If you were to measure all the electromagnetic activity in Earth’s atmosphere, you’d notice peaks at different points along the spectrum.
There’s one such peak at 50Hz, caused by radiation from mains electricity grids worldwide, but it’s a series of peaks seen in the Extremely Low Frequency (ELF) part of the spectrum – that’s the band from 3Hz to 30Hz – that we’re more interested in.
These peaks occur at the Schumann resonances, which are the resonant frequencies of the Earth-ionosphere cavity, and they represent standing electronic waves that encircle our planet as a result of lightning activity.
Find out more about magnetic fields and the electromagnetic spectrum
Understanding how these waves are generated and what factors influence them enables scientists to monitor global thunderstorm activity.
Since thunderstorms become more common as temperature rises, it has been suggested in recent years that tracking Schumann resonances could also be a useful way of monitoring climate change.
Actually, all sorts of claims have been made about Schumann resonances – mostly with very little scientific evidence to back them up.
But first let’s look at how these standing electromagnetic waves form in the first place.
Earth's natural frequencies
Our planet and its atmosphere can be viewed as a multi-layered sphere. Earth’s inner and outer core, mantle and crust form the layers beneath our feet, while above ground the atmosphere consists of several more layers.
- Troposphere (up to 12km, and the bit where we live)
- Stratospere (12-50km, includes the ozone layer)
- Mesosphere (50-80km, very cold)
- Ionosphere (80-700km, warmer)
- Exosphere (700-10,000km)
Within the ionosphere is a layer of warmer air that has been ionised by incoming solar radiation, which is known – sensibly enough – as the ionosphere.
This layer, like the Earth’s crust, is highly electrically conductive – whereas the other atmospheric layers in-between aren’t.
The ‘Earth-ionosphere cavity’, then, is effectively a spherical layer of non-conductive air sandwiched between two conductive layers, in which electromagnetic waves (such as those generated by lightning) can bounce around.
And that layer has a series of resonant or ‘natural’ frequencies.
As you may hopefully remember from school science lessons, everything has natural frequencies to which it is susceptible, and at which frequencies it will vibrate more strongly, or ‘resonate’.
Troops break step when marching over bridges, for instance, just in case their marching tempo happens to match the bridge’s natural frequency, which could cause it to collapse.
Another good example is blowing over the neck of a bottle.
The sound of air rushing out from your lips is basically white noise – it contains a whole load of different frequency sounds, all jumbled up together.
But when you blow over the bottleneck, the air in the bottle will respond to its own natural frequency from within that melange and become excited, causing the familiar whistle- or hum-like sound.
Read more about how Earth's atmosphere affects your astro images and why the sky is blue.
Resonating frequencies on Earth
Back now to lightning. At any given moment, there are around 2,000 thunderstorms active on Earth, generating approximately 50 lightning bolts per second worldwide.
These lightning bolts emit electromagnetic radiation at a wide range of frequencies, but most of those pass through the conductive outer layers of the Earth-ionosphere cavity harmlessly: that is, they’re absorbed by the ground or pass harmlessly out into space.
For electromagnetic waves of the right frequencies, however, the Earth-ionosphere cavity acts like a waveguide, causing them to bounce back and forth between the ground and the ionosphere.
This creates standing electromagnetic waves that encircle the Earth.
The frequencies of these waves are approximately 7.83, 14.3, 20.8, 27.3 and 33.8Hz – known as the Schumann resonances.
They take their name from Winfried Otto Schumann, the German physicist who first described them in the 1950s.
However, these frequencies do vary slightly, for a variety of reasons.
The precise altitude of the ionosphere, for instance, fluctuates with climate and weather conditions.
This will change the overall shape/volume of the Earth-ionosphere cavity, in turn altering its natural frequencies and hence the wavelength and frequency of the standing waves.
Find out more about electric currents on exoplanets
Schumann resonances, science and myth
An entire pseudoscience industry has sprung up around Schumann resonances online.
It took us all of a second’s searching, for instance, to find one website that purported to track Schumann resonances every 15 minutes, claiming that fluctuations therein are responsible for everything from depression to poor skin.
But there is little to no evidence that the Schumann resonances have any effect on biological life (human or otherwise) whatsoever, and any claims regarding their impact on mental or physical health should be taken with several shovels’ worth of salt.
But Schumann resonances do exist – and could, in theory, exist on any other planet with a multi-layered atmosphere like Earth’s.
This makes them a useful tool for astronomers, because if the electromagnetic spectogram of a planet’s atmosphere shows a similar set of peaks to those seen here on Earth, it’s a strong indicator of lightning activity on that planet – even if such is not detectable using optical equipment.
