Abstract
Abstract. Schumann resonance (SR) frequency variation has been studied using Magnetotelluric (MT) data recorded in one of the world's toughest and generally inaccessible Himalayan terrain for the first time in the author's knowledge. The magnetotelluric data, in the form of orthogonal time varying electric and magnetic field components (Ex, Ey, Bx and By), recorded during 10 March–23 May 2006, in the Himalayan region, India, at elevations between 1228–2747 m above mean sea level (amsl), were used to study the SR frequency variation. Electromagnetic field components, in the form of time series, were recorded at 64 Hz sampling frequency at a site located away from the cultural noise. Spectral analysis of time series data, at a frequency resolution of 0.03 Hz, has been performed using Fast Fourier Transform (FFT) algorithm. Spectral stabilization in three Schumann resonance modes is achieved by averaging the power spectral magnitude of 32 data segments, each with 2048 sample data. Amplitude variation in the Schumann resonance frequency associated with day-night, sunrise and terminator effect was observed. Average diurnal variation in the first three Schumann resonance frequencies associated with magnetic field components is presented. The maximum frequency variation of about 0.3, 0.4 and 0.7 Hz was observed in the first, second and third mode, respectively. The frequency variations observed in electric and magnetic field components also show phase shift and varying attenuation. The SR frequency variation has been used to define the ionospheric electron density variation in the Himalayan region, India.
Highlights
Schumann (1952) has predicted an extremely low frequency (ELF) resonance in the earth-ionospheric waveguide
Characteristics of Schumann resonance frequencies depend on the characteristics of their source, location of the observation point with respect to the source, and ionospheric electron density/conductivity behaviour
The spectral analysis of magnetotelluric data recorded from the Garhwal Himalayan region, India, has been conducted to study the Schumann resonance frequency variation
Summary
Schumann (1952) has predicted an extremely low frequency (ELF) resonance in the earth-ionospheric waveguide. Assuming the perfectly conducting earth and ionospheric boundaries, different modes of resonance frequencies can be derived from the following relation (Schumann, 1952), fn. For a local region, assuming the average constant source distribution, Schumann resonance frequency variations can be used to determine the average conductivity profile of the ionosphere (Tran and Polk, 1979a, b). By incorporating different realistic ionospheric electrical conductivity models, characteristics of resonance frequencies can be estimated (Tran and Polk, 1979a, b; Sentman, 1983). The changes in SR frequencies are sensitive to a general increase/decrease of conductivity within the outer layer (Satori et al, 2005) It has been found, experimentally, that the variations in the N-S and E-W magnetic components occur in antiphase for the first Schumann resonance frequency (Roldugin et al, 2004a). The SR frequency variations are used to determine ionospheric electron density distribution assuming constant source distribution in the Indian Garhwal Himalayan region
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