JOHN THE OBSCURE ™

By John Ruch

© 2006

 

The Sound of the Sun

 

            I hate the Sun.

            So it’s responsible for all life on Earth. So what?

            I’ve never hesitated at chomping down on the hand that feeds me, but this probably is the zenith of hypocrisy. After all, I do like nature in general and the tropics especially. I hate being cold. And my preference for the nocturnal is probably more misanthropic than anti-solar.

            But if the Sun is what keeps life going, it’s also the abusive parent that will eventually end it all when it runs low on fuel and expands enormously, frying the entire planet like a 7-Eleven hot dog. Meanwhile, it’s merely an insanely hellish thermonuclear fireball that, despite maintaining a mean distance of 93 million miles, is hot and bright enough to kill me within a few hours.

            Mega-annoying.

            If Earth was the cafeteria of an aging high school with too few books and lousy teachers—and let’s face it, it is—then the Sun would be the class bully. Out of sheer dumb brutishness, it pounds on you all day long. Don’t dodge the blows and you’ll burn to death, or genetically mutate into a mass of tumors.

            Several years ago, probably while ducking-and-covering from this perpetual ultra-Hiroshima in the sky, I wondered if the Sun was capable of hitting us with more than just electromagnetic radiation. What if outer space was filled with some kind of acoustic medium instead of just a near-vacuum? Could we hear the Sun?

            Naturally, I expected that we could. I imagined the trillion World War IIIs rumbling in the belly of our Sol Inflictus would translate here into something akin to distant thunder or a Satanic chuckle.

            On a recent day with temperatures in the 90s, I decided my fevered imagination was no longer answer enough. I started asking around about the sound of the Sun, hypothesizing an acoustic medium with the same characteristics of air at sea level, but no other physical effects.

            This is, of course, a ridiculous scenario, as a few astrophysicists wasted no time in pointing out. But if somebody had asked such a silly question sooner, the discipline of helioseismology, which has revolutionized solar knowledge, could be more than a mere 35 years old.

            Because it turns out the Sun is literally throbbing with sound, its surface oscillating in five-minute standing waves amid millions of other, more chaotic tones. The vast majority of it is far below the threshold of normal human hearing and dies quickly, but some solar astronomers told me it’s conceivable the frequencies would register faintly on Earth.

            Indeed, there’s been a recent claim that one frequency actually does make it to Earth via an actual medium that hadn’t occurred to me—the solar wind.

            It also appears possible, within my hypothetical universe, for a solar flare to cause a sonic shockwave major enough to give Earth a jolt. They certainly do a hell of a job on the solar surface.

            Sci-fi aside, my general instinct was right: the Sun is a sonic monstrosity, too. The late planetary scientist and author Carl Sagan loved to describe humanity as “starstuff pondering the stars,” but he didn’t factor in hatred and disgust as possible conclusions.1 The Sun was glaring today; I found myself glaring back. (Metaphorically, of course; otherwise it would have blinded me just for looking at it.)

            Astrophysicists, however, tend to wax poetic about Sun sound. While the acoustics in fact have the “broad-spectrum white noise” profile of a jet engine and a general inaudibility to humans, you more regularly find musical metaphors such as “solar chimes.”2 Two major oscillation-observation programs have gone by the acronyms GONG and MUSICOS.3

            Well, scientists have reason to sing the praises of solar acoustics. The accidental discovery of the phenomenon in the 1960s enabled solar study on a grand scale and caused major changes in models of the orb’s density, temperature, composition, magnetism and rotational velocity, among other things. Today, astronomers routinely produce complete maps of the Sun—including the invisible opposite side—with reverse-engineered acoustic holography based on the sound waves. (As we, of course, do not hear the sounds and cannot measure them directly, they are traced based on the literal, relatively minute oscillations of the solar surface, which heaves and falls perhaps 80 to 100 feet based on the sound waves.)

