I wasn’t just watching this weekend’s aurora display but listening to it, too. No, I didn’t hear the ‘swishing’ or ‘crackling’ reported by some. Despite seeing hundreds of displays ranging from meek to wild, I’ve yet to actually hear any auroral audio. There is some evidence however that electrophonic transduction can convert otherwise very low frequency (VLF) radio waves given off by the aurora into sound waves through conductors. Wire eyeglass frames, grass and even hair can act as transducers to convert radio energy into low-frequency electric currents that can vibrate an object into producing sound. Similar ‘fizzing’ sounds have been recorded by meteor watchers that may happen by the same process.
Of course another reason people might hear auroras is they imagine a soundtrack. It’s a psychological thing – you see a spectacular display of auroral light and in your head hear sounds your imagination might expect like crackling and whooshing. Given that the aurora is never closer to the ground than 50 miles, the air is far too thin to transmit any weak sound waves that might be produced to your ears.
If you’re like me and hard of auroral hearing, a small VLF radio receiver will do the job nicely. This handheld device converts very low frequency radio waves produced from the interaction of the solar electrons and protons with the Earth’s magnetic field into sounds you can listen to with a pair of headphones.
We’re used to waves of light which are very, very short, measuring in the millionths of an inch long. The pigments in our retinas convert these waves into visible images of the world around us. Radio waves given off by auroras and other forms of natural ‘Earth energy’ like lightning range from 19 to 1,800 miles long or longer! To make them available to our senses we use a radio receiver. I fire up a little unit called a WR-3 that I bought for $60 back in the mid-1990s. The components are housed in a small metal box with a whip antenna and powered by a 9-volt battery. The on-off switch also controls the volume. Plug in a set of headphones and you’re ready to listen. That’s all there is to it.
The receiver picks up lots of things besides aurora including a big ‘unnatural’ hum from alternating or AC current in power lines and home appliances. It creates a loud, continuous buzz in the headphones. You’ll need to be at least a quarter mile from any of those sources in order to hear the more subtle music of the planet. I drive out to a open ‘radio quiet’ rural area, turn on the switch and raise the antenna to the sky. Don’t stand under any trees either. They’re great absorbers of the low frequency radio energy you’re trying to detect.
The first thing you hear will be the pops, crackles and sizzles of distant lightning called sferics which are similar to what you’ve probably heard on your car radio during a thunderstorm.
Lightning gives off lots of energy in the long end of the radio spectrum. When that energy gets ducted through the upper layers of Earth’s atmosphere called the ionosphere over distances of several thousand miles, it emits another type of sound called ‘tweeks‘ which remind a listener of pings or dripping water. Flurries of tweeks have an almost musical quality like someone plucking the strings of a piano.
When those same lightning radio waves enter Earth’s magnetosphere and interact with the particles there, they can cycle back and forth between the poles traveling tens of thousands of miles to create what are called whistlers. These things sound really eerie. After their long journey, the higher frequency waves arrive before the lower frequency ones causing them to spread out in tone. What you hear are a series of descending whistles that remind me of the whistling sound of bombs released from a plane like you’d see in a World War II movie. Tweeks are very brief; whistlers last anywhere from 1/2 to 4 seconds or longer.
Sometimes you’ll hear dozens of whistlers one after the other. There there’s the aurora. When conditions are right, a VLF receiver can pick up disturbances in Earth’s magnetic bubble spawned by auroras called ‘chorus‘ or ‘dawn chorus’. Talk about strange. Sometimes there are swooshing sounds, but chorus primarily sounds like a pond full of peeping frogs or a flock of birds singing at sunrise. Click on the links above to hear the different sounds.
Saturday night was a bonanza for natural radio. I heard all the phenomena described above. A line of passing thunderstorms provided a bounty of crackles and tweeks early on. As they thunderheads drifted far to the east, whistlers were heard a couple seconds after the appearance of every distant lightning flash. Most of these lasted 3 to 4 seconds, meaning they’d traveled back and forth between Earth’s magnetic poles. How astonishing it was to sense our planet’s magnetosphere through sound.
When the aurora was strongest starting around 1 a.m., the chorus rose from the background noise of lightning sferics and burbled and chirped along for nearly an hour. Later, through the aurora was still active, the sounds mysteriously disappeared.
I realize this radio stuff may not be everyone’s piece of pie, but if you’re interested in listening to VLF and in particular the aurora, basic receivers are available through at least several online sites shown below. I’ve only used the WR-3 and can’t speak for the others, but they all run around $125. Oh, and by the way, don’t use one when there’s a lightning storm nearby. Holding a metal aerial under a thundercloud is not recommended!
More on natural radio can be found HERE. Things to keep in mind when considering a purchase are whether you have access to an open area 1/2 mile from a power line and away from homes. You’ll also need patience. Many nights you’ll only hear lightning crackles from distant storms thousands of miles away peppered by the occasional ping of a tweet. Whistlers may not appear for weeks at a time and then one night, you’ll hear them by the hundreds. But if you regularly watch the sky, it’s so easy to take the radio along and ‘give a listen’ for some of the most curious sounds you’ll ever hear. It’s just one more way to be in touch with the home planet.