ELF · the Earth–ionosphere cavity
Schumann resonance
Lightning somewhere on Earth strikes dozens of times every second. Its lowest-frequency energy travels around the planet between the ground and the ionosphere, and the waves that line up with themselves after a full lap add up. The result is a set of weak, natural radio resonances near 8, 14, 20 and 26 Hz.
Live: Cumiana station, Italy
How to read it
- The bottom panel is the vertical electric field from 0 to 105 Hz (scale on the right). The Schumann resonances are the faint horizontal bands near 8, 14, 20 and 26 Hz.
- The top panel is a geophone (a ground-vibration sensor), 0.2 to 30 Hz. It is not radio.
- The strong line at 50 Hz is Italy's mains power, not a resonance. Vertical streaks are nearby lightning or local interference.
Other live spectrograms, on their own sites (their terms don't allow showing them here):
The modes
| Mode | Ideal cavity | Measured | Q | Width (f/Q) |
|---|
Ideal: a perfectly conducting Earth and ionosphere, fn = c / (2πa) · √(n(n+1)), computed here with a = 6371 km. Measured: Balser & Wagner (1960) as tabulated by Cole (1965). Mode 5 is omitted because its value is illegible in the only scan we could check.
The real cavity resonates lower and with broader peaks than the ideal one because the ionosphere is not a perfect mirror: ELF waves reflect over a range of heights and lose energy to its finite conductivity. Day–night and north–south differences in the ionosphere spread the peaks further (Price 2016, §2).
What drives it
Lightning. About 2,000 thunderstorms are active at any moment, producing roughly 50 flashes per second worldwide (Price 2016, §4). Below 100 Hz the waves lose only about 0.5 dB per 1,000 km, so a flash anywhere on Earth contributes everywhere.
Because the big thunderstorm regions take turns with the time of day, the resonance strength follows a daily cycle. At Mitzpe Ramon, Israel, one magnetic component peaks around 09:00 and 20:00 UTC (South-East Asia and South America) and the other around 14:00 UTC (Africa) (Price 2016, §4, Fig. 6).
The fields are tiny: a few picotesla magnetically, about a millionth of the Earth's steady magnetic field (Salinas et al. 2022), and millivolts per metre electrically (Price 2016, §3). You need 5 to 10 minutes of data to see the peaks clearly.
Space weather moves it a little
The resonances depend on the lower ionosphere, so solar events show up in them. Sátori et al. (2016) found solar X-ray flares raised the mode frequencies, by about 0.3 Hz during the Bastille Day event (14 July 2000) and about 0.2 Hz in the fundamental during the Halloween storms of 2003. Solar proton events pushed them down, by up to about 0.8 Hz in the vertical electric field on Bastille Day. The amplitudes stayed largely unaffected, because lightning sets them.
For radio amateurs
The Schumann band is far below any amateur allocation, but it is the same Earth–ionosphere waveguide that carries VLF. Navies used it for submarine communication: the US ELF transmitter ran at 76 Hz and the Russian one at 82 Hz (Price 2016, §1). The closest hams get is experiments just below 9 kHz, such as W4DEX's 8.971 kHz signal copied in the UK in 2014 (ARRL News). That is VLF, about a thousand times higher than the Schumann modes.
Is the Earth's frequency rising?
No. Posts claiming "Schumann spikes" or a rising "Earth frequency" usually show the Tomsk spectrogram. The Tomsk operators explain on their own site that the white vertical bands are local thunderstorm discharges, the thin horizontal lines are man-made noise from power lines and substations, and black bands mean no data.
József Bór of the HUN-REN EPSS institute, which runs the Nagycenk observatory in Hungary, describes the frequencies as "remarkably stable" (2026). He adds that a link between the resonances and headaches or anxiety has not been scientifically established. The real variations are the small shifts above: tenths of a hertz, driven by the Sun and the ionosphere.
Sources
- C. Price, "ELF Electromagnetic Waves from Lightning: The Schumann Resonances," Atmosphere 7(9):116, 2016. doi:10.3390/atmos7090116 (open access).
- R. K. Cole Jr., "The Schumann Resonances," Radio Science, J. Res. NBS 69D(10):1345, 1965, Table 1. NIST PDF.
- M. Balser & C. A. Wagner, "Observations of Earth–Ionosphere Cavity Resonances," Nature 188:638, 1960. doi:10.1038/188638a0.
- W. O. Schumann, Z. Naturforsch. A 7:149, 1952 (the original prediction). doi:10.1515/zna-1952-0202.
- G. Sátori, E. Williams, C. Price et al., "Effects of Energetic Solar Emissions on the Earth–Ionosphere Cavity of Schumann Resonances," Surveys in Geophysics 37:757, 2016. doi:10.1007/s10712-016-9369-z.
- A. Salinas et al., "Schumann resonance data processing programs and four-year measurements from Sierra Nevada ELF station," Computers & Geosciences 165:105148, 2022, §1. doi:10.1016/j.cageo.2022.105148.
- HUN-REN EPSS, "The Earth's 'Hum': What Does Schumann Resonance Actually Tell Us?", 2026. epss.hun-ren.hu.
- Tomsk space observing system, spectrogram legend: sos70.ru.
- ARRL News, 6 June 2014: W4DEX 8.971 kHz signal received in the UK. arrl.org.
- Live image: Cumiana VLF monitoring station, R. Romero, vlf.it.