Lightning as the Source
The Schumann resonance has no single point source. It is excited by lightning discharges all over the world. Each flash radiates a broadband electromagnetic pulse; the component at the cavity's resonant frequencies (about 7.83, 14.3, 20.8 Hz, …) is reinforced as it travels around the Earth. So the Schumann resonance is essentially a global average of thunderstorm activity.
How Many Lightning Flashes?
Globally, there are on average about 50 lightning flashes per second (including cloud-to-ground and in-cloud discharges). This number varies with season and climate. The distribution of storms—over continents vs. oceans, day vs. night—affects which resonant modes are excited most strongly and how the amplitude varies over the day.
Effect on Amplitude
When thunderstorm activity is high (e.g. in the tropical and subtropical storm belts), the amplitude of the Schumann resonance tends to increase. When activity is low or shifted to regions that couple less efficiently to the global cavity, amplitude can decrease. So the "strength" of the resonance you see in spectrograms reflects where and how much lightning is happening, not a change in Earth's fundamental frequency.
Daily and Seasonal Variation
Because lightning distribution depends on solar heating and atmospheric circulation, there are daily and seasonal patterns. For example, there is often more activity over land in the afternoon and over certain regions in summer. These patterns show up in Schumann amplitude time series and are normal, not anomalous.
Relation to Space Weather
Ionospheric disturbances (e.g. from geomagnetic storms or solar flares) can also affect the cavity and thus the Schumann resonance—both amplitude and, to a smaller extent, frequency. So Schumann data can complement space weather monitoring. Cosmic Radar shows Schumann information together with the Kp index and space weather events so you can see how lightning-driven resonance and space weather interact on a given day.
Sources and further reading
- NOAA Space Weather Prediction Center – Space weather and geomagnetic activity
- Cumiana VLF Station – Schumann resonance and lightning-driven amplitude
- NASA – Lightning and atmospheric electricity – Global lightning and climate