Study of the variations in the Schumann resonances parameters measured in a southern Mediterranean environment

Highlights

• Shumann Resonances

• Winter and autumn SRs values predominate over spring and summer recordings.

• Max and min mean diurnal values appear close to Universal lightning centers peaks.

• SRs mean power increases with lightning activity.

• Lightning activity of absolute amplitudes increases for SRs modes 3 and 5.

Abstract

Schumann resonances (SRs) provide a unique tool for exploring continuous and long-term monitoring of global environmental parameters, although their detection and study is a very complex task. This paper presents diurnal and seasonal variations of principal parameters, namely the resonance frequency, the Q-factor, and the power of SRs, in the North-South (NS) magnetic field component for the first five modes from Extra Low Frequency (ELF) measurements. These were acquired at the observation site located at Doliana, Kalpaki, in the Region of Epirus, North West Greece. These variations are the first ones reported for the principal parameters in the South Eastern Mediterranean area and correspond to the one-year measurement period from 2016 to 2017. The comparison with variations at several observations sites is also discussed. Moreover, correlation with global lighting centers, as well as local lightning activity, and SRs parameters variation is attempted.

Introduction

Schumann resonances (SRs) are quasi standing electromagnetic waves created in the spherical cavity between the surface of the earth and the low layers of the ionosphere with a height of around 50 to 60 km. This spherical cavity is a “natural” waveguide that acts as a resonance cavity of the Extra-Low Frequency (ELF) electromagnetic waves (Schumann, 1952; Balser and Wagner, 1960). Study and analysis of the SRs parameters remains one of the primary fields of research activity, known for its interdisciplinary nature. The main source of these resonances are the global lightings exciting the electromagnetic field inside this cavity (Balser and Wagner, 1962; Williams, 1992). The magnetic component of SRs exhibits elliptical polarization and mode splitting (Sentman, 1987; Sentman, 1989). Spectral analysis of SRs during solar proton events has revealed that the peak resonance frequencies are declined according to various observations (Roldugin et al., 2001; Shvets et al., 2005). During this period, the amplitudes of SRs become several times greater than the background level due to sprites and elves (Huang et al., 1999), which refer to transient luminous events. On the other hand, the peak resonant frequency for the first SRs mode is enhanced during solar X-ray bursts.

SRs is one of the most promising tools in a variety of fields related to electromagnetics (Price, 2016). Specifically, SRs can be used as a sensitive measure of temperature fluctuations in the tropical atmosphere linked to the lightning flash rate (Williams, 1992). A strong correlation has also been found between SRs intensity and the global ground temperature has been found to give a latitude interval that exceeds ±45°. Comparative measurements of SRs stations at a distance comparable to their wavelength has revealed the different nature ofconvection inSouth America and Africa (Sátori et al., 1999). An increase in SRs intensity was observed before the 2004 Mid-Niigata Prefecture earthquake and the 2007 Noto Hantou earthquake, followed by a decrease after the occurrence of the earthquakes, implying a connection between SRs and earthquake activity. Extreme Electromagnetic Events (EEEs) with frequencies limited in range around 20–25 Hz, which overlap with normal SRs spectra, have been detected before and after three medium-magnitude earthquakes in Northern Greece (Florios et al., 2019; Christofilakis et al., 2019). Similar results were obtained during the 1999 Taiwan earthquake, i.e. an increase in the intensity of the fourth SRs mode, as well as a shift of its peak frequency. The SRs intensities have been found to vary with the position of thunderstorm regions related to El Nino and La Nina phenomena. SRs observations of electrical and magnetic fields have been used to infer the global lightning activity through the inversion method (Heckman et al., 1998). An inverse problem solution was taken into account based on the decomposition of the average power spectra of SRs, in order to reveal the worldwide lightning intensity, and distance distribution technique was used for reconstruction of the global lightning distance profile from the background SRs signal (Shvets, 2001). Moreover, a two-stage inverse problem was solved for locating the global lightning source distribution from SRs signals variations of the global lightning distribution revealed from three-stations SRs measurements (Shvets et al., 2010).

Additionally, considerable work has been done in the field of lightning activity over Greece. A preliminary study was conducted by Nastos et al. (2014) who studied the spatio-temporal analysis of lightning activity over Greece for one year using data from the Greek meteorological service. Based on an extended 4-y database, Matsangouras et al. (2016) estimated the cloud-to-ground lightning activity over Greece. Significant work was also done by Proestakis et al. (2016) who combined lightning activity and aerosols in the Mediterranean region.

The objective of this work was to measure SRs in a new ELF station located in Greece, and to derive the parameters that describe their characteristics, in order to study the temporal variability of these parameters in terms of daily and seasonal changes. Moreover, correlation of SRs with universal and local lightning activity was attempted. Even today, continuous recording of SRs and precise definition of the three principal parameters, for as many Schumann modes as possible, remains a challenging issue that requires accurate scientific experimental setups, isolated from man-made Electromagnetic Fields (EMFs) (Panagopoulos and Chrousos, 2019; Parra et al., 2015). Additionally, the dissemination of SRs parameters variations, covering a large area where seasonal and diurnal variations were never measured before, could shed light to global geophysical and environmental phenomena described above. Section 2 presents the ELF measurement station. The results are presented and discussed in Section 3. Finally, the conclusions are drawn in Section 4.