Listen to the sky

Exploring the strange wonders of very low frequency radio.

I’ve always loved the sound of static, for some reason. Leaving the radio tuned halfway between stations, voices floating in and out of the noise, has always been a favorite thing to listen to on late-night drives. More recently I’ve been taking frequent weekend trips to listen to an entirely different kind of static: naturally occurring radio signals. A sferic, short for “Radio Atmospheric Signal,” is the radio signal generated by a lightning strike, a short but intense burst of wide-spectrum radio waves sent into the ether. With the right receiving equipment, you can pick up a chunk of that signal with frequencies in the human hearing range and play it back as audio. 

The sounds are mostly a dense cacophony of pops, clicks, and tweeks, with an odd whistle if you’re out at the right time of year and get particularly lucky. If the aurora is particularly strong, you might hear chirps and whistles like a chorus of birds. In the same way that AM radio signals can be picked up several states away from where they’re broadcasting, these sorts of very low frequency (VLF for short) radio signals can bounce off the ground and upper atmosphere, allowing them to travel around the world, or even through the magnetic field lines of the earth if conditions are just right. The farther a signal travels, the more distorted it becomes. In the most extreme cases, sferics sound to the human ear like ethereal, ghostly whistles. 

VLF listening needs the right conditions to get the best results. It’s best to be away from dense grids of power lines, as the receivers have to be incredibly sensitive and the interference can be intense. It also works best late at night, as the radio frequencies involved propagate best when the ionosphere is calm. I’ve taken to tracking space weather to keep tabs on when conditions are right—times of high geomagnetic activity caused by increased solar activity. We’re currently in the middle of the upswing of the 11 year solar cycle, with this cycle looking to have much higher sunspot activity—much to many a radio nerd’s delight.

Madison is well suited to this sort of delicate radio listening. While you won’t get a clear signal in the middle of most neighborhoods, the abundance of parks and natural areas makes it quite easy to find a quieter place to listen to the sky. Anywhere reasonably far away from high voltage transmission lines works well, with a good view of the open sky. 

Right in Madison we have spots like Cherokee Marsh or the UW-Madison Arboretum that work fantastically well. I’ve taken my kit along bike routes like the Cannonball Path or Military Ridge to see what’s there, and even during the day I can get a few pops and clicks coming through. Just off the Monona lake loop there’s Capital Springs Recreation Area, or Pheasant Branch Conservancy and Governor Nelson State Park on the Mendota side of town. 

Outside of Madison, going for hikes along the Ice Age Trail, or natural areas like Hemlock Draw, provides fantastic conditions for listening.  Several of these spots are close enough to power lines that  an unavoidable hum creeps through, but with enough trees blocking any overhead lines the signals are clear enough to not be too overpowering. There’s also an undeniable magic to listening to these sorts of things that we can’t normally hear that lets me personally ignore some of the less ideal conditions.

The kit required for this sort of listening is a fair bit removed from typical radio equipment. With standard ham radio receivers, it is possible to make long random wire antennas that can pick up these signals, though a setup like that requires quite a lot of room to make. Pre-made receivers exist. I have one made by Stephan P. McGreevy, who builds and sells them on eBay when he has time. His work and library of recordings he’s made through the years can be found at the magnificent web1.0 time capsule auroralchorus.com. For those more DIY-inclined, an antenna like a LOM Electrosluch Prezior can be created with the instructions found on the company’s github page.  

You can also access a livestream of various VLF feeds from around the world at any time at http://abelian.org/vlf/, with receivers as far-flung as Australia giving us a chance to listen to winter sounds in the middle of northern hemisphere summer. Space missions have also found these sorts of signals even on other planets, and the YouTube channel Space Audio has several videos of these extraterrestrial signals.