AURA/NSF)

It turns out that the entire Sun is ringing like a bell, with global oscillations that may continue for weeks. Each of the 10 million sound waves reverberates around the interior before it reaches the surface. Waves of different frequencies descend to different depths. On their return journey, they are influenced by changes in temperature, density, and composition, just like seismic waves inside Earth.

The lower‐pitched waves, with a frequency of about 3 MHz (a 5‐minute period), have been used to probe the solar interior and even to make images of the far side of the Sun, when they give advance warning of flares and active regions before they appear around the limb and start to impact Earth.

One of the most significant results has been the recognition of a small change in sound speed at a radial distance from the center of 71.3%, marking the lower boundary of the convection zone. Measurements of the speed of the waves indicate a helium composition of 23–26% in the convection zone – consistent with other calculations of helium abundance.