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High‐speed video data were used to analyze the initiation and propagation of needles and their associated flickering events observed in positive cloud‐to‐ground (+CG) flashes. Needles occurred during the return‐stroke later stage and the continuing current. They initiated near the lateral surface of the positive leader channel and extended almost perpendicular to that channel. Flickering events are recoil type streamers (or leaders) that retrace the channels created by needles. Needles are caused by the radial motion of negative charge from the hot core of the positive‐leader channel into the positive corona sheath surrounding the core, when the core is rapidly recharged (its radial electric field reversed) by the return‐stroke process and during the following continuing current. Needles could evolve into self-propagating negative leaders. Self-propagating negative leaders evolving from needles exhibit stepped development via connection of space stems/leaders to the tips of negative leaders. The occurrence of space stems/leaders and their connection to the tips of needles resulting in the continuous extensions of needle channels play an important role in the evolution of needles into self-propagating negative leaders., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

We present observations of X-rays from laboratory sparks created in the air at atmospheric pressure by applying an impulse voltage with long (250 &micro, s) rise-time. X-ray production in 35 and 46 cm gaps for three different electrode configurations was studied. The results demonstrate, for the first time, the production of X-rays in gaps subjected to switching impulses. The low rate of rise of the voltage in switching impulses does not significantly reduce the production of X-rays. Additionally, the timing of the X-ray occurrence suggests the possibility that the mechanism of X-ray production by sparks is related to the collision of streamers of opposite polarity.

The important data supporting the conclusion of the manuscript is submitted to Geophysical Research Letters.

We analyze currents of 30 positive flashes recorded on the Säntis tower, Switzerland, from May 2010 to January 2012. The currents were classified into two types. Type 1 events exhibit a large, unipolar main pulse with a risetime of tens of microseconds, followed by subsidiary peaks separated by millisecond-scale intervals. The main pulse in three flashes was preceded by a slowly rising ramp lasting several milliseconds and was followed by a relatively steady current with superimposed positive and negative pulses, constituting the first direct evidence of M components of both polarities. In four of the five type 1 flashes, the main current was preceded by pulse bursts, presumably due to attempted negative leaders. Type 2 events are characterized by a millisecond-scale waveform with large, oscillatory pulse trains on its rising portion. These pulse trains are inferred to be due to upward negative stepped leaders. Peak currents of type 2 flashes are associated with the fast pulses. Our positive flashes of both types have a median peak current of 11.1 kA and a median duration of 80 ms, consistent with data recorded in Austria. Our measured median transferred charges are about 6 times larger than those at Switzerland's Monte San Salvatore and in Japan and about 3 times larger than in Austria. Eight type 2 flashes in our data set transported over 500 C of positive charge to ground. Our five type 1 events appear to be similar (except for the pulse duration) to their counterparts examined by Berger et al. (1975). Our type 2 events are “classical” upward flashes.

We present a study focused on pulses superimposed on the initial continuous current of upward negative discharges. The study is based on experimental data consisting of correlated lightning current waveforms recorded at the instrumented Santis Tower in Switzerland and electric fields recorded at a distance of 14.7 km from the tower. Two different types of pulses superimposed on the initial continuous current were identified: (1) M-component-type pulses, for which the microsecond-scale electric field pulse occurs significantly earlier than the onset of the current pulse, and (2) fast pulses, for which the onset of the field matches that of the current pulse. We analyze the currents and fields associated with these fast pulses (return-stroke type (RS-type) initial continuous current (ICC) pulses) and compare their characteristics with those of return strokes. A total of nine flashes containing 44 RS-type ICC pulses and 24 return strokes were analyzed. The median current peaks associated with RS-type ICC pulses and return strokes are, respectively, 3.4 kA and 8 kA. The associated median E-field peaks normalized to 100 km are 1.5 V/m and 4.4 V/m, respectively. On the other hand, the electric field peaks versus current peaks for the two data sets (RS-type ICC pulses and return strokes) are characterized by very similar linear regression slopes, namely, 3.67 V/(mkA) for the ICC pulses and 3.77 V/(mkA) for the return strokes. Assuming the field-current relation based on the transmission line model, we estimated the apparent speed of both the RS-type ICC pulses and return strokes to be about 1.4 x 10(8) m/s. A strong linear correlation is observed between the E-field risetime and the current risetime for the ICC pulses, similar to the relation observed between the E-field risetime and current risetime for return strokes. The similarity of the RS-type ICC pulses with return strokes suggests that these pulses are associated with the mixed mode of charge transfer to ground.

