2014

Schulz W., S. Pedeboy, M. M. F. Saba:
LLS Detection Efficiency of Ground Strike Points

32nd International Conference on Lightning Protection (ICLP), Shanghai, China, 2014

During the last years the ground strike points of a flash got more attention because it was realized that risk estimation should not be performed with flash densities but with ground strike point densities. In countries with a lightning location system (LLS) the ground flash densities are normally derived from the LLS data. Recently an effort has been made to derive also ground strike points and ground strike point densities from LLS data [1], [2]. Detection efficiencies (DE) for flashes are well understood and often used to correct the ground flash densities. In this paper we show that also ground strike point densities determined with a LLS exhibit a DE. We further present a theoretical estimation of this DE, which we validate with real data from video and E-field measurements.

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Defer E., O. Bousquet, J. Ribaud, J. Pinty, S. Coquillat, W. Rison, P. Krehbiel, R. Thomas, W. Schulz:
Properties of the Lightning Activity at Storm Scale during HyMeX SOP1 Campaign : Comparison Between an Isolated Storm (05 Sept 2012), a Multi-cellular

XV International Conference on Atmospheric Electricity (ICAE), Norman, Oklahoma, USA, 2014

During the HyMeX (Hydrology cycle in the Mediterranean Experiment) SOP1 (Special Observation Period 1; September-November 2012) campaign, the New Mexico Tech Lightning Mapping Array in conjunction with four European operational lightning detection networks (ATDNET, UKMO; EUCLID; LINET, nowcast; ZEUS, NOA) recorded the total lightning activity over South-Eastern France. We present here observations collected during three different weather situations: one isolated thunderstorm occurring on the 5th of September, a multi-cellular system on the 24th of September (HyMeX IOP6 case) and the 14 November tornadic cell. So far the analysis of the lightning data has been focusing on some specific parameters or features like flash density, convection surge or intra-cloud ratio. We first briefly describe the instrumentation operated during the field campaign and the methodology applied to analyze the data. Some properties of the lightning activity (e.g. flash rate, intra-cloud ratio, altitude of flash triggering) are then discussed according to the type and the stage of convective clouds and related to the properties of the parent clouds as derived from concurrent radar and satellite observations. Further investigations on relating the lightning activity to the cloud properties are currently underway and results will be presented during the conference.

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Anderson G., H.-D. Betz, O. Bousquet, S. Coquillat, J. Delanoe, V. Ducrocq, T. Farges, L.-J. Gallin, V. Kotroni, P. Krehbiel, K. Lagouvardos, G. Molinié, P. Ortéga, S. Pedeboy, J.-P. Pinty, J.-F. Ribaud, E. Richard, W. Schwarzenboeck, W. Rison, W. Schulz,

XV International Conference on Atmospheric Electricity (ICAE), Norman, Oklahoma, USA, 2014

The PEACH (Projet en Electricité Atmosphérique pour la Campagne HyMeX) project is the Atmospheric Electricity component of the decadal HyMeX (Hydrology cycle in the Mediterranean Experiment) project. PEACH aims at measuring and analyzing the lightning activity and electrical state of thunderstorms over the Mediterranean Sea for the HyMeX Community. During the SOP1 (Special Observation Period 1; September-November 2012), records of four European operational lightning detection networks
(ATDNET, UKMO; EUCLID; LINET, nowcast; ZEUS, NOA) and the NMT Lightning Mapping Array were used to document the total lightning activity over South-Eastern France. Other research instruments such as electric field sensors (ALDIS; LA; NMT), video cameras (ALDIS; ONERA), micro-barometer and microphone arrays (CEA) were deployed to characterize the properties of the lightning flashes as well as the electrical state of parent thunderclouds. All these observations are used to describe the evolution of the electrical activity during the life cycle of SOP1 storms in conjunction with microphysics and kinematics description of the parent storms as derived from groundbased radar, ground-based and airborne in situ observations. Cloud models (WRF; MESO-NH with electrification and lightning schemes) are used to interpret the observational-based results. We will first present an overview of the observations collected during the SOP1. Results of flash-scale, storm-scale and regional-scale analysis will then be discussed. We will also introduce some of the products that will be made available to the HyMeX Community. Finally we will discuss on the next steps of the PEACH project. The French MISTRALS program and the ANR IODA-MED project support the PEACH project. Additional supports came from Université de Toulouse, the French LEFA-IDAO program, GOES-R Visiting Program.

Vergeiner C., E. Defer, P. Laroche, W. Rison, P. Krehbiel, R. Thomas, W. Schulz, G. Anderson, S. Pedeboy, F. Malaterre, S. Soula:
Investigation of Lightning Properties during HyMeX SOP1 Storms based on Distinct Observation Data Sets – First Analysis and

8th HyMeX Workshop, 2014

During HyMeX (Hydrology cycle in the Mediterranean Experiment) SOP1 (Special Observation Period 1), a number of observational systems such as a Lightning Mapping Array (HyLMA), Operational Lightning Locating Systems (OLLSs), Slow Antennas (SLAs), a Fast Antenna as a part of the Video and Field Recording System (VFRS) and an Electric Field Mill (EFM) were operated in order to record the electrical state of thunderstorms and the lightning activity. This unique lightning dataset in Europe provides key information to investigate the properties of the lightning flashes. Each observation system/instrument itself has its advantages, disadvantages as well as limits. Taking simultaneous records of different systems/instruments for the same lightning flash into account, a more precise interpretation of each flash component can be done and several uncertainties, caused by the limits of the systems/instruments, can be eliminated. The temporal and spatial development of lightning discharges based on HyLMA, OLLS, SLA, VFRS and EFM data are investigated and interpreted. To start the analysis we chose the 24th September IOP6 case which was characterised by a remarkable and well-sampled lightning activity over the HyMeX SOP1 Cevennes-Vivarais domain. The analysis procedures, the current state of the analysis as well as some interesting cases will be discussed.

Gallin L.-J., T. Farges, R. Marchiano, F. Coulouvrat, E. Defer, W. Rison, W. Schulz, M. Nuret:
Acoustic characterization of lightning discharges

8th HyMeX Workshop, 2014

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Pédeboy S., E. Defer, W. Schulz:
Performance of the EUCLID network in cloud lightning detection in the South-East France

8th HyMeX Workshop, 2014

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Defer E., A. Berne, O. Bousquet, J. Delanoë, P. Krehbiel, S. Pedeboy, J.-P. Pinty, J.-F. Ribaud, W. Rison, W. Schulz, J. Schwarzenboeck, R. Thomas, J. Van-Baelen:
Preliminary results on the study of the lightning activity and the cloud properties recorde

8th HyMeX Workshop, 2014

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Chum J., G. Diendorfer, T. Sindelarova, J. Base, F. Hruska:
Narrow infrasound pulses from lightning ; are they of electrostatic or thermal origin?

European Geosciences Union, General Assembly (EGU), 2014

Narrow ∼(1–2 s) infrasound pulses that followed, with ∼11 to ∼50 s delays, rapid changes of electrostatic field were observed by a microbarometer array in the Czech Republic during thunderstorm activity. The angles of arrival (azimuth and elevation) were analyzed for selected distinct events. Comparisons of distances and azimuths of infrasound sources from the center of microbarometer array with lightning locations determined by EUCLID lightning detection network show that most of the selected events are most likely associated with intra-cloud (IC) discharges. Preceding rapid changