16.04.2017

For centuries, some observers have claimed that shooting stars or meteors hiss as they arc through the night sky. And for just as long, skeptics have scoffed on the grounds that sound waves coming from meteors should arrive several minutes after the light waves, which travel nearly a million times faster. Now, scientists have proposed a theory to explain how our eyes and ears could perceive a meteor at nearly the same time. The hypothesis might also explain how auroras produce sound, a claim made by many indigenous peoples living at high latitudes.

Meteors release huge amounts of energy as they disintegrate in the atmosphere. They also produce low frequency radio waves that travel at the speed of light. Some scientists have suggested that those radio waves produce the sound that accompanies meteors. The waves can cause everyday objects—including fences, hair, and glasses—to vibrate, which our ears pick up as sound between 20 and 20,000 Hertz. This phenomenon, called electrophonics, is a well-known principle: “The conversion from electromagnetic waves to sound waves … is exactly how your radio works,” says Colin Price, an atmospheric scientist at Tel Aviv University in Israel and co-author of the new study. “But in this case nature provides the conversion between electromagnetic waves and acoustic waves.”

But nailing down that scenario isn’t easy. Reports of noisy meteors are relatively scarce—there were only 40 last year, according to the American Meteor Society (AMS). And because most of these “hearings” have been made by amateur sky watchers, it’s difficult to find audio recordings to back them up. “[We’ve] never had [a recording] cross our path,” says David Meisel in Geneseo, New York, executive director of AMS. Moreover, a key question remains to be answered: How do the meteors produce low-frequency radio waves in the first place?

Now, Price and Michael Kelley, a physicist at Cornell University, have developed a model to answer that question. As a meteor streaks through Earth’s atmosphere, it ionizes the air around it, splitting it into heavy, positively charged ions and lighter, negatively charged electrons. The ions follow the meteor, whereas the electrons are deflected by Earth’s magnetic field. That separation of positive and negative charges in the meteor’s wake produces a large electric field that drives an electrical current. And it’s that current that launches the radio waves, Price and Kelley hypothesize in an upcoming issue of Geophysical Research Letters. The size of the meteor and its speed through the atmosphere would control the frequency of the radio waves, they predict.

Earlier this year, another research team presented a different hypothesis to explain how meteors make sound. That team proposed that visible light from a meteor heats up materials such as hair and glasses, which then vibrate and produce sound waves. But this theory requires a “huge” light source, Price says. Only meteors as bright as the full moon could emit enough light to produce such sound waves. But according to the new theory, all meteors generate radio waves that can produce sound, some of which our ears are capable of picking up.

Price and Kelley suggest that their model might also explain reports of “clapping” sounds accompanying auroras, the colorful light displays created when charged particles from the sun collide with molecules in Earth’s atmosphere at high latitudes. These sounds feature prominently in stories of native peoples of the northern United States, Greenland, and Canada, but they have largely been dismissed by scientists. “Auroras also create radio waves that can easily reach the ground,” Kelley says.  

The new hypothesis is “reasonable,” says Meers Oppenheim, an astronomer at Boston University not involved in the study. But it’s difficult to simulate what’s truly going on 100 kilometers up in Earth’s atmosphere as tiny particles of dust whiz by at 50 or more kilometers per second. “The devil lies in the details, and no one seems to have truly worked through those,” he says.

Quelle: AAAS