We present and discuss current waveforms associated with 13 bipolar lightning flashes recorded at the Santis Tower during the period from June 2010 to January 2015. During this period, a total of 427 flashes were recorded, of which 13 (3%) were classified as bipolar flashes. The majority of the recorded flashes (10 out of 13) exhibited a polarity reversal during the initial continuous current, therefore belonging to Category I according to the classification proposed by Rakov (2003). Of the three remaining flashes, two were characterized by different polarities of the initial stage current and the following return stroke or strokes (Category II), and one flash involved return strokes of opposite polarity within the same flash (Category III). In Category I bipolar flashes, the polarity reversal is usually assumed to be associated with in- cloud processes. However, two of our 10 Category I flashes appeared to be each a sequence of two upward discharges of opposite polarity, initiated from the tower within tens of milliseconds of each other. This is the first time that such a sequence has been observed from the same tower. We suggest that the traditional classification of bipolar flashes should be modified to distinguish between two types of Category I flashes: those in which the polarity reversal during the initial stage is associated with in- cloud processes (Category Ia) and those in which the polarity reversal is due to initiation of two opposite- polarity leaders from the tower (Category Ib).

We present an analysis of measured current waveforms associated with positive and bipolar flashes recorded on the Säntis tower Switzerland from May 2010 till January 2012. The overall number of recorded flashes in the considered period was 201 of which 30 were of positive polarity (effectively transported positive charge to ground) and three were bipolar. The recorded positive flashes were mainly concentrated in the summer months August being the month during which most of them occurred (6 events in 2010 and 16 events in 2011). The observed positive flashes are characterized by a median peak current of 11.8 kA and a median flash duration of 80 ms. These values are consistent with those associated with the data recorded at the Gaisberg Tower in Austria. On the other hand the amount of transferred charge is substantially larger in our dataset with a median value of 169 C (6 times as large as the values obtained in Monte San Salvatore and in Japan and 3 times as large as the value obtained in Austria). Eight flashes out of 30 transported positive charge to the ground in excess of 500 C. The obtained results confirm also the findings of Saba et al. according to which positive lightning flashes may combine high peak currents with high charge transfers (or flash durations). The three bipolar flashes recorded during the considered time period occurred during one storm occurred on August 27 2011. The relative number of bipolar flashes (1.5) is found to be lower than the values reported in previous studies in Austria and in Switzerland (3 to 6). On the basis of the classification proposed by Rakov two of the three observed bipolar flashes belonged to Type 2 and one to Type 3.

