Listening to the Sounds of the Sun

You could say that Tim Larson, Seth Shafer, and Elaine diFalco were brought together by the Sun. Now the three of them are sharing the sounds of the Sun with scientists, musicians, and the general public through a unique effort called the Sonification of Solar Harmonics (SoSH) Project.

Credit: Mike Giles (via Unsplash)

In the early 1960s, scientists realized that the surface of the Sun oscillates, expanding and contracting in regular cycles of about five minutes. Eventually, they identified the source of this motion: as sound waves beneath the Sun’s surface tried to escape, they instead bounced back inside the Sun. That might sound a little creepy, but don’t worry—it’s science.

The hottest part of the Sun is its core, where intense pressure fuses hydrogen into helium and releases energy in the process (thanks to E=mc2). This energy travels outward, first via radiation and then through convection as it nears the surface of the Sun. The convection zone is a rough place, full of turbulence. Just as turbulent skies make for a bumpy airplane ride, turbulence in the convection zone makes for a bumpy trip to the surface of the Sun. Hot plasma rises to the surface and radiates its energy into space. The cooled plasma then descends back into the interior. This turnover at the surface creates sound waves.

Diagram of the Sun. Credit: Kelvinsong (CC BY-SA 3.0)

Most of the time we think of sounds as noises thatare audible to the human ear, but that’s actually a narrow definition. Sound waves are disturbances in pressure that travel away from their source, regardless of whether there is an ear around to hear them.

When a lady down the street looks up and calls “hello” to you, she creates disturbances in air pressure (vibrations) that travel away from her mouth and are picked up by your ears. In the convective layer of the Sun, turbulence plays the role of your neighbor and plasma plays the role of the air. Rather than hear these sound waves, we can see their visible impact as they reach the surface of the Sun and are reflected back inward.

Sound waves are characterized by frequency—the number of cycles that pass by a stationary point in one second. Our ears perceive frequency as pitch. The five-minute cycle of the Sun’s surface corresponds to a frequency of about 0.0033 hertz. You might think that cycle reflects when or how the sound waves are created, but it actually reflects something much bigger—the physical properties of the Sun.

Guitar strings, drum membranes, and flute bodies all have what’s called resonant frequencies, frequencies at which they naturally vibrate. When you pluck a string, hit a membrane, or blow into a tube, you create sound waves with various frequencies. However, the frequencies that match the instrument’s resonant frequencies are amplified. They dominate the sound. The Sun is also a resonator, and its strongest resonant frequency is at 0.0033 hertz.

The resonant frequencies of an object depend on the object’s physical properties. For example, the resonant frequencies of guitar strings depend on mass, length, and tension. For each such frequency, the string has a corresponding shape. This shape, together with its frequency, is called a harmonic.

Different strings on a guitar oscillate at different frequencies, creating the various shapes seen here. Credit: Kinja

The Sun has many harmonics that each vibrate at a characteristic frequency. The harmonics influence the motion of the Sun’s surface in small but measurable ways. Furthermore, different regions of the Sun have different harmonics. By measuring the motion of the solar surface, you can separate these harmonics from one another and make inferences about the structure and composition of the Sun’s interior. That was the focus of Tim Larson’s research as a physics PhD student at Stanford University. “I spent all day every day studying sound waves and never listened to them,” says Larson, who now teaches at Moberly Area Community College.

It’s not that he didn’t want to. Human ears are sensitive to sounds in the frequency range of about 20-20,000 hertz. At 0.0033 hertz, the Sun’s dominant resonance is way too low for us to hear. Translating the data into an audible range while preserving its integrity isn’t a simple task. Although he tried, Larson didn’t have much luck convincing his colleagues that it was a worthwhile effort.

Range of audible frequencies for different species. Even elephants can’t hear a frequency below 0.0033 Hz. Credit: Codeelectron 

But then Elaine diFalco reached out. As a graduate student in music composition at the University of North Texas (UNT), she was interested in using data from the Michelson Doppler Imager (MDI) and the Helioseismic and Magnetic Imager (HMI)—the instruments whose data on the Sun Larson was studying—for musical purposes.

“I became an astronomy enthusiast a few years back, and the topic became so pervasive in my mind, that when I began working on my masters, I drew inspiration from it for my compositions,” she explains.

Seth Shafer, a recent UNT graduate (now a professor at the University of Nebraska at Omaha), joined Larson and diFalco, and the SoSH Project was born. The team has now built an interactive tool that takes real data from the Sun and maps it to clips that you can hear. It does this by shifting the frequencies into the audible range while preserving the relationships between the harmonics.

Three of the sun’s harmonics are represented in this audio file; they first sound in sequence and then simultaneously. The data was taken by the Michelson Doppler Imager. The frequencies are transposed by a factor of 90,000. And the file plays in about 35.6 seconds. The difference in volume for each mode corresponds to its amplitude as measured on the Sun. Credit: SoSH Project. To hear more sounds of the sun, visit the SoSH Project website.

The SoSH tool is interactive and free, so composers, scientists, and even you can download the interface and create your own Sun songs. You can mess around with different parameters, such as playback rate and frequency range, and hear their influence. You can also listen to several sample clips on the SoSH Project website. Looking forward, the group is thinking about planetarium shows, virtual reality experiences, smartphone apps, and maybe someday even sonifying a solar flare.

Solar flare. Credit: NASA (via Giphy)

Experiencing the sounds of the Sun is definitely a cool experience, but it’s one that may also have scientific merit. “Because scientists never listen to their (already acoustic!) data, they simply have no idea what they might learn from a sonification that they can’t find in plots and charts,” says Larson. SoSH is also great for composers, according to DiFalco. She plans to compose “a sort of solar concerto” based on the musical intervals of the Sun.

“Music is an art and the great challenge is to create something that is aesthetically intriguing while remaining true to whatever system you’re using,” she says.

–Kendra Redmond

Kendra Redmond is a freelance science writer and editor. After earning a master’s degree in physics, she’s worked for years in science education and communication, regularly contributing to Physics Buzz and other science news outlets, which you can find on her Facebook and LinkedIn. Kendra lives in Bloomington, MN with her husband and three kids.

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