Global distribution of intense lightning discharges and their seasonal variations

In order to study the temporal and regional variation of lightning occurrences and their relation to sprite activity and climate variability, we have analysed the 1-100 Hz ELF magnetic field waveform data obtained at the Syowa station in Antarctica, Onagawa in Japan and Esrange in Sweden for a one year period from September 2003 to August 2004. We have selected totally 1.7 x 105 events of transient Schumann resonances from the ELF magnetic field data whose amplitude exceeds 40 pT at all stations. Then, the lightning locations are estimated by a triangulation method with an estimation error of 0.5 Mm. It is found that in the summer season (from June to August) the lightning occurrence rates are higher in the northern hemisphere than in the southern hemisphere with large enhancements in North America, South-East Asia and the northern part of Africa. On the other hand, in the winter season (from December to February) these rates are higher in the southern hemisphere with large enhancements in South America, Australia and the southern part of Africa. These features are consistent with the results of global lightning measurements from space conducted by the Optical Transient Detector and the Tropical Rainfall Measuring Mission satellite. Then, we have also calculated the charge moment value (Q · dl) of lightning discharges using ELF magnetic field waveform data and have estimated the distribution function of charge moments for positive cloud-to-ground (+CG) discharges and for negative cloud-to-ground (-CG) discharges. It is found that the shape of the distribution function for both +CG and -CG discharges is almost the same for all seasons and that the distribution function of -CG discharges has a steeper slope at high Q · dl than that of +CG discharges.