Looking Inside The

tte interior of the Sun is as opaque as a stonewall, and there is no way you can see inside it! But we can illuminate, or sound, the hidden depths of the Sun by recording oscillations of its visible disk, which depend on conditions inside the Sun.

Some of the oscillations are created by sound waves that have moved just beneath the part of the Sun we can see; others arise from sounds that have traveled deep into the Sun's interior. We can therefore use them to open a window into the Sun and look at various levels within it. Moreover, the information can be combined to create a picture of the Sun's large-scale internal structure, somewhat in the manner of an X-ray CAT scan that probes the inside of a human's brain.

Geophysicists similarly construct models of the Earth's interior by recording earthquakes, or seismic waves, that travel to different depths; this type of investigation is called seismology. Most earthquakes occur just beneath the Earth's surface when massive blocks of rock slip and crunch against one another, tte reverberations move out and propagate throughout the terrestrial interior, like ripples spreading out from a disturbance on the surface of a pond.

Solar oscillations arise from a persistent, random turbulence in the outer convec-tive regions of the Sun, and similarly shake it to its very center. In fact, astronomers use the name helioseismology to describe such studies of the Sun's interior; it is a hybrid name combining the Greek words helios for the "Sun" and seismos for "earthquake" or "tremor."

Seismic waves move in all directions through the Earth and their arrival at various places on the Earth's surface is recorded by seismometers. By combining the arrival times of waves that have traveled through the Earth to various points on the surface, seismologists can pinpoint the origin of the waves and trace their motions through the Earth, ttis enables them to construct a profile of the Earth's interior. In analogy with terrestrial seismology, it is now possible to use observations of the five-minute solar oscillations to isolate sound waves penetrating to a given depth. Ms has resulted in precise and detailed information about the properties of the solar interior, rivaling our knowledge of the inside of the Earth.

Of course, unraveling the internal constitution of either the Earth or the Sun is not quite as simple as it might seem. One geophysicist has compared seismology to determining how a piano is constructed by listening to one falling down a flight of stairs, tte situation is even worse for the Sun, where the observed five-minute oscillations result from millions of different sound waves, with new ones starting up and old ones dying away all the time. One therefore has to measure the wavelength, or frequency, of numerous solar oscillations with incredible precision, and then compare them with theoretical expectations using large computer programs and simplified models of the Sun. Differences between the observed and predicted wavelengths can then be used to test and refine the models.

FIG. 4.5 Radial variations ofsound speed Just as scientists can use earthquake measurements to determine conditions under the Earth's surface, measurements of the Sun's oscillations, and the sound waves that produce them, can be used to determine the internal structure of the Sun. This composite image shows the extreme ultraviolet radiation of the solar disk (orange) and internal measurements of the speed of sound (cutaway). In the red colored layers in the solar interior, sound waves travel faster than predicted by the Standard Solar Model (yellow), implying that the temperature is higher than anticipated. Blue corresponds to slower sound waves and temperatures that are colder than expected. The conspicuous red layer, about a third of the way down, shows unexpected high temperatures at the boundary between the turbulent outer region (convective zone) and the more stable region inside it (radiative zone). The disk measurements were made at a wavelength of 30.38 nanometers, emitted by singly-ionized helium, denoted He II, at a temperature of about 60,000 kelvin using the Extreme-ultraviolet Imaging Telescope, abbreviated EIT, aboard the SOlar and Heliospheric Observatory, or SOHO for short, and the MDI/SOI and VIRGO instruments on SOHO made the sound measurements over a period of 12 months beginning in May 1996. (Composite image courtesy of Steele Hill and SOHO, a project of international cooperation between ESA and NASA.)

By considering a sequence of waves with longer and longer wavelengths, that penetrate deeper and deeper, it is possible to peel away progressively deeper layers of the Sun and establish the radial profile of the sound speed (Fig. 4.5). A small but definite change in sound speed marks the lower boundary of the convective zone, indicating that it is located at a radius of 71.3 percent, or extends to a depth of 28.7 percent, of the radius of the visible Sun. Moreover, this convective zone depth, measured by the helio-seismology technique in 1991, agreed at the time with calculations using the Standard Solar Model of the solar interior.

Nevertheless, there has never been perfect agreement between the measured sound speed and the predictions of the model. Helioseismology measurements of the sound speed just below the convective zone were substantially higher than those of the models. And the accord on the zone's depth was slightly disrupted by measurements, in 2003, oflow abundances for the lighter metals in the solar disk.

tte Sun appears to contain 30 to 40 percent less carbon, nitrogen, oxygen, neon and argon than previously believed, ttese elements provide an opacity that impedes the outward flow of radiation, like dirt that blocks light flowing through a window. And when these opacities are used in detailed models of the Sun, they predict slightly different values for the speed of sound than those measured by helioseismology at the same locations.

tte solar models constructed with the revised, lower element abundances and corrected opacities yield a depth of the solar convection zone of 27.4 percent, at a radius of 72.6 percent of the Sun's radius, disagreeing with the measured value by helioseismol-ogy by a small but significant 1.3 percent, ttere is also a discord between the helium abundance calculated by the new model and the amount inferred from helioseismology; the new value is lower, at 23 percent by mass, rather than about 25 percent as previously thought. So there is room for improvement in our solar models, perhaps by consideration of magnetic fields, rotation or mixing that were previously omitted in the calculations.

Fortunately, the discrepancies occur at a temperature range that is too cool and distant from the Sun center to significantly affect the helioseismic measurements of the Sun's hotter, deeper temperatures. So the model calculations of the amount of neutrinos emitted by the Sun remain the same.

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