Solar Cycle-Modulated Deformation of the Earth–Ionosphere Cavity

The Earth–ionosphere cavity resonator is occupied primarily by the electromagnetic radiation of lightning below 100 Hz. The phenomenon is known as Schumann resonances (SR). SR intensity is an excellent indicator of lightning activity and its distribution on global scales. However, long-term measurements from high latitude SR stations revealed a pronounced in-phase solar cycle modulation of SR intensity seemingly contradicting optical observations of lightning from satellite, which do not show any significant solar cycle variation in the intensity and spatial distribution of lightning activity on the global scale. The solar cycle-modulated local deformation of the Earth–ionosphere cavity by the ionization of energetic electron precipitation (EEP) has been suggested as a possible phenomenon that may account for the observed long-term modulation of SR intensity. Precipitating electrons in the energy range of 1–300 keV can affect the Earth–ionosphere cavity resonator in the altitude range of about 70–110 km and modify the SR intensities. However, until now there was no direct evidence documented in the literature supporting this suggestion. In this paper we present long-term SR intensity records from eight stations, each equipped with a pair of induction coil magnetometers: five high latitude (|lat| \u3e 60°), two mid-high latitude (50° \u3c |lat| \u3c 60°) and one low latitude (|lat| \u3c 30°). These long-term, ground-based SR intensity records are compared on the annual and interannual timescales with the fluxes of precipitating 30–300 keV medium energy electrons provided by the POES NOAA-15 satellite and on the daily timescale with electron precipitation events identified using a SuperDARN radar in Antarctica. The long-term variation of the Earth–ionosphere waveguide’s effective height, as inferred from its cutoff frequency, is independently analyzed based on spectra recorded by the DEMETER satellite. It is shown that to account for all our observations one needs to consider both the effect of solar X-rays and EEP which modify the quality factor of the cavity and deform it dominantly over low- and high latitudes, respectively. Our results suggest that SR measurements should be considered as an alternative tool for collecting information about and thus monitoring changes in the ionization state of the lower ionosphere associated with EEP

Correlation between airtemperature and thunderstorm activity in Africa according to the ELF measurements in Antarctica, Arctica and Ukraine

Purpose: Search for the connection of seasonal variations in characteristics of the Earth-ionosphere global resonator with air temperature in Africa. Comparison of results obtained in Arctica, Antarctica and in the midlatitudes of the Northern Hemisphere with the surface temperature of African continent. Checking the effectiveness of the point source model for describing the seasonal change in the position of regions with the greatest thunderstorm activity. Design/methodology/approach: The method of correlation analysis of time series was used. According to the long-term monitoring of the natural noise of the extremely low frequency (ELF) range at the Ukrainian Antarctic Station (UAS), at the Low Frequency Observatory of the Institute of Radio Astronomy, National Academy of Sciences of Ukraine in Martove village (Ukraine), as well as at the SOUSY observatory (Spitsbergen), seasonal changes in the level of the first mode of Shumann resonance was restored by the activity of the African thunderstorm center. The average air temperature in the African continent over the same period was estimated according to the global network of meteorological stations. When estimating the intensity of the resonance maximum of extremely low frequency radiation, a correction has been introduced for the distance to the source of lightning discharges. Findings: The presence of a strong relationship between the surface air temperature of the equatorial and sub-equatorial regions of Africa and the intensity of the Schumann resonance generated by the African thunderstorm center is shown. It is shown that the model of an effective point source adequately describes the seasonal behavior of the African thunderstorm center. Conclusions: The developed technique can be applied at various receiving points for studying all continental thunderstorm centers. Such an approach will be useful for developing the concept of using the Schumann resonator as a “global thermometer”. Simultaneous observations in several receiving points can become promising also for estimating shorter (several days) variations in global temperature