            Naturally, astrophysicists are now monitoring the acoustic activity of other stars, though this astroseismology is much harder to detect and may not even exist (or work in the same way) in larger stars, which are the kind we can best observe. The much simpler radial pulsation of some large stars was already known for centuries and hypothesized as acoustic by the 1870s.

            While I was right about solar sound, I was wrong about its nature. It is not directly caused by the nuclear fusion reaction occurring in the core. Indeed, a minute’s thought about basic physics should have told me as much. Instead, it was Prof. Robert Rosner, a theoretical astrophysicist at the University of Chicago, who pointed out that the Sun’s furnace is not explosive like thermonuclear bombs, but rather a steady, contained burning. Furthermore, the reason the fusion is happening is the massive density of the body itself. As Tom Burns, director of the Perkins Observatory in Delaware, Ohio, pointed out, any sound from the core would have about 430,000 miles of solar plasma to get through, which seems like quite a damper; indeed, it takes at least thousands of years for light to work its way to the surface. (There are likely other physical problems such as the acoustical cut-off point in a given atmosphere, but they aren’t worth going into after the aforementioned.)

            Instead, the sounds in the Sun are created by, and largely contained within, its convection zone. This is a layer of the Sun, roughly 110,000 miles thick, just below the surface layers, where the plasma is cool enough for heat to start moving by convection. That means it churns something like a pot of boiling water. (Solar convection is actually highly complex and poorly understood.) Relatively hot packets of plasma rise while relatively cool packets of plasma fall. It is the jostling of these areas together that creates solar sound, again vaguely like the sound of boiling water.

            (Actually, acoustic transmission in the Sun may not be like familiar terrestrial sounds; in a plasma, the wave may be transmitted not by individual molecules banging into each other, but rather by their electrical fields doing so. The result, however, is the same.4 Also, motion in the Sun also includes gravity waves, but these appear to be a minor component and insignificant compared to the massive energy of the acoustic waves.)

            It would more accurate to say the sound is like the shaking of the sides of the pot holding boiling water. The majority of the sound is trapped inside the convection zone. It functions as seismic activity, akin to earthquakes. (Indeed, Earth is also always throbbing with seismic activity and thus has a constant sound of its own.)

            The acoustic result is “a form of very low-pitched noise, with many tones playing at once,” as Jack B. Zirker describes it in “Journey from the Center of the Sun.”5 Kenneth R. Lang in “The Cambridge Encyclopedia of the Sun” describes the sound as like a jet engine, “but much, much louder.”6

            Low-frequency sounds predominate because of the massive size of the plasma quantities moving around, I was told by Jack Harvey, the world-renowned helioseismologist at the Kitt Peak National Observatory in Arizona. (If you look at a close-up photograph of the Sun, you can see a surface that looks something like shattered car glass. Each little “granule” is a packet of plasma moving by convection. The average one is about 900 miles across.7) The Sun is the biggest woofer in the solar system.

            Most of this noise stays in the convection zone because of acoustical physics. The low density of plasma at the top of the zone and the high temperature (with increased speed of sound) and greater rotational velocity of the plasma at the bottom of the zone severely limit the ability of the medium to carry acoustic energy, thus acting as cut-off points. Only a few select frequencies can make it through either barrier, with the rest refracting back into the convection zone or dying altogether.

            That also means the convection zone can act as a resonance chamber, like the body of an acoustic guitar. Most of the Sun’s noise dies quickly or is disrupted by other sounds. But sounds that match the natural frequencies of the zone itself create massive standing waves that give the surface a fairly predictable throb. The best-known is the five-minute frequency.8 (That’s about 3.3 mHz, compared to about 20 Hz for the bottom range of human hearing.) It’s powerful enough that it actually affects the output of sunlight, which throbs minutely along with the oscillation.

            OK, I’ll admit it. By now, I want to hear the darn thing. (Or at least have some device on Earth hear it for me.) It’s pretty neat stuff. So what happens if we stick our slab of air in there?

