Studies on the Schumann resonance frequency variations

Schumann resonances are the resonant frequencies of electromagnetic radiations from lightning propagating in the earth - ionosphere wave-guide. It is generally around 8 Hz and it's higher harmonics are also present (Sentman, 1987; Roldugin et.al, 2004). The recorded data of Schumann resonances at Dharali in Himalaya at an elevation of 2591 m above sea level were used for the present study. The horizontal electric field components E_x (North-South direction) and E_y (East-West direction) were obtained by measuring the voltage drop between pairs of non-polarizing electrodes deployed in electrical contact with the soil. Induction Coil magnetometers were used to measure the horizontal H_x and H_y components. A description of the instruments is given by Israil et.al (2008). The measurements were made in frequency range 1 to 30 Hz for the present study, which was sufficient to cover first three resonance modes. It has been found that power spectrum shows maximum during noontime and minimum during nighttime. The frequency shifts in X and Y components of electric and magnetic fields are not in the same phase. Frequency changes in different components vary from 1.9 to 2.6%, 1.8 to 3.2% and 1.4 to 3.3% respectively in first three Schumann resonance modes. Ionospheric anisotropy resulting in the frequency splitting in the X and Y magnetic field components of the Schumann resonance modes were also observed at a spectral resolution of 0.03 Hz.