Response of the Earth–ionosphere cavity resonator to the 11-year solar cycle in X-radiation

Global changes in the fundamental Earth's Schumann resonances (SR) have been identified on the time scale of the 11-year solar cycle (SC). The first multi-station, multi-field component, multi-mode experimental evidence for solar X-ray modulation of the Earth–ionosphere waveguide is presented. Long-term SR observations covering one solar minimum and two solar maxima, accumulated under different experimental conditions in Antarctica, Hungary and the United States, have been analyzed. The variations of both SR frequencies and quality ( Q ) factors display pronounced maxima during solar maxima #22 and #23, and corresponding minima during the solar minimum of 1996–1997. The observed variations are interpreted on the basis of the uniform extremely low frequency (ELF) mode theory and the presence of two characteristic ionospheric layers responsible for ELF propagation (Greifinger and Greifinger, 1978: Radio Science 13, 831–837; Mushtak and Williams, 2002: Journal of Atmospheric and Solar Terrestrial Physics 64, 1989–2001). The hundredfold changes in the solar X-ray flux dominate the variations in the conductivity profile within the upper characteristic ELF layer (the 90–100 km portion of the E-region), with a negligible effect on the lower characteristic layer (the 50–60 km portion of the D-region). The general increase/decrease of this conductivity by up to one order of magnitude over the solar cycle is responsible for changes in the wave phase velocity and, consequently, in the observed SR frequencies by several tenths of Hz. The change in the scale height of the conductivity profile by several tenths of percent modulates the reflection/dissipation effects within the upper characteristic layer and, therefore, the Q-factors of the resonator. The physical role of the X-radiation in influencing SR is also demonstrated on short ( ∼ 1 h ) time scales, when occasionally variations in X-radiation comparable to the 11-year solar cycle are experienced.