Characteristics of Upward Lightning Flashes

In addition to the general aims of lightning research such as lightning physics and meteorology, the study of upward lightning is of particular importance in protection of tall objects such as wind turbines and telecommunication towers. It also helps us in better understanding the lightning initiation process and its role in the earth- atmosphere electrical balance. Within this context, this thesis presents an analysis on various aspects of upward lightning discharge (negative, positive, bipolar) using experimental observation and theoretical modeling for better understanding of its initial stage, the propagation of its electromagnetic field along irregular terrain and its interaction with the ionosphere. Our investigation on the superimposed impulsive components of the initial stage of upward negative flashes revealed that they can transfer net negative charges to ground by both M-component and return stroke modes of charge transfer, which can be distinguished by their associated electric field signature. Moreover, we investigated the ability of Lightning Location Systems (LLSs) to locate and detect upward negative flashes. Different aspects of upward negative flashes which might affect the evaluation performance of LLSs were discussed. It is found that LLSs tend to overestimate the peak current values of RS pulses of upward negative flashes. Using full-wave numerical simulation, it is demonstrated that this overestimation is mainly due to electric field enhancement by wave propagation along mountainous terrain around Säntis Tower. Using simultaneous channel-base current and electric field records of upward positive flashes, we observed that two types of pulsations can be distinguished during the course of progression of upward negative leaders which are very similar to ¿Classical PBPs¿ and ¿Narrow PBPs¿ of the initial stage of downward negative leaders suggesting a general similarity between upward and downward negative leaders. We present and discuss current waveforms associated with 13 bipolar flashes recorded at the Säntis Tower during the period from June 2010 to January 2015. We have found two flashes of our data base each characterized by a sequence of two upward leaders of opposite polarity within the same flash, a scenario that has never been reported from previous observations at instrumented towers. The obtained results suggest that the traditional classification of bipolar flashes should be revisited. We present simultaneous channel-base current and wideband electric field waveforms at 380 km distance from the strike point associated with upward flashes initiated from the Säntis Tower. The dataset presented in this study represents, to the best of the Author¿s knowledge, the first simultaneous records of lightning currents and distant fields associated with natural upward flashes featuring ionospheric reflections. The data are used to infer the characteristics of the ionospheric layers. We present a full-wave 2D FDTD analysis of the field propagation including the effect of the ionospheric reflections and the results are compared with the experimental data. Furthermore, we present a novel semi-analytical simplified approach based on the ray tracing concept to estimate radiated electric fields associated with lightning return strokes, taking into account ionospheric reflections

Common fixed point theorems in modular G-metric spaces

The purpose of this paper is to prove the existence of the unique common fixed point theorems of a pair of weakly compatible mappings satisfying $\Phi$-maps in modular G-metric spaces

An Analysis of Current and Electric Field Pulses Associated With Upward Negative Lightning Flashes Initiated from the Säntis Tower

International audienceWe present a study on the characteristics of current and electric field pulses associated with upward lightning flashes initiated from the instrumented Säntis Tower in Switzerland. The electric field was measured 15 km from the tower. Upward flashes always begin with the initial stage composed of the upward-leader phase and the initial-continuous-current (ICC) phase. Four types of current pulses are identified and analyzed in the paper: (1) return-stroke pulses, which occur after the extinction of the ICC and are preceded by essentially no-current time intervals; (2) mixed-mode ICC pulses, defined as fast pulses superimposed on the ICC, which have characteristics very similar to those of return strokes and are believed to be associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC-carrying channel at relatively small junction heights; (3) " classical " M-component pulses superimposed on the continuing current following some return strokes; and (4) M-component-type ICC pulses, presumably associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC-carrying channel at relatively large junction heights. We consider a data set consisting of 9 return-stroke pulses, 70 mixed-mode ICC pulses, 11 classical M-component pulses, and 19 M-component-type ICC pulses (a total of 109 pulses). The salient characteristics of the current and field waveforms are analyzed. A new criterion is proposed to distinguish between mixed-mode and M-component-type pulses, which is based on the current waveform features. The characteristics of M-component-type pulses during the initial stage are found to be similar to those of classical M-component pulses occurring during the continuing current after some return strokes. It is also found that about 41% of mixed-mode ICC pulses were preceded by microsecond-scale pulses occurring in electric field records some hundreds of microseconds prior to the onset of the current, very similar to microsecond-scale electric field pulses observed for M-component-type ICC pulses and which can be attributed to the junction of an in-cloud leader channel to the current-carrying channel to ground. Classical M-component pulses and M-component-type ICC pulses tend to have larger risetimes ranging from 6.3 to 430 μs. On the other hand, return-stroke pulses and mixed-mode ICC pulses have current risetimes ranging from 0.5 to 28 μs. Finally, our data suggest that the 8-μs criterion for the current risetime proposed by Flache et al. is a reasonable tool to distinguish between return strokes and classical M-components. However, mixed-mode ICC pulses superimposed on the ICC can sometimes have considerably longer risetimes, up to about 28 μs, as observed in this study

