From the many transients that were observed at the Astronomical and Geophysical Observatory AGO of Comenius University near Modra, western Slovakia, in the vertical electric field component mainly during May and June ofa peculiar group of events could be recognized. According to the waveform analysis, these peculiar events in most cases consist of two overlapping transients with a characteristic time difference of 0.
The natural origin of the peculiar events was verified from the NCK data and the source location was determined from the second transient. The results suggest that the two consecutive transients originated in the same thunderstorm.
Furthermore, the phase spectrum analysis indicates that the sources have coherently excited the Earth-ionosphere cavity.
These findings seem to support the idea that electromagnetic waves orbiting the Earth might trigger lightning discharges.
The possibility that electromagnetic waves may trigger discharges was first considered by Nikola Tesla. This inversion method involves a propagation model for the Earth-ionosphere cavity with day-night asymmetry. The Earth is now populated with more than 30 calibrated ELF receivers making continuous time series observations.
This circumstance is exploited in the present study to verify the findings of the ELF inversion method. Detailed comparisons will be shown over Universal Time for selected days.