Detection efficiency of the VLF World-Wide Lightning Location Network (WWLLN): initial case study

An experimental Very Low Frequency (VLF) World-Wide Lightning Location Network (WWLLN) has been developed through collaborations with research institutions across the world, providing global real-time locations of lightning discharges. As of April 2006, the network included 25 stations providing coverage for much of the Earth. In this paper we examine the detection efficiency of the WWLLN by comparing the locations from this network with lightning location data purchased from a commercial lightning location network operating in New Zealand. Our analysis confirms that WWLLN favours high peak current return stroke lightning discharges, and that discharges with larger currents are observed by more stations across the global network. We then construct a first principles detection efficiency model to describe the WWLLN, combining calibration information for each station with theoretical modelling to describe the expected amplitudes of the VLF sferics observed by the network. This detection efficiency model allows the prediction of the global variation in WWLLN lightning detection, and an estimate of the minimum CG return stroke peak current required to trigger the network. There are strong spatial variations across the globe, primarily due to station density and sensitivity. The WWLLN is currently best suited to study the occurrence and impacts of high peak-current lightning. For example, in 2005 about 12% of the global elve-producing lightning will have been located by the network. Since the lightning-EMP which produce elves has a high mean rate (210 per minute) it has the potential to significantly influence the ionosphere on regional scales.