Publicações

The present work consists on the report of one stepped leader event whose initial development becomes visible above cloud base. The leader starts to propagate with speeds of the order of 10^6 m/s simultaneously with the emission of electric field pulses normally associated with the preliminary breakdown phase. This event has been observed in São José dos Campos, southeastern Brazil, with the help of one high-speed camera (set to record 4000 frames per second), GPS time-synchronized, and three flat antenna electric field sensors (two configured as fast antennas, with gains different by a factor of 10, and one configured as a slow antenna). Lightning location system data provided an estimation of distance between the instruments and ground contact point (approximately 14.6 km), which allowed us to develop a photogrammetric analysis based on the recorded imagery data. The leader leaves the cloud opaque region around 4800 m above ground (instead of emerging from its base, about 2000 m above ground), propagating with speeds above 1 x 10^6 m/s and emitting a particularly intense luminosity. At heights between 3900 and 3600 m, the leader begins to slow down, presenting speeds in the 5-6 x 10^5 m/s range and a considerable decrease in it luminous intensity. Finally, when the leader tip reaches heights below 3000 m from the ground, its propagation speeds oscillates between 2 and 4 x 10^5 m/s. When the photogrammetry data (leader tip speed and height) is temporally correlated with the electric field waveforms, one can notice that the initial, fast propagation phase (with speeds of the order of 10^6 m/s) occurs simultaneously with the emission of pulses usually associated in literature with preliminary breakdown processes. Some of the pulses were so intense that they saturated the higher-gain fast electric field sensor and could also be sensed by the slow electric field sensor. A detailed analysis of these initial pulses show a bipolar structure in which one or more small nega.