Sounds in the Stratosphere
Months of research hung on a 500-foot tall balloon.
Daniel Bowman had watched it slowly fill with air for hours, hoping that his small box of equipment would survive the stratosphere.
For the past two years, Bowman, a doctoral candidate in geophysics at UNC-Chapel Hill, has strapped microphones and other sound sensors to the High Altitude Student Platform (HASP), a NASA balloon that can reach heights of up to 23 miles.
In August 2015, he traveled for the second time more than 1,600 miles from Chapel Hill to Fort Sumner, New Mexico, where everything was now out of his hands. All he could do was ready for launch and watch the balloon slowly lift, shrinking into a tiny white dot in the cloudless sky.
Bowman is among the first to record infrasound in the stratosphere, a sound with a frequency lower than humans can hear.
He believes the sounds humans make, from the rush of planes to the dull hums of air conditioners, could be heating the atmosphere, but it could take five to 10 years to prove his theory and understand the implications — that civilization may affect the environment in ways previously unseen.
Bowman said the atmosphere we know is two-dimensional, but he hopes his research can help people understand the atmosphere as a three-dimensional space.
“I like the atmosphere because I can see it,” he said. “I can see what’s going on. Solid earth, you can’t really see and it doesn’t change over a human lifetime. The atmosphere you can see it and turn around and five minutes later it is completely different.”
According to Bowman’s theory, acoustic waves carry energy into the atmosphere, but they can’t travel forever into space. When they get to the thermosphere, the uppermost part of the atmosphere, they lose that energy, and the energy gets transferred to heat.
Scientists have tried to calculate how much energy is being transferred to the atmosphere from the ground, but Bowman said measuring acoustic waves from the Earth’s surface does not provide the most accurate results. Taking to the sky is the first step to finding the answer.
Bowman wasn’t always a balloon man.
When he first came to UNC-CH, Bowman started studying volcanoes and the sound waves surrounding volcanic eruptions. He started by measuring sound on the ground, but found he was missing some signals.
“I started to think of ways to put microphones in the air,” he said.
Bowman began ballooning. At first he tried tethering the balloons and setting off explosions under them to simulate a volcanic eruption. All of the balloons snapped free. He needed to find a way for a balloon to fly freely while collecting data.
He found the HASP balloon by accident in an email from a NASA listserv, one he would have normally sent to junk mail. This was his chance to test a free-flying balloon. What he recorded temporarily shelved his volcano research.
“We thought, ‘OK, we’ll do this project, we’ll record stuff — I bet someone has done this before — and if nothing is there, we’ll write a paper about how nothing is there, and we’ll move on with our lives,’” he said. “We got the data back and started analyzing it, and it was kind of like, ‘What the hell is this?’
“I’m still planning to go back to the volcanoes, but if you catch a big fish, you tend to let the smaller ones go.”
What does the atmosphere sound like? It is not the question on the forefront of most people’s minds, but to Bowman, it is the question that drives him.
It sounds like the wind howling through a jet engine right before takeoff. Eerie, high-pitched wails are broken up by static and scratching noises like turning the dial of an old radio. Sometimes there is a blip of something — a half-buried frequency among the garbled noise — but it quickly dissipates.
Acoustic waves in the atmosphere is a subject with scant documentation. The last known experiments on the subject were conducted in the mid-1960s by a team from the University of Michigan, but from their research Bowman could not say what the sounds he recorded mean or how they are affecting the atmosphere.
“It was kind of unclear of what they were trying to do,” he said. “I think the idea was just exploration. They did 20 to 30 balloon launches, but the documentation is unclear of what they found. I don’t know if the data even exists anymore — it may be gone.”
Finding the answers to his questions means more balloons and going to even greater heights. Bowman thinks he can achieve this with solar-powered balloons — or what he likes to call the “solar-powered garbage bag.”
“We think we can break the world record in solar power flight with paint drop cloth and packing tape,” he said. “People in a gym could build a 40-foot solar balloon and possibly break all the records of solar balloons, solar-powered airplanes — everything.”
