Schumann Resonance spectral characteristics: A useful tool to study Transient Luminous Events (TLEs) on a global scale

© International Conference on Atmospheric Electricity, ICAE 2014 The background Schumann Resonance (SR) spectra require a natural stabilization period of ~10-12 minutes for the three modal parameters, namely, the frequency, intensity and Q-factor to be derived from Lorentzian fitting. Before the spectra are computed and the fitting process is initiated, the raw time series data need to be properly filtered for local cultural noise, narrow band interference as well as large transients in the form of global Q-bursts. Mushtak and Williams [2009] describe an effective technique named as Isolated Lorentzian (I-LOR), in which, the contribution from local cultural and various other noises are minimized to a great extent, and enabling the problem of inter-modal interference to be more effectively addressed in the SR background spectra. An automated technique based on median filtering of time series data and the rejection of events exceeding 16 core standard deviations (CSD) (where 'core' pertains to the central portion of the "spectral power content") from the average of the period of interest has also been developed by Mushtak et al. [2012]. This cleaning of data before obtaining the modal parameters is essential for work related to the background SR, for example, finding the source strength of tropical 'chimney' regions by inversion of multi-station data. The methodology used for removing the effect of Q-bursts from background SR spectra could also be used to search for big sprite-producing positive lightning flashes in mesoscale convective systems worldwide. These special lightning flashes are known to have greater contribution in the ELF range (below 1 kHz) compared to negative CG strikes [Cummer 2006]. The global distributions of these Q-bursts have been studied by Huang et al., [1999] and Hobara et al. [2006] by wave impedance methods from single station ELF measurements at Rhode Island, USA. The present work aims to demonstrate the effect of Q-bursts on SR spectra using GPS time-stamped observation of TLEs and average energy data from the VLF World Wide Lightning Location Network (WWLLN). It is observed that the Q-bursts selected for the present work do alias with the background spectra over a five second period, through the amplitudes of these Q-bursts are far below the 16 CSD limit so that they do not strongly alias the background spectra of 10-12 minute duration. The extent of this aliasing is yet to be investigated thoroughly. It is expected that the spectral ELF methodology could be used effectively to detect TLEs globally with a small number of networked stations, especially during daylight conditions, when optical measurements of sprites are not possible.