Harmonies of the Sphere
The Greek mathematician Pythagoras imagined a universe described by numbers. When he talked about the “harmonies of the spheres,” he meant music that enlightened individuals might hear resulting from the translucent shells that carried the celestial objects. Kepler continued this line of thought and applied it to the elliptical orbits of the planets. These ideas sound archaic but they’re not misguided. The universe contains many periodic and oscillatory phenomena, and the formalism to understand them involves studying the resulting harmonic frequencies.23 Situations as different as planet, moon, and ring systems and their orbits and interactions, the spiral arms of the Milky Way, and the interactions between matter and radiation in the early universe are well described in terms of coupled frequencies and harmonics. We’ve seen beautiful examples of this in the complex gravitational dance of Saturn’s rings and moons, discussed in the Cassini chapter.
A hundred years ago, nobody imagined that the Sun could be studied in terms of harmonics. Apart from the sunspot “blemishes,” the surface seemed smooth and featureless. Solar properties vary smoothly through the region we see as the “surface.” The edge marks the distance out from the center at which the density of gas reduces to the point where light no longer interacts with particles and travels freely. Inside this region, which is called the photosphere, light is trapped and so the Sun’s interior is hidden from view. The first inkling that the interior was pulsating came when George Ellery Hale built his heliostat on Mount Wilson in the early twentieth century. High-magnification photographs showed a surface mottled with fine structure, and time sequences revealed that the Sun’s surface was a seething sea on which the pairs of sunspots floated like large lily pads. The field of helioseismology matured as the century progressed, and solar scientists identified thousands of different oscillatory modes—the Sun “rings” like a bell. The Sun is also like an echo chamber (plate 12). Sound travels through the plasma and sets up standing waves, like the vibrations of the head of a drum or the air inside an organ or woodwind instrument.24
Just as seismologists can infer the internal structure of the Earth from the way sound waves and earthquake tremors pass through the planet, helioseismologists can study the Sun by seeing how interior sound waves manifest at the surface. This work has led to measurements of the density, temperature, and chemical abundance of the interior, as well as inference of the age of the Solar System and the constancy of the gravitational constant.25 The workhorse instrument on SOHO is the Michelson Doppler Imager since it shows the oscillations of the entire Sun. This instrument discovered a layer about a third of the way to the Sun’s center where the orderly interior, within which energy flows radially, transitions to the turbulent outer region, where energy moves in convective loops. This is the place where the solar magnetic field is created. Just as large-scale flows like the Gulf Stream and the jet stream are important for the Earth’s climate, SOHO data have shown that such flows are important for solar weather.
SOHO’s data is of such high quality that 3D maps of the Sun were derived for the first time. The maps answered questions that puzzled Galileo: how deep do sunspots extend and how can they survive for weeks at a time? The answers: they are fairly shallow but they are rooted in places where the plasma converges and strongly flows downward. SOHO scientists have managed the amazing trick of holographically reconstructing features on the far side of the Sun.26 All these images are available daily on the web. In fact, a plethora of solar data is available online, since a small armada of satellites is monitoring our life-giving star all the time.