IX International Symposium on Lightning Protection 26 th -30 th November 2007 – Foz do Iguaçu, Brazil HIGH-SPEED VIDEO OBSERVATIONS OF POSITIVE LIGHTNING Marcelo M. F. Saba Maurício G. Ballarotti Leandro Z. S. Campos Osmar Pinto Jr. INPE - Instituto Nacional de Pesquisas Espaciais msaba@dge.inpe.br mgballa@dge.inpe.br leandro.zanella@gmail.com osmar@dge.inpe.br Av. dos Astronautas, 1758 – São José dos Campos – SP – Brazil – 12227-010 Abstract - Although positive flashes are usually less frequent than negative lightning, their strokes may have high peak currents followed by long continuing current combining two threatening characteristics for lightning protection. The highest directly measured lightning currents and the largest charge transfers to ground are thought to be associated with positive lightning. This study presents some characteristics of natural cloud-to-ground positive lightning from video images obtained with high-speed cameras (up to 8000 frames per second), from electric field measurements, and from information given by the Brazilian Integrated Lightning Detection Network. The characteristics presented in this work are: stroke and channel multiplicity, percentage of single strokes, average interstroke time interval, continuing current and flash duration, leader speed, and peak current distribution. Besides, we present, for the first time in literature concerning positive lightning, the presence of M- components during the continuing current. We also present measurements of 2-D average velocity for four cloud-to- ground positive leaders. 1 INTRODUCTION The highest directly measured lightning currents and the largest charge transfers to ground are thought to be associated with positive lightning. Recently, [1] showed that their strokes may have high peak currents followed by long continuing current, and thus combine these two threatening features for lightning protection. Although positive flashes are usually less frequent than negative lightning, their special characteristics make the understanding of positive lightning an important issue. This study presents some of them. 2 INSTRUMENTATION AND MEASUREMENTS Two high-speed digital video cameras (up to 8000 frames per second) were used by the Atmospheric Electricity Group (ELAT) to record images of cloud-to-ground flashes in Southeast and South Brazil between January 2003 and April 2007. Each high-speed camera has a trigger system that detects a signal from an external source. We can set the trigger point to determine how many frames we want to record before the event. Each trigger pulse was initiated manually, depressing a hand- held switch when a flash occurred. All high-speed video recordings had a 1 s pre-trigger time and a total recording time of 2 seconds. The pre-trigger time of 1 s proved to be long enough to prevent the missing of first strokes. Also, the total recording time of 2 s was long enough to capture the whole flash. All images were GPS synchronized, time stamped and without any image persistence. For more details on the accuracy of high-speed cameras techniques for lightning observations see Ref. [2]. In order to identify the stroke location, polarity and peak current, we used the BrasilDAT data [more information on the characteristics of the network is found in Ref. [3]. The stroke matching between camera and network was done by GPS time synchronization (timing accuracy for each GPS system less than 1 millisecond). The information about the polarity of each individual stroke is available even if the location of the stroke is not computed by the system. A fast electric field flat-plate antenna was also used in this study to observe some flashes. Its bandwidth was from 306 Hz to 1.5 MHz and the sample rate used was 5 MS/sec. The acquisition module used was a National Instruments PCI-6110, 12-Bit with 4 analog inputs. Similarly to the high-speed camera, the triggering system of the acquisition module was manually operated and GPS-synchronized. 2.1 Data A total of 39 downward cloud-to-ground positive flashes were recorded by the high-speed camera during 12 different thunderstorm days in two different regions of Brazil. In the Southeast region, 18 flashes were recorded during 8 thunderstorm days, whereas in the South region, 21 flashes were recorded during only 4 thunderstorm days. The observing sites and their coordinates were: Cachoeira Paulista (22.68 o S; 44.98 o W) and São José dos Campos (23.21 o S; 45.87 o W) – São Paulo State, and São Martinho do Sul (29.44 o S; 53.82 o W) and Uruguaiana (29.81 o S; 57.00 o W) – Rio Grande do Sul State. The high frequency of positive flashes in the south, together with some of their characteristics, was already reported in a recent study [4]. In general, the low number of positive