Between the nearly perfectly conducting terrestrial surface and ionosphere, a resonating cavity is formed. Broadband electromagnetic impulses, like those from lightning flashes, fill this cavity, and create globally the so-called Schumann resonances at frequencies 5 - 50 Hz (Schumann, 1952; Bliokh et al., 1980; Sentman, 1987). The nominal average frequencies observed are 7.8, 14, 20, 26, 33, 39, and 45 Hz with slight diurnal variation (the three first lines can be seen in the figure displaying data from northern Finland, the Kilpisjärvi station). It has been suggested that the intensity of these lines could be used to monitor the global lightning activity, especially after the local time modulation by changes in D region height has been removed (e.g., Sentman and Fraser, 1991).
Note that also the ionospheric Alfvén resonator -related spectral resonance structures (SRS) are formed due to the lightnings, and cover partly the same frequency range. In the figure here only the two first SRS lines are clearly visible. Finally, the strong spike in the figure just below 17 Hz is due to the Swedish railroads! Standard magnetometers are not able to measure the Schumann resonances, and even the search coil (i.e., pulsation) magnetometers are most often sampled at about 0.1 Hz that does not allow such studies. Special equipment are thus needed (see, e.g., Sentman and Fraser, 1991). That is also true if one wants to study lightning activity with Schumann resonances in other planetary systems: this might work at least for Venus and Jupiter!
- Bliokh, P. V., A. P. Nikolaenko, and Y. F. Filippov, Schumann Resonances in the Earth-Ionosphere Cavity, Peter Perigrinus, London, 1980.
- Sentman, D. D., Magnetic polarization of Schumann resonances, Radio Science, 22, 595-606, 1987.
- Schumann, W. O., Uber die strahlungslosen Eigenschwingungen einer leitenden Kugel, die von einer Luftshicht und einer Ionosphärenhulle umgeben ist, Z. Naturforsch., 7a, 149, 1952.
- Sentman, D. D. and B. J. Fraser, Simultaneous observations of Schumann resonances in California and Australia: Evidence for intensity modulation by the local height of the D region, J. Geophys. Res., 96, 15973-15984, 1991.