In upward flashes, charge transfer to ground largely takes place during the initial continuous current (ICC) and its superimposed pulses (ICC pulses). ICC pulses can be associated with either M-component or leader/return-stroke like modes of charge transfer to ground. In the latter case, the downward leader/return stroke process is believed to take place in a decayed branch or a newly created channel connected to the ICC-carrying channel at relatively short distance from the tower top, resulting in the so-called mixed mode of charge transfer to ground. In this paper, we study the electromagnetic fields associated with the M-component charge transfer mode using simultaneous records of electric fields and currents associated with upward flashes initiated from the Säntis Tower. The effect of the mountainous terrain on the propagation of electromagnetic fields associated with the M-component charge transfer mode (including classical M-component pulses and M-component-type pulses superimposed on the initial continuous current) is analyzed, and compared with its effect on the fields associated with the return-stroke (occurring after the extinction of the ICC) and mixed charge transfer modes. For the analysis, we use a 2D FDTD (2-Dimentional Finite-Difference Time-Domain) method, in which the M-component is modeled by the superposition of a downward current wave and an upward current wave resulting from the reflection at the bottom of the lightning channel (Rakov et al. 1995 model) and the return stroke and mixed mode are modeled adopting the MTLE (Modified Transmission Line with Exponential Current Decay with Height) model. The finite ground conductivity and the mountainous propagation terrain between the Säntis Tower and the field sensor located 15 km away at Herisau are taken into account. The effects of the mountainous path on the electromagnetic fields are examined for ‘classical’ M-component and M-component-type ICC pulses. Use is made of the propagation factors defined as the ratio of the electric or magnetic field peak evaluated along the mountainous terrain to the field peak evaluated for a flat terrain. The velocity of the M-component pulse is found to have a significant effect on the risetime of the electromagnetic fields. A faster travelling wave speed results in larger peaks for the magnetic field. However, the peak of the electric field appears to be insensitive to the M-component wave speed. This can be explained by the fact that at 15 km, the electric field is still dominated by the static component which mainly depends on the overall transferred charge. The contribution of the radiation component to the M-component fields at 100 km accounts for about 77% of the peak electric field and 81% of the peak magnetic field, considerably lower compared to the contribution of the radiation component to the return stroke fields at the same distance. The simulation results show that neither the electric nor the magnetic field propagation factors are very sensitive to the risetimes of the current pulses. However, the results indicate a high variability of the propagation factors as a function of the branch-to-channel junction point height. For junction point heights of about 1 km, the propagation factors reach a value of about 1.6 for the E-field and 1.9 for the H-field. For a junction height greater than 6 km, the E-field factor becomes slightly lower than 1. The obtained results are consistent with the findings of Li et al. (2016b) in which an electric field propagation factor of 1.8 was inferred for return strokes and mixed mode pulses, considering that junction points lower than 1 km or so would result in a mixed-mode of charge transfer, in which a downward leader/return-stroke like process is believed to take place. It is also found that the field enhancement (propagation factor) for return stroke mode is higher for larger ground conductivities. Furthermore, the enhancement effect tends to decrease with increasing current risetime, except for very short risetimes (less than 2.5 s or so) for which the tendency reverses. Finally, model-predicted fields associated with different charge transfer modes, namely return stroke, mixed mode, classical M-component, and M-component-type ICC pulse are compared with experimental observations at the Säntis tower. It is found that the vertical electric field waveforms computed considering the mountainous terrain are in very good agreement with the observed data. The adopted parameters of the models that provide the best match with the measured field waveforms were consistent with observations. The values for the current decay height constant adopted in the return stroke and mixed mode models (1.0 km for the return stroke and 0.8 km for the mixed-mode pulse) are lower than the value of 2.0 km typically used in the literature.

We present simultaneous current and wideband electric field waveforms at 380 km associated with upward lightning flashes initiated from the Santis Tower, Switzerland. To the best of our knowledge, the dataset presented in this study includes the first simultaneous records of lightning currents and associated fields featuring ionospheric reflections for natural upward flashes, and the longest distance at which natural upward lightning fields have been measured simultaneously with their causative currents. The intervals between the groundwave and skywave arrival times are used to estimate ionospheric reflection heights during day and night times using the so-called zero-to-zero and peak-to-peak methods. During daytime, the mean ionospheric reflection heights, obtained using the two different delay estimation approaches, are about 78 and 76 km, corresponding to the D layer. The mean reflection height at nighttime is about 90 and 89 km, corresponding to the E layer. We present a full-wave, finite-difference time-domain (FDTD) analysis of the electric field propagation including the effect of the ionospheric reflections. The FDTD simulation results are compared with the measured fields associated with upward flashes initiated from the Santis Tower. It is found that the model reproduces reasonably well the measured waveforms and the times of arrival of the one-hop and two-hop skywaves relative to the groundwave.

A magnetic head comprising half-cores separated by a non-magnetic layer forming a working gap of the head. The line of intersection of the outer faces of the half-cores and the working surface of the magnetic head is represented by a smoothly varying curve characterized by a specific function causing a monotonically decreasing variation of the integral magnetic conductivity along the generating line of the working surface of the head as measured from the plane of the working gap of the head. In accordance with a method of fabricating the magnetic head, each half-core is machined by a shaped abrasive wheel so as to remove a portion of the body of the half-core in the zone where it is aligned with the working surface of the magnetic head on the side opposite the plane of the working gap of the head. The axis of revolution of the abrasive wheel is caused to move reciprocally at a certain angle to the plane of the working gap and is displaced during each such reciprocal pass by the cutting depth towards the half-core. The recesses formed in the course of removal of the portion of the body of each half-core are filled with a non-magnetic adhesive material.

