Q-bursts as Tools to Detect Structures of Anisotropic Conductivities in the Earth's Crust

Intense lightning discharges produce Q-bursts, i.e. brief electromagnetic (EM) radiation in the extremely low frequency (ELF, 3-3000 Hz) band. A recording system can detect Q-bursts produced almost anywhere on Earth, because the attenuation rate of ELF-band waves is low in the Earth-ionosphere waveguide. The azimuth of the source of a Q-burst can be unambiguously deduced from its time series recorded in the horizontal magnetic and vertical electric components of the atmospheric EM field. ELF data-based source azimuths of Q-bursts recorded in the Széchenyi István Geophysical Observatory (NCK, 16.7 E, 47.6 N) in Hungary were compared to true source azimuths. True source azimuths were calculated after identifying the parent lightning stroke of each Q-burst in the records of the World Wide Lightning Location Network (WWLLN) using the detection times of the ELF events. This study is based on 342 and 245 Q-bursts recorded at NCK station on August 1 and 2, 2012. It was found that ELF data-based source azimuths differ systematically from true source azimuths. The error of ELF data-based azimuths could be decomposed into time dependent and static components. The largest error term at NCK station is static but it varies with the true azimuth of the source. This variation of the source azimuth error correlates to the azimuthal variation of the horizontal conductivity gradient in the Earth's crust inferred from magnetotelluric surveys at NCK. Similar results were found by Füllekrug and Sukhorukov (GRL, 1999) for two other ELF stations in Germany and in the USA. It was suggested that large-scale anisotropies in the conductivity of the Earth's crust below the observatory may cause the azimuthal variation of the ELF data-based source direction error. In addition to supporting this view, the results of this study point out that the effect provides a new technique to study the Earth's crust and that the performance and limitation of the method should be quantitatively evaluated.