Inferring global lightning activity based on multi-station Schumann Resonance measurements – the Nelson-Pracser approach
Résumé
Lightning activity is now recognized as a climate variable indicating the changing climate of the Earth, however its continuous monitoring on global scale is still a challenging issue. In the lowest part of the ELF band (< 100 Hz) lightning-radiated electromagnetic waves travel several times around the globe before losing most of their energy. Therefore all individual lightning discharges (intracloud and cloud-to ground alike) contribute to the measured electromagnetic field, which enables the investigation of global lightning activity with only a few (1-20) receiver stations. The global interference of such waves is known as the Schumann resonances (SR), characterized by the peak frequencies of ~8, ~14, ~20 Hz.
Here, we present a new inversion algorithm designed to reconstruct the distribution and the average charge moment change of global lightning activity in absolute units (C2 km2/ s) from the SR electromagnetic field components measured at two sets of distant observation sites (6 stations with identical instrumentation and 18 with different characteristics) on the globe. Forward modeling is based on the analytical solution of the two-dimensional telegraph equation for a uniform but lossy Earth-ionosphere cavity while a linearized inversion algorithm has been implemented for the inverse calculation which is based on the singular value decomposition of the Jacobian. Lightning activity is determined for guess locations which are selected based on GLD360 lightning stroke locations. The reliability of our algorithm will be demonstrated via synthetic tests and inversion results based on real observations for a few days will be presented, too.