Armed with his giant garbage bag, a sprinkle of coal to power it and a small camera in a Tupperware box, Bowman launched a prototype from the parking lot behind his office in November. It peaked at 72,000 feet before landing just outside Raleigh.
“I think it’s one of the most nerve-wracking things I have ever done,” he said. “Even on the NASA balloons you are not sure if it’s going to work as planned. In my own balloons, I’ve seen a lot of failures. Simply getting a balloon off the ground and clear the trees and clear the buildings and into the sky is 90 percent of the battle. It’s a great feeling to see it go, and it’s an even greater feeling to see it report back its final resting place.”
GPS tracking and smaller sensors have made high-altitude ballooning easier, but the only thing certain in Bowman’s experiments is uncertainty.
Bowman’s sensors on the HASP balloon picked up strange, unexplained noises he thinks may have come from a thunderstorm the balloon passed over, the sound of the ocean and sounds from air conditioners on Earth in his original 2014 launch.
“He has, over time, focused on this unexplained noise, but unless you’re absolutely sure, you can’t say what it is,” said Rachana Gupta, associate director of Electrical and Computer Engineering Design Center at N.C. State.
She and N.C. State graduate student Scott Johnson spent months creating the sensors Bowman mounted on the HASP balloon, and thinking of every possible thing that could go wrong. They had never built anything for a high-altitude balloon before.
Jake Anderson, a graduate student at Boise State University, went with Bowman to the HASP balloon launch to test his own infrasound sensors.
“There’s so much suspense leading up to it,” Anderson said. “You get up really early, you get everything organized on the balloon and then you wait and wait and wait. They have all this rigorous procedure they have to go through. If the weather is not perfect they can call off at any time. The whole time, you’re just hoping, ‘please launch today, please launch today.’”
Anderson developed infrasound sensors that are smaller, lighter and cheaper than many of the existing sensors. He eventually wants to use his sensors to measure infrasound over erupting volcanoes.
“You become more willing to do risky projects and possibly get results you wouldn’t be able to get with the expensive instruments,” he said. “I’m not willing to risk my advisor’s really expensive equipment. He would be pretty mad at me if I blew up $5,000 of his gear.”
After Bowman got the HASP data back almost two months after the 2015 launch, he found the sensors from both teams picked up interfering sounds.
Gupta thinks the sounds Bowman recorded could be from the wind, electrical noises from the other equipment on the balloon or even the balloon itself.
“One of the biggest challenges of this research is finding out what’s real and what’s not,” Bowman said.
Bowman spends most of his time in his office surrounded by computer monitors trying to interpret the slow trickle of data he records from each balloon he launches.
According to the yellowing sign on his door, Bowman works in the seismology office. Though the study of earthquakes brings soil and tectonic plates to mind, he said this is where his advisor puts anyone in the geology department working with sound.
He has managed to cram several plastic storage buckets and crates of equipment, two shoulder-high helium tanks and a work table in the small space.
Cables are strewn across his desk, and he finds himself untangling them every few minutes. In the back corner, there is an old sink filled with empty containers and a counter cluttered with more nests of wires, broken computer monitors, stacks of papers and a stick of deodorant. Posters of the planets — Pluto included — hang across the chalkboard smudged with layers of equations erased and rewritten over and over.
It wouldn’t be a real lab if it were clean, he said.
Here, he assembles balloon packages sent all over the world. One under his desk will be shipped to New Zealand where NASA will launch another balloon in March. The balloon will be drifting over Antarctica for about 100 days.
“The ballooning and infrasound community is so far apart that there’s no transfer, and I think that’s the only reason why this hasn’t been done,” Bowman said. “The will is there, the technology is there, once you bring them together, it’s not really that complicated, but the two elements haven’t come together until we had this opportunity.”
The atmosphere is considered one of the most beautiful, yet unforgiving places on Earth. It burns during the day and freezes at night. The sky is black, and the air is thin. The Earth begins to curve, and the clouds cast wispy shadows on the ground. It is the one place no human could ever go.
“Over the years my perception of the atmosphere has changed,” he said. “I feel like I’m at the bottom of this gigantic ocean that’s flowing over us, and there’s all this stuff going on, and we’re just at the bottom looking up.”