Electromagnetic fields associated with the M‐component mode of charge transfer

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 theM‐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 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, https://doi.org/10.1029/95JD01924 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 theM‐component pulse is found to have a significant effect on the risetime of the electromagnetic fields. A faster traveling 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, Azadifar, Rachidi, Rubinstein, Paolone, et al. (2016, https://doi.org/10.1109/TEMC.2015.2483018) 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 μs 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

On the classification of self-triggered versus other-triggered lightning flashes

We present in this paper lightning current measurements and LMA (Lightning Mapping Array) data associated with upward flashes observed at the Sàntis Tower during Summer 2017. The LMA network consists of six stations located in the vicinity of the tower at distances ranging from 100 m to 11 km from it. 20 flashes simultaneously recorded by the current measurement system and the LMA are analyzed. Based on the lightning activity derived from the European Lightning Detection Network (EUCLID) in an area within 30 km from the tower and in a 5-second time window before the start of the flash, all the 20 flashes were classified as 'self-triggered' (ST). However, the investigations based on the LMA data reveal that 3 of the flashes were preceded by nearby activity and should be therefore classified as 'other-triggered' (OT) flashes. The results suggest that the number of OT flashes inferred from LLS data can be underestimated. © 2018 IEEE.Peer ReviewedPostprint (published version

LMA observation of upward flashes at Säntis Tower: preliminary result

Lightning striking tall towers is mainly of the upward lightning type, which is characterized by the absence of a first return stroke and the presence of an initial continuous current (ICC) with or without superimposed pulses. The Säntis Tower is 124 m tall and it is located on the top of the Säntis Mountain (2502 m ASL) in the eastern Swiss Alps. The Tower location exhibited the highest lightning flash density in Switzerland during the period from 1999 to 2006. The Tower was instrumented in May 2010 for the measurement of lightning current parameters. In order to complement these data, a Lightning Mapping Array (LMA) was deployed around the Tower during the Summer 2017. The LMA system locates the sources of radio emissions in the very high frequency range (VHF, 60-66 MHz) in three dimensions by a time-of-arrival analysis. The LMA system allows detailed analysis of individual flashes, through the mapping of the lightning channels in the cloud with sufficient time resolution and spatial precision to locate the origin and propagation of each flash. With the help of the LMA, we intend to further investigate the initiation and propagation characteristics of upward lightning emerging from the Tower. From June 29 to August 15, 2017, 33 upward flashes initiated from the Säntis tower were registered by the LMA network. These records are the first set of VHF total lightning mapping obtained in Switzerland. Preliminary results of the campaign are presented in this paper. © 2018 IEEE.Peer ReviewedPostprint (published version

Polarimetric radar characteristics of lightning initiation and propagating channels

In this paper we present an analysis of a large dataset of lightning and polarimetric weather radar data collected in the course of a lightning measurement campaign that took place in the summer of 2017 in the area surrounding Säntis, in the northeastern part of Switzerland. For this campaign and for the first time in the Alps, a lightning mapping array (LMA) was deployed. The main objective of the campaign was to study the atmospheric conditions leading to lightning production with a particular focus on the lightning discharges generated due to the presence of the 124¿m tall Säntis telecommunications tower. In this paper we relate LMA very high frequency (VHF) sources data with co-located radar data in order to characterise the main features (location, timing, polarimetric signatures, etc.) of both the flash origin and its propagation path. We provide this type of analysis first for all of the data and then we separate the datasets into intra-cloud and cloud-to-ground flashes (and within this category positive and negative flashes) and also upward lightning. We show that polarimetric weather radar data can be helpful in determining regions where lightning is more likely to occur but that lightning climatology and/or knowledge of the orography and man-made structures is also relevant.Peer ReviewedPostprint (author's final draft