In this paper, statistical analysis of lightning-induced voltages on distribution lines accounting for lossy ground is proposed. The analysis is performed by means of a Monte Carlo-based probabilistic approach. It aims at deducing parametric distributions that best fit the probability density functions of the induced voltages. A lightning model based on step-function channel-base current is employed.

CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior Este trabalho propõe maneiras de otimizar a detecção e o registro de formas de onda de campos elétricos radiados por descargas atmosféricas. Como resultado das técnicas de otimização desenvolvidas nesta Tese, é apresentado o “Sistema de detecção e armazenamento de formas de onda de raios” ou “Lightning Detection and Waveform Storage System - (LDWSS) ”, como um dispositivo otimizado para o registro de campos eletromagnéticos gerados por raios. Os principais pontos aprimorados foram: custo do dispositivo; ampliação do alcance dinâmico de detecção; desenvolvimento de um sistema móvel; possibilidade de detecção multi-banda e; calibração para inferir, de forma remota, a corrente de pico dos raios detectados. O sistema desenvolvido foi validado, a partir de comparações com dados de raio induzido, no Centro Internacional para Pesquisa e Teste com Raios (International Center for Lightning Research and Testing - ICLRT), dados da Rede Nacional Norte Americana de Detecção de Raios (National Lightning Detection Network - NLDN) e dados do Observatório de Raios em Gainesville (Lightning Observatory in Gainesville - LOG). A principal aplicação do sistema é na investigação da física e dos efeitos das descargas atmosféricas, principalmente na região Amazônica. Como resultado da utilização do sistema obteve-se: melhor entendimento sobre as descargas intra-nuvem do tipo Compact Intracloud Discharge - CID; as características da altura da refletividade da ionosfera calculada utilizando formas de onda de raios intra-nuvem e nuvem-solo; a criação de um banco de dados contendo mais de oito mil formas de onda do campo elétrico de diferentes tipos de raios e; a primeira estação de registro contínuo de formas de onda de campo elétrico devido a raios na Amazônia, localizada no CENSIPAM regional Belém. Palavras-chave: Descargas atmosféricas. Otimização. Formas de onda. Amazônia. LDWSS. This work aims the application of optimization techniques to detect and record lightning electromagnetic waveforms. As a result of the optimization techniques developed on this Thesis, it is presented the “Lightning Detection and Waveform Storage System - (LDWSS)”. The main optimized points were: cost of the device; improvements of its detection dynamic range; development of a mobile device; possibility to detect in a multi-band way; and calibration in order to infer peak currents from remote measurements of lightning electric fields. The developed system was validated through comparison of a return stroke data in lightning triggered using the rocket-and-wire technique at the International Center for Lightning Research and Testing - ICLRT, of National Lightning Detection Network – NLDN data, and data from the Lightning Observatory in Gainesville - LOG. The main usage of the optimized system is on the investigation of lightning physics and effects, mainly in the Amazon region. As a result of using the system were obtained: a better understanding about Compact Intracloud Discharge - CID; the characteristics of ionosphere heights computed using intraclound and cloud-to-ground lightning electric field waveforms; the conception of a lightning electric field waveform database containing more than 8 thousand waveforms of different types of lightning; and the implementation of the first continuous lightning electric field measurement system in the amazon region, localized at CESIPAM, Belem, PA. UFPA - Universidade Federal do Pará

in this paper, we present a new tool for lightning-induced voltage calculations. The tool, a circuit model which can be integrated into power systems simulators, is based on the theory developed by Andreotti et al. (2001, 2009, 2013). In Part II, the accuracy, stability, and efficiency of the new tool are demonstrated via comparison with other solutions/codes found in the literature and with experimental data

in the companion paper (Part I), a new tool for lightning-induced voltage calculations, called CiLIV, has been presented. In this paper (Part II), predictions made with the new tool are compared to experimental data and to outputs of analytical procedures/numerical codes found in the literature.

We present CiLIV (Circuit for Lightning Induced Voltage), a new tool for lightning induced voltage calculations. The tool can be integrated into power systems simulators, and is based on the theory proposed by Andreotti et al. (2001, 2009, 2013). The accuracy, stability and efficiency of the new tool has been demonstrated by comparison with other solutions/codes found in the literature and with experimental data.