            Let’s first plow through the pessimism. Only relatively high-frequency sounds—roughly above 5 mHz—can make it out of the convection zone and into the Sun’s atmosphere. As Harvey notes, “current thinking is that their strength is quickly dissipated into heat.” (Indeed, acoustic energy may help heat the chromosphere.) So there may not be much “living” sound very far from the surface.

            There may be other acoustic problems. Astronomy professor James Lattimer of the State University of New York at Stony Brook noted that air has its own acoustic cut-off point based on its density, which might be exceeded. Burns noted the sheer quantity of 93 million of miles of air as a damper, and also that the Earth is a very small target that would receive only a tiny portion of the Sun’s acoustic energy output.

            And, as sound intensity decreases in inverse proportion to distance, I ran into much skepticism that solar sound could make it here at any appreciable level.

            But this is still the Sun we’re talking about. It’s hard to imagine it wimping out.

            “If we could place a microphone close to the solar surface, I’m fairly certain that it would pick up audible noise that would be mainly low frequencies,” Harvey told me. “How strong it would be, I don’t know. My intuition is that it would be very faint at the distance of the Earth, assuming that it could travel between us.”

            Jeff Kuhn, associate director of the University of Hawaii’s Institute for Astronomy and an expert solar observer, went at it from the angle of the Sun’s raw power.

            “Its acoustic power in this case [i.e., an atmosphere between Earth and the Sun’s photosphere] could be 1019 watts or about one-10 millionth of the Sun’s luminous heat and light power. Thus, the total acoustic intensity reaching the Earth would be less than about 0.0001 watt/m2, which is larger than the faintest acoustic intensity the ear can hear….Thus, if we could hear very low frequencies, this power is about the intensity of the environment of a loud party.”

            And for that matter, “The Sun does produce some energy in the audible range, which leaks out. This intensity would be at least a million times fainter but might be audible as a faint whisper.”

            The Sun’s whisper…It sounds almost gentle. It’s an appealing idea. As Kuhn’s analysis seems to factor in only the loss of power over distance, and not atmospheric acoustic cut-offs and heat losses, I think it’s suspect. But then, so’s my entire hypothesis.

            Rosner and Harvey told me to chuck my Astronomical Unit of air and think about another medium—literally: the interplanetary medium (commonly known as the solar wind), the stream of charged particles spewed from the Sun that constantly surround the Earth.

            Harvey noted that some recent research claims sound around 1 mHz in fact does propagate through the solar wind from the Sun to Earth. (Considering that the Sun itself creates the solar wind, I guess this means the medium is the message.)

            Rosner went even further, declaring that the solar wind even carries audible frequencies. But, he explained, we don’t hear them because their “energy flux” (as he noted, that’s ergs per second per square centimeter) is “very tiny, and furthermore, the impedance match between the IPM [interplanetary medium] and our atmosphere for such waves is very, very poor.” In short, it’s too tenuous a medium, and those few dozen miles of Earth atmosphere are enough to spoil everything, let alone 93 million miles worth.

            It occurred to me that perhaps, given an air atmosphere, the fiery loops of plasma in the Sun’s corona—the things that look like flames in photos—could make some noise. I didn’t ask about that specifically, but I’m guessing their sound would be insignificant compared to the general convection rumbling. Also, they consist of plasma flowing along strong magnetic fields, which tend to scatter sound. (Indeed, the powerfully magnetic sunspots consume large amounts of acoustic energy that pass through them.)

            But a solar flare could be a different story. Flares are massive explosions on the corona caused by superheated plasma.9 They can burst out for 100,000 miles or more and release energy equivalent to millions of nuclear bombs.

