What Are the Harmonics?
The cavity between Earth's surface and the ionosphere supports standing electromagnetic waves at several discrete frequencies. The fundamental (first mode) is about 7.83 Hz. The higher modes (harmonics) correspond to two, three, four, and five wavelengths fitting around the Earth. These are the Schumann resonance harmonics.
Frequency Table
| Mode | Approximate frequency | |------|------------------------| | 1 | 7.83 Hz | | 2 | 14.3 Hz | | 3 | 20.8 Hz | | 4 | 27.3 Hz | | 5 | 33.8 Hz |
The exact values depend on ionosphere height and conductivity and can vary slightly (e.g. ±0.1–0.2 Hz) with time of day and space weather.
Why These Frequencies?
For a standing wave in a spherical cavity, the circumference must equal an integer number of wavelengths. So:
- Mode 1: one wavelength fits once around → ~7.83 Hz
- Mode 2: two wavelengths fit → ~14.3 Hz
- Mode 3: three wavelengths → ~20.8 Hz
- and so on.
The frequency spacing is roughly constant because the cavity is large and the modes are well separated.
Amplitude of the Modes
The fundamental (7.83 Hz) is usually the strongest and most stable. Higher modes are generally weaker and more sensitive to cavity geometry and lightning distribution. In spectrograms, you often see the first few modes clearly; higher ones may be noisier or less visible.
In Practice
When you look at Schumann resonance data (e.g. from Cumiana or Tomsk), the bands at 7.83, 14.3, 20.8 Hz (and sometimes 27.3, 33.8 Hz) are these harmonics. Cosmic Radar shows Schumann information in the daily report together with the Kp index so you can relate resonance activity to geomagnetic conditions.
Sources and further reading
- Cumiana VLF Station – Live Schumann resonance spectrograms
- NOAA SWPC – Geomagnetic activity and Kp index