American Scientist

Anyone who's watched one of the sci-fi flicks from the 1950s, say Forbidden Planet, can recall the haunting electronic sounds used to evoke the mystery of outer space. The whirring hum of a flying saucer, the stealthy footfalls of an alien creature, the scream of an extraterrrestrial weapon—all preyed on our fears of the unknown. But these were merely echoes from the human imagination. Nothing in the natural world could make such spooky noises . . . could it?

Most civilians don't know it, but atmospheric scientists have been tuning in to eerily similar sounds for decades. The sounds can be heard by way of very-low-frequency (VLF) receivers that pick up radio waves in the Earth's atmosphere and convert them into audible signals. They've been given quirky names—sferics, tweeks and whistlers—that aptly portray their weirdness to the human ear.

The sources of the sounds are actually well understood, but no less strange for being so. It turns out that most of the sounds are various manifestations of impulsive radio emissions from lightning. Listening to a tweek or a whistler conjures up nothing like a bolt of lightning in the mind's eye, however. Sferics, for example, sound like a careless animal snapping twigs as it walks through the woods, or perhaps the sound of bacon frying (in a lot of grease). Tame interpretations, to be sure.

A sound is better heard than described in writing, and readers are urged to visit a Web site—www.spaceweather.com/glossary/inspire.html—where they can listen to a live broadcast of atmospheric VLF signals 24 hours a day. Scientists at NASA’s Marshall Space Flight Center in Huntsville, Alabama, have set up the online VLF receiver as part of an educational program called INSPIRE, or Interactive NASA Space Physics Ionosphere Radio Experiments. Budding atmospheric scientists can even build their own VLF receiver (for less than $100) and take part in a global network of monitoring stations. (Visit the INSPIRE Web site at image.gsfc.nasa.gov/poetry/inspire/index.html). Who knows? Maybe some young scientist will detect a signal from the Krell. —Michael Szpir