[1] Based on analyses of high-speed video recordings of cloud-to-ground lightning in Brazil and the USA, the characteristics of positive cloud-to-ground (+CG) leaders are presented. The high frame rates permitted the average, 2-dimensional speeds of development along the paths of the channels to be resolved with good accuracy. The values range from 0.3 to 6.0 Â 10 5 ms À1 with a mean of 2.7 Â 10 5 ms À1. Contrary to what is usually assumed, downward +CG leader speeds are similar to downward ÀCG leader speeds. Our observations also show that the speeds tend to increase by a factor of 1.1 to 6.5 as they approach the ground. The presence of short duration, recoil leaders (RLs) during the development of positive leaders reveal a highly branched structure that is not usually recorded when using conventional photographic and video cameras. The existence of the RLs may help to explain observations of UHF-VHF radiation during the development of +CG flashes. Citation: Saba, M. M. F.,

[1] From analysis of digital high-speed video records of 233 negative cloud-to-ground (CG) lightning flashes associated with 27 thunderstorms in southeastern Brazil, various lightning properties have been determined. The analysis of the video records showed that although 20% of them were single-stroke flashes and the average number of strokes per flash was 3.8, a significant variation was observed in these parameters from storm to storm. In a smaller subset containing 138 flashes, 70 (51%) had multiple terminations on the ground. As 138 flashes produced 235 different strike points, the average number of strike points per CG flash was 1.70. Considering that in this study the missing of strokes is practically negligible, we can say from the average multiplicity and from the average number of strike points per flash that each ground contact point is, on average, struck 2.2 times. The 608 time intervals between strokes of 186 negative multiple-stroke flashes presented a geometric mean value of 61 ms. Although time intervals preceding subsequent strokes that create a new termination tend to be greater than intervals between subsequent strokes that follow the previously formed channel, the difference was not statistically significant. A strong positive correlation between the number of subsequent strokes in a flash and the flash minimum duration may indicate that some processes concerning the time requisite for the channel decay and for the positive leader in the cloud to provide more charges for the next stroke do not permit multiple strokes to occur under a certain minimum time duration.

[1] This study revisits the sprite polarity paradox, first manifest by observations that exceptional cloud-to-ground flashes with negative polarity generally did not produce detectable sprites. The paradox is here resolved by the Transient Luminous Event (TLE) known as the halo, which on account of its inferior brightness (0.3 MR versus 1.5 MR) and substantially shorter duration (1 ms versus 10-100 ms) in comparison with the sprite, is not readily detectable in ground-based video cameras with standard field duration (16.7-20 ms). Observations with improved temporal resolution (ISUAL (Imager of Sprites and Upper Atmospheric Lightnings) from space and PIPER (Photometric Imager of Precipitated Electron Radiation) observations from the ground) provide evidence that flashes with negative polarity dominate the global halo population, and that the halo numbers are more than sufficient to account for the previously missing TLEs. The evidence for lightning polarity-dependent TLEs (sprites, positive and halos, negative) is attributable to the well established but incompletely understood contrast in the behavior of negative and positive lightning flashes to ground.

Why lightning sometimes has multiple discharges to ground is an unanswered question. Recently, the observation of small plasma structures on positive leaders re-ignited the search. These small plasma structures were observed as pulsing radio sources along the positive leader length and were named “needles”. Needles may be the missing link in explaining why lightning flickers with multiple discharges, but this requires further confirmation. In this work we present the first optical observations of these intriguing plasma structures. Our high-speed videos show needles blinking in slow motion in a sequential mode. We show that they are formed at unsuccessful leader branches, are as bright as the lightning leaders, and report several other optical characteristics.

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