            A 1996 average flare was the first ever directly observed having a massive acoustic impact on the Sun. Directly beneath it, a series of concentric ripples spread across the Sun’s surface. While looking pretty in a photo, these were waves estimated at about 10,000 feet high. They spread out at least 75,000 miles. The “sunquake” was, in Earth terms, estimated as an 11.3 on the Richter scale.10

            These seismic waves were strange. It’s still unclear exactly what caused them, with the main theory being a particle beam produced by the flare impacting and energizing the surface. The waves also moved strangely, increasing in velocity as they went, rather than decreasing as is normal in Earth’s atmosphere.

            Whatever happened, it was impressive. If we had some viable acoustic medium, it seems likely that solar flares might be audible and/or smack us with a similar shockwave. In the case of a shockwave, Lattimer said, “this could travel supersonically through the atmosphere with much less dissipation [than normal sound at that distance] and could result in intense sound when it hit the Earth.”

            The nice thing about asking a goofy, petulant question is gaining new respect and wonder for the subject. The Sun ringing like a bell, acoustic energy riding an evanescent solar wind, two-mile-high sunquake waves—they’re all astonishing, magical truths.

            Oh, and did I mention horrifying as well? When I’m cowering under my Panama hat tomorrow, I’m going to think about that flare thing and be grateful that the Sun is only seen and not heard.

           

            1 In “Cosmos.”

            2 The jet engine comparison and quote are from Loren Acton, a former astronaut and current physics professor and head of the Solar Physics group at Montana State University, in an e-mail to me. The chimes analogy is from “The Restless Sun” by Donat G. Wentzel, who later admits, “The Sun is not very musical.”

            3 The National Solar Observatory’s Global Oscillation Network Group and the French-led Multi-Site Continuous Spectroscopy program. Heartbeat/pulse analogies are also fairly common. For a rare idiosyncratic analogy to a giant bowl of Jell-O (presumably orange-flavored), see the Stanford University press release at www.stanford.edu/dept/news/pr/95/951120soholaunch.html. No one has ever compared it to the wingbeats of a self-consuming, suicidal demon, so let me be the first.

            4 As explained in “A Web-Based Java Applet for the Visualization of Helioseismological Raytracing” by Andy Ganse, engineer at the University of Washington’s Applied Physics Laboratory, March 14, 2003, available at http://staff.washington.edu/aganse/helioseis/helioseis_rtapplet.html.

            5 Zirker’s work in this book and in “Sunquakes: Probing the Interior of the Sun” was the primary source for my understanding of convection zone acoustics.

            6 Scientists are able to track and reproduce simulacrums of the solar sounds on observational equipment, and then speed up recordings of the sounds to an audible frequency. Under this guise, you can find Web sites offering recordings of Sun sounds. Indirect and artificial, this is, to me, nothing but a parlor trick.

            7 Zirker, “Sunquakes.”

            8 Wentzel in “Restless Sun” notes that most observers are interested only in overall surface motion and thus calculate average oscillations, eliminating much tonal detail and variation from their reports; the five-minute oscillations are themselves somewhat irregular. Incidentally, some groundbreaking oscillation observation occurred at France’s Pic du Midi observatory, a James Bond villain HQ-type facility with a Web site worth checking out for kicks: www.gillesvidal.com/picdumidi[1].htm.

            9 Zirker, “Sunquakes.”

            10 Figures from Lang, “Cambridge Encyclopedia of the Sun.”

 

Significant sources not cited in the text or footnotes include: “A Concise History of Solar and Stellar Physics” by Jean-Louis Tassoul and Monique Tassoul; http://observe.arc.nasa.gov/nasa/ootw/1996/ootw_961113/ob961113.html (“Observation of the Week” column [now defunct], Nov. 13, 1996, NASA’s Observatorium site); http://solar-center.stanford.edu/singing/singing.html#sing (Stanford [University] Solar Center); Peter Sturrock, professor emeritus of applied physics and expert on solar physics, Stanford University; “The Sun” (“Voyage Through the Universe”), Time/Life Books, ed.; www.noao.edu/outreach/press/pr00/pr0003.html (National Optical Astronomy Observatory press release, March 30, 2000). Posted Aug. 13, 2006.

 

 

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