Propagation velocity of VLF EM waves from lightning discharges producing Q-bursts observed in the range 10–15 Mm

The electric fields in ELF (extremely low frequency) to VLF (very low frequency) range were observed in Kochi, Japan (latitude 33.3° north, longitude 133.4° east) on 27 December 2007. The SODs (source-to-observer distances) for 72 Q-bursts of positive polarity obtained in 2 h after 0401.00 UT were estimated by analyzing the waveforms. The VLF component from lightning discharges producing the Q-bursts was detected preceding ELF at the SODs less than 16 Mm (mega-meters). Using time separation between arrivals of VLF and ELF waves in the SOD range 10–15 Mm, the propagation velocity of VLF waves in the Earth-ionosphere spherical guide is estimated as 295 ± 3 Mm/s in comparison with the velocity of ELF waves (Schumann resonance frequency range) of 265 ± 1 Mm/s (Ogawa and Komatsu, 2007).