Lightning Makes the Signal, and Lightning Ruins It

Lightning Makes the Signal, and Lightning Ruins It

The Earth is struck by lightning dozens of times every second, and it rings.

That is not a metaphor I reached for. It is the mechanism. The Schumann resonance, the thing people call Earth's heartbeat, is made of lightning and nothing else. Every flash dumps electromagnetic energy into the gap between the ground and the ionosphere, that gap behaves like a cavity, and a cavity that size has a note.

Here is the part that took me a while to sit with. The same lightning that creates the signal is also the worst thing that can happen to a measurement of it. Far away, lightning is the instrument being played. Close to the antenna, lightning is a hand slammed flat across the keys. One phenomenon, two completely opposite roles, and the only thing that decides which one you get is where the receiver happens to be standing.

That is the whole post.

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How a flash becomes a resonance

A lightning stroke is a current pulse. It radiates electromagnetic energy across an enormous span of frequencies, from the crack you hear on an AM radio all the way down into the extremely low frequency band, below about 100 Hz.

Most of that energy goes nowhere interesting. The ELF portion does something else. Those wavelengths are far too long to be absorbed neatly by the ground or to escape upward through the ionosphere, so they get trapped between the two and travel sideways, around the curve of the planet, in a shell of air roughly 100 km thick and 40,000 km around.

Now the geometry takes over. A wave that travels all the way around the world and arrives back where it started out of step with itself cancels out. A wave whose wavelength divides evenly into the planet's circumference arrives back in step and reinforces itself. Run that for every frequency lightning produces and you get a filter made of nothing but the size of the Earth: energy drains away everywhere except at a few specific frequencies, where it accumulates.

Those surviving frequencies are the Schumann resonances. Winfried Otto Schumann worked them out on paper in 1952, before anyone had measured them, purely from the dimensions of the planet and the height of the conducting layer above it. That prediction is one of my favorite things in geophysics. He did not detect the Earth's note. He calculated what it had to be.

If you want the fuller introduction to the phenomenon itself, I wrote a plain explainer here, and a closer look at the fundamental in what is 7.83 Hz.

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The five modes

The spacing between them, close to 6.5 Hz, is not a coincidence and not a tuning choice. It falls out of the spherical geometry of the cavity, the same way the overtones of a bell fall out of the shape of the bell.

This is also why one particular claim will not stop bothering me. These frequencies are set by the size of the planet and the height of the ionosphere. They do not rise. They wander by small fractions of a hertz as the ionosphere breathes between day and night and across seasons, and that is all the room physics gives them. What actually moves, sometimes dramatically, is amplitude: how much energy is sitting in those bands. Amplitude is a measure of how much lightning is happening and where. Reading an amplitude change as a frequency change is the single most common error in this whole field, and I took it apart properly in is Earth's frequency rising.

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How much lightning, actually

The number you will see repeated everywhere is 100 lightning strikes per second. It comes from a 1925 estimate built on world thunder-day records, which was a reasonable thing to do in 1925 and is not how anyone counts anymore.

Satellites count now. The Optical Transient Detector, in orbit from 1995, watched flashes directly from above for five years. Christian et al. (2003) put the global rate at 44 plus or minus 5 flashes per second, about 1.4 billion a year, well under half the old figure. Cecil et al. (2014) merged that record with the Lightning Imaging Sensor aboard TRMM and refined the mean to about 46 per second.

Forty-four is less than a hundred, and somehow it does not feel smaller. Nearly 80 percent of it happens in the tropics. Roughly ten times more of it happens over land than over ocean. The classic estimate of about 2,000 thunderstorms active at any given moment is older and softer than the satellite flash counts, but the picture it paints holds up: at every instant, on the night side and the day side at once, a couple of thousand storms are quietly keeping the planet in tune.

The ring gets louder in August

This is the detail I did not expect and now cannot stop telling people.

The global flash rate is not constant through the year. Cecil et al. found it swinging from about 35 flashes per second in February to about 60 in August. The reason is unglamorous and lovely: most of the planet's land is in the northern hemisphere, land makes far more lightning than ocean, and northern summer therefore puts more of Earth's surface under convection than any other time of year.

