Solar Cycle-Modulated Deformation of the Earth–Ionosphere Cavity

Abstract

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| > 60°), two mid-high latitude (50° < |lat| < 60°) and one low latitude (|lat| < 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.

Long‐Term Schumann Resonance Dynamics Based on Horizontal Magnetic Field Data at Mikhnevo Observatory During 2016–2020

“…Observed monthly average SR parameters represent clear seasonal variations, related to global lightning activity. Such SR seasonal variations with NH summer annual maxima appear similar to a number of mid and low latitude SR observations (Bozóki et al., 2021; Musur & Beggan, 2019) and are in accordance with optical observations of global lightning activity (H. Christian et al., 2003). Interannual dynamics of the first resonance frequency for both magnetic components and quality factors in the H WE component at Mikhnevo station represent good correlation with solar X‐ray long‐term intensity variations, similar to Sátori et al.…”

“…To overcome it high-quality experimental SR data is required. The most promising approach is to use long-term multistation data (Bozóki et al, 2021;Neska & Sátori, 2006;A. P. Nickolaenko et al, 2011;A.…”

“…In the paper by Bozóki et al. (2021) authors suggest that observed qualitative differences of SR intensities from numerous stations may be partly related to problems with the absolute calibration.…”

Long-Term Observations of Schumann Resonances at Portishead (UK)

“…The observed inter-annual trend (solid red line in Figure 8a) decreases between June 2015 and June 2018, which corresponds to the SD extrema (maximum and minimum, respectively) recorded during the 5-year period, and subsequently becomes nearly constant. An exceptional increase of the SR intensity for the period 2015/2016 has been documented by other studies, that considered the SR modes average peak frequency and intensity measured at various observatories distributed globally [20,24,37]. In analogy with other SR amplitude observations taken at similar mid-high latitudes (i.e., Eskdalemuir [25]), the steady decline in the inter-annual SD is found to be in phase with the transition from the maximum to the minimum of the 11-year solar cycle (Figure 8c).…”

“…Satellite observations do not show a solar phase-related variation in the intensity and distribution of global thunderstorm activity (i.e., the primary origin of SR) [38]. Solar cycle-related long-term changes in the SR frequency and power are thus attributed to the modification of the earth-ionosphere medium, and in particular to a reduction of local ionospheric height resulting from increased solar X-rays and energetic electron precipitation (EEP) near to periods of maximum solar activity [37,39]. In the absence of data covering an entire solar cycle, however, a univocal interpretation of the long-term trend observed in the BTD data is still challenging.…”