Solar seismology

Solar seismology, or helioseismology, is the study of the oscillations that take place within the Sun. These periodic vibrations originate in the Sun’s convective zone. By analyzing the motion of these oscillations, scientists can image the solar interior and develop more accurate models of the Sun.

Overview

In 1960, Robert Leighton of the California Institute of Technology discovered that small areas of the Sun’s surface oscillated up and down with a period of about five minutes. He observed that the spectra of various points on the Sun’s surface were Doppler-shifted alternately toward shorter and longer wavelengths. The Doppler effect shifts electromagnetic radiation to shorter wavelengths (termed “blueshifts”) when the source moves toward the observer and to longer wavelengths (termed “redshifts”) when the source moves away from the observer. However, he was not able to tell whether the oscillations occurred only in small regions or over the entire Sun.

A few years later, it was found that the entire Sun vibrates. Like many other scientific discoveries, it happened while trying to measure something else. Astronomers were attempting to measure the oblateness of the Sun to discriminate between Einstein’s general theory of relativity and a competing theory of gravity—the Brans-Dicke scalar-tensor theory. Since the mid-1800s, it had been known that Mercury’s orbit around the Sun precesses; that is, the elliptical orbit itself slowly rotates about the focus occupied by the Sun. Most of the observed precession could be explained by the other planets gravitationally perturbing Mercury according to Sir Isaac Newton’s laws of gravity and motion, but a small residual was unaccounted for. The existence of an unknown planet, the legendary Vulcan, inside the orbit of Mercury was suggested to provide the extra gravitational perturbation. Searches were made, and some even reported spotting Vulcan, but none of the “discoveries” were confirmed.

In the early 1900s, Einstein developed his general theory of relativity. One of its consequences is that mass distorts space-time in its vicinity. Thus, Mercury’s orbit would precess because of the curvature of space-time around the Sun. The amount predicted by general relativity for Mercury’s precession matched exactly the observed discrepancy, and this was hailed as major evidence supporting the theory.

A half-century later, Carl Brans and Robert H. Dicke developed an alternative to general relativity called the Brans-Dicke scalar-tensor theory. It, too, predicted the precession of Mercury’s orbit if the Sun were slightly oblate—that is if the Sun had a small equatorial bulge due to its rotation. In the early 1960s, Dicke’s team at Princeton University claimed to have measured a small oblateness of the Sun using a telescope of their own design and construction. A few years later, Henry Hill of the University of Arizona built a telescope that was designed specifically to detect a distortion in the shape of the Sun. He found no evidence of any distortion, but after further study, Hill and his colleagues discovered periodic oscillations of the Sun’s entire surface.

Since then, it has been determined that the entire surface of the Sun is in a state of constant oscillation, with periods varying between minutes and hours. It might be said that the Sun is ringing like a bell being struck continuously. The origin of these vibrations is the convective zone beneath the photosphere or “visible surface” of the Sun. Bubbles of hot gas rise in convection cells, carrying heat from the Sun’s interior to the surface. The tops of these convection cells produce the granulation seen in images of the photosphere. The hot bubbles rise toward the surface, accompanied by a tremendous roar. These sound waves oscillate through the Sun and cause its surface to rise and fall periodically as they are reflected by the bottom of the photosphere. As the waves travel back downward into the Sun, they encounter higher temperatures and pressures. These changing physical conditions result in the waves’ velocity being increased, which eventually causes the waves to refract or bend upward toward the surface. When they reach the surface, again, the waves are reflected back downward. The depth that the wave reaches depends upon its wavelength. The wavelength also determines how far a wave will travel around the Sun between reflections from the surface.

The Sun’s interior is conducting waves with virtually millions of different wavelengths and frequencies. Some waves have the exact wavelength necessary to make an even number of bounces before they return to where they began. Astronomers categorize these waves by the number of times that they reflect from the surface in one complete circuit of the Sun. For example, a wave with the designation I-4 strikes the surface in three places before it bounces back to its starting position. Once it returns to its origin, it has struck the surface of the Sun four times. Scientists have found that waves with low I numbers travel deep into the Sun and can reveal physical characteristics there, while waves with higher I numbers probe the shallower zones closer to the Sun’s surface.

Knowledge Gained

On Earth, geologists use seismic waves (produced naturally by earthquakes or artificially by setting off explosions or by “thumping” the ground) as probes of the Earth’s interior. Astronomers use solar seismic waves in an analogous way to image the interior of the Sun, which cannot be observed directly. Prior to this new development in solar physics, knowledge of the processes that occur within the Sun and the locations of various boundaries within the Sun came only from computer-generated models of its interior.

The generally accepted model of the Sun’s interior is called the Standard Solar Model (SSM). Observed frequencies and wavelengths of observed solar oscillations, for the most part, are consistent with the SSM to within 0.1 percent. For example, within the Sun’s core, hydrogen nuclei are fused together to form helium nuclei. This process converts a small fraction of the input mass into energy that keeps the Sun shining. The SSM indicates the central temperature needed to maintain this reaction at the right rate to account for the Sun’s Luminosity is about fifteen million kelvins. Vibration patterns observed on the Sun’s surface but that travel through the deep interior are consistent with this temperature.