So the Schumann resonance has a season. The cavity is driven harder in the northern summer and more gently in the northern winter. The planet's background hum is louder in August than in February, every year, for reasons that come down to where the continents happen to be.

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Three chimneys and a daily rhythm

Global lightning is not spread evenly around the equator. It concentrates over three tropical landmasses that atmospheric physicists call chimneys: the Americas, Africa, and the Maritime Continent (Southeast Asia through Indonesia and northern Australia). Africa is the largest contributor of the three.

Each chimney fires in its own local afternoon, when the sun has had all day to build convection. Because those three regions are spread around the globe in longitude, their afternoons arrive at different universal times, and the Schumann amplitude measured at any single station rises and falls three times a day in a pattern that tracks them: broadly, the Maritime Continent near 09 UT, Africa near 15 UT, the Americas near 21 UT.

The related measurement in atmospheric electricity, the Carnegie curve, shows the same idea in the direct-current side of the global circuit: a single daily rise and fall in the fair weather electric field that looks the same whether you measure it in the Pacific or the Atlantic, because it is not local at all. It is the whole planet's thunderstorm population being counted at once. (The two curves do not agree on which chimney dominates, which is a real and unresolved argument in the literature, and worth knowing about before anyone tells you the daily rhythm is simple.)

What I like about this: a spectrogram in Siberia is quietly reporting on afternoon storms over the Congo. Nothing has to travel to you as weather. The cavity does the summing.

More on the heartbeat framing, and where I think it earns its keep, in Earth's heartbeat.

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Why distance flips lightning from signal to noise

Now the turn.

Everything above describes lightning at a distance of thousands of kilometres. At that range, the energy arriving at an antenna has already made a long trip through the cavity, and the cavity has done its work on it. The off-resonance frequencies have cancelled themselves out along the way. What reaches the coil is pre-shaped: narrow bands at 7.83, 14.3, 20.8 and up. On a spectrogram, that reads as thin horizontal lines, steady, sitting in the same place hour after hour.

Move the same lightning to fifteen kilometres away and none of that has happened yet. The antenna receives the direct pulse, the raw broadband impulse of the stroke itself, before the planet has had any chance to filter it. Flat energy across the entire band. On a spectrogram that reads as a vertical pillar, floor to ceiling, white.

Same physics. Same lightning. Opposite signature, decided entirely by geometry.

And the imbalance is brutal, because of how faint the real thing is. The global Schumann signal at ground level is on the order of one picotesla. That is roughly a millionth of a millionth of a tesla, tens of millions of times weaker than the Earth's static magnetic field you would measure with a compass. It is the summed contribution of every storm on the planet, spread thin by the trip. A single stroke a few kilometres away is not slightly stronger than that. It is orders of magnitude stronger, and it does not need to be filtered by anything to arrive.

Then the storm adds its extras. Rain and snow hitting the sensor housing carry charge and generate precipitation static. Wind moves the coils, and a moving coil in Earth's magnetic field manufactures a voltage of its own. None of that is the ionosphere. All of it lands in the same recording.

Which is how you get an image where the horizontal bands vanish completely under a solid white column. Nothing about that column is global. It is one thunderstorm, over one antenna, for a few hours.

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The consequence nobody can dodge

If lightning is both the source and the contaminant, then every honest Schumann tracker has to answer one question: what do you do when the two collide?

You cannot rescue the number. When a sensor is pegged at maximum across the entire band, the hour contains no recoverable information about the global cavity. Anything you print for that hour is invented. So the choice is narrow: publish a fake spike, or detect the contamination and say so.

Removing locally contaminated intervals before estimating resonance parameters is standard practice in the field, not something I made up for an app. Tatsis et al. (2021) ran an ELF detector in Greece against local lightning records and found a clear, statistically significant relationship between nearby strikes and the recorded ELF signal, concluding plainly that local lightning has to be removed from the background. Research stations have been doing versions of this for decades.

