Vadim Mushtak

ELF propagation parameters for uniform models of the Earth–ionosphere waveguide

Abstract Uniform models for the Earth–ionosphere cavity are considered with particular attention to the physical properties of the ionosphere for the extremely low frequency (ELF) range. Two consistent features have long been recognized for the range: the presence of two distinct altitude layers of maximum energy dissipation within the lower ionosphere, and a “knee”-like change in the vertical conductivity profile representing a transition in dominance from ion-dominated to electron-dominated conductivity. A simplified two-exponential version of the Greifinger and Greifinger (1978) technique widely used in ELF work identifies two slopes in the conductivity profile and, providing accurate results in the ELF communication band (45– 75 Hz ), simulates too flat a frequency dependence of the quality factor within the Schumann resonance frequency range (5– 40 Hz ). The problem is traced to the upward migration, with frequency increasing, of the lower dissipation layer through the “knee” region resulting in a pronounced decrease of the effective scale height for conductivity. To overcome this shortcoming of the two-exponential approximation and still retain valuable model analyticity, a more general approach (but still based on the Greifinger and Greifinger formalism) is presented in the form of a “knee” model whose predictions for the modal frequencies, the wave phase velocities and the quality factors reasonably represent observations in the Schumann resonance frequency range.

Schumann Resonance Signatures of Global Lightning Activity

This chapter is concerned with the Earth’s Schumann resonances (SR) and their application to understanding global lightning. The natural electromagnetic waves in the SR frequency range (5 Hz to approx. 60 Hz) radiated by lightning discharges are contained by the Earth-ionosphere cavity. This cavity excitation by lightning can occur as a single energetic flash (a ‘Q-burst’), or as an integration of a large number of less energetic flashes (the ‘background’ resonances). In principle, continuous observations of SR parameters (modal amplitudes, frequencies, and quality factors) provide invaluable information for monitoring the worldwide lightning activity from a single SR station. Relationships between the variation of SR intensity and global lightning activity are shown. Connections between the change of diurnal modal SR frequency range and the areal variation of worldwide lightning are demonstrated. The temporal variation of the diurnal SR frequency patterns characteristic of the global lightning dynamics is also presented. Distortions of ELF waves propagating between the lightning sources and the observer are theoretically discussed based on the TDTE (two-dimensional telegraph equation) technique, focusing on the role of the day-night asymmetry of the Earth-ionosphere cavity. Theoretical and observational results are compared. Both instruments for SR observations and spectral methods for deducing SR parameters are reviewed. Experimental findings by SR on global lightning variations on different time scales (diurnal, seasonal, intraseasonal, annual, semiannual, interannual, 5-day, long-term) are summarized. The growing use of SR measurements as a natural diagnostic for global climate change is emphasized.