Direct observation shows that the Sun’s surface rotates differentially—the equatorial region has a shorter rotation period than the polar regions. Helioseismology provides a way to study the internal rotation of the Sun, and it turns out to be quite complex. At shallow depths, there is a zonal flow with alternating bands moving faster and slower than the average. Just below the surface are wide rivers moving more slowly near the equator and faster near the poles, just the opposite of what the surface itself does. The base of the convection zone oscillates in rotational speed with a period of 1.3 years, sometimes moving faster than the surface and sometimes more slowly than the surface by about 10 percent. The deeper radiative zone, including the core, rotates reasonably uniformly with a period of about twenty-seven days.

Context

The discovery that the surface of the Sun is oscillating was made in the early 1960s. At the time, scientists were gathering data on the oblateness of the Sun. As it turned out, there was no measurable oblateness of the Sun, but subsequent observations revealed an alternating Doppler shift in the solar spectra taken from various points on the Sun’s surface. This Doppler shift provided evidence of periodic oscillations. Further investigations revealed that the Sun is ringing as if it were a large bell that is continuously being struck. The millions of different wavelengths and frequency combinations of waves are believed to originate within the Sun’s convective zone. In this region, the tremendous heat from the interior is carried outward toward the surface by rising bubbles of hot gas. The sound waves given off by this movement of hot gases cause the Sun to vibrate.

The Global Oscillations Network Group (GONG) has provided extensive observations of solar oscillations. GONG consists of an array of fifty-millimeter-aperture refracting telescopes at various locations around the Earth. These locations were selected to ensure that at least two telescopes could gather data from the Sun at all times, enabling astronomers to differentiate real oscillations, together with their amplitudes and periods, from noise produced by instrumentation or Earth’s rotation. Each of the telescopes contains a Fourier tachometer. This device is capable of measuring extremely small Doppler shifts at more than sixty-five thousand different points on the surface of the Sun. By observing these shifts, astronomers can determine oscillation periods of these various points and form a detailed picture of the solar disk. This large amount of data is necessary to determine the paths that the waves follow through the Sun and convert that information into a model of the solar interior. The Solar and Heliospheric Observatory (SOHO), launched by the European Space Agency (ESA) in 1995 and on station between the Earth and the Sun, also provides continuous monitoring of solar oscillations. Scientists have also analyzed over ten years of data collected by the Solar Dynamics Observatory which showed global oscillations of the Sun over much longer periods.

Helioseismic waves act as probes of the solar interior that enable solar scientists to map the interior of the Sun and solve some of the perplexing problems in solar physics. By observing oscillations visible on the Sun’s surface, astronomers are assembling a detailed picture of the solar interior, in much the same way that geologists gain knowledge of Earth’s interior by the study of seismic waves traveling through our planet. For example, solar oscillations provide information about the temperature and density of the Sun from its surface down to its core. They reveal the rotational speeds of the internal layers of the Sun. Helioseismic imaging helps delineate the boundaries between the convective zone, radiative zone, and the core itself. Solar seismology also provides clues as to how energy is transferred from the solar surface to the chromosphere and corona. (It is believed that intense magnetic fields, along with acoustic shock waves from the tops of convecting cells, are responsible for high temperatures in the chromosphere and the corona—up to two million kelvins in the corona.) In sum, the accumulation of helioseismic data over the years is helping astronomers form an accurate model of the conditions and processes occurring both on the solar surface and in the interior.

Bibliography

Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. New York: Addison-Wesley, 2008.

Cox, A. N., W. C. Livingston, and M. S. Matthews, eds. Solar Interior and Atmosphere. Tucson: University of Arizona Press, 1991.

Fraknoi, Andrew, David Morrison, and Sidney Wolff. Voyages to the Stars and Galaxies. Belmont, Calif.: Brooks/Cole-Thomson Learning, 2006.

Freedman, Roger A., and William J. Kaufmann III. Universe. 8th ed. New York: W. H. Freeman, 2008. .

“Global Oscillations Network Group - NSO.” National Solar Observatory, 2023, nso.edu/telescopes/nisp/gong. Accessed 19 Sept. 2023.

Gribbin, John R. Blinded by the Light: The Secret Life of the Sun. New York: Harmony Books, 1991.

Lopresto, James Charles. “Looking Inside the Sun.” Astronomy 17 (March, 1989): 20-28.

Mitton, Simon. Daytime Star: The Story of Our Sun. New York: Charles Scribner’s Sons, 1981.

Planck, Max. “Long-Period Oscillations of the Sun Discovered - Astronomy.” Phys.org, 20 July 2021, phys.org/news/2021-07-long-period-oscillations-sun.html. Accessed 19 Sept. 2023.

Schneider, Stephen E., and Thomas T. Arny. Pathways to Astronomy. 2d ed. New York: McGraw-Hill, 2008.