ResonanceOne does it too. I wrote the detection method, the thresholds, what the app holds and how it marks those hours, in why the Schumann resonance chart turns white. The current state of the guard, live, is on the interference status page, and today's reading is on Schumann resonance today.

The short version: white pillars mean local weather at one station, never a global event.

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Frequently Asked Questions

What causes the Schumann resonance?

Lightning. Every flash radiates electromagnetic energy, and the portion in the extremely low frequency band gets trapped in the cavity between Earth's surface and the ionosphere. Wavelengths that fit evenly around the planet's circumference reinforce themselves, so energy concentrates near 7.83, 14.3, 20.8, 27.3 and 33.8 Hz. No lightning would mean no Schumann resonance.

How many lightning strikes hit the Earth every second?

About 44 plus or minus 5, based on five years of satellite observation (Christian et al., 2003), refined to roughly 46 per second when a second satellite instrument was merged in (Cecil et al., 2014). The widely repeated figure of 100 per second is a 1925 estimate from thunder-day records and is more than twice too high.

Does lightning make the Schumann resonance spike?

Both yes and no, and the difference is distance. Distant lightning raises the amplitude of the resonant bands, which is the real signal doing what it is supposed to do. Lightning close to the measuring antenna floods the entire frequency range with a raw broadband pulse, which is not a resonance at all and cannot be read as one. On a spectrogram the first looks like brighter horizontal bands, the second looks like a solid vertical white column.

Why does the Schumann resonance chart turn white?

Almost always because a thunderstorm is passing over the measuring station. A nearby strike is orders of magnitude stronger than the globally integrated signal and saturates the sensor from top to bottom of the frequency axis, joined by rain static and wind moving the coils. It is local weather at one antenna, not a planetary event. The full detection story is in the whiteout post.

Does the Schumann resonance change with the seasons?

The amplitude does. Global lightning swings from roughly 35 flashes per second in February to roughly 60 in August (Cecil et al., 2014), because most of Earth's land sits in the northern hemisphere and land produces far more lightning than ocean. So the cavity is driven harder in the northern summer. The frequencies themselves stay where the size of the planet puts them.

Can a thunderstorm near the station affect the reading?

Yes, and it is the dominant source of bad Schumann data. This is why single-station readings need an interference check before they are published, and why removing locally contaminated intervals is established practice in Schumann resonance research (Tatsis et al., 2021).

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References

• Schumann, W.O. (1952). On the free oscillations of a conducting sphere which is surrounded by an air layer and an ionosphere shell. Zeitschrift fur Naturforschung A, 7:149-154.
• Christian, H.J. et al. (2003). Global frequency and distribution of lightning as observed from space by the Optical Transient Detector. Journal of Geophysical Research: Atmospheres, 108(D1), 4005. https://doi.org/10.1029/2002JD002347
• Cecil, D.J., Buechler, D.E. & Blakeslee, R.J. (2014). Gridded lightning climatology from TRMM-LIS and OTD: Dataset description. Atmospheric Research, 135-136, 404-414. https://doi.org/10.1016/j.atmosres.2012.06.028
• Price, C. (2016). ELF Electromagnetic Waves from Lightning: The Schumann Resonances. Atmosphere, 7(9), 116. https://doi.org/10.3390/atmos7090116
• Nickolaenko, A. & Hayakawa, M. (2002). Resonances in the Earth-Ionosphere Cavity. Kluwer Academic Publishers, Dordrecht.
• Tatsis, G. et al. (2021). Correlation of local lightning activity with extra low frequency detector for Schumann Resonance measurements. Science of the Total Environment, 787, 147671. https://doi.org/10.1016/j.scitotenv.2021.147671

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I did not expect to find the lightning explanation more moving than the mystical one. I did. A planet wrapped in a thin shell of air, struck forty-four times a second, humming at a pitch you could have calculated in 1952 with a pencil if you knew how big the Earth was. That is enough. It does not need help.

Thanks for reading this far. There is a real person behind this, and you can write to me anytime.

ResonanceOne tracks Schumann resonance, Kp index and solar activity in one Activity Index, and holds the reading when a storm sits over the station instead of publishing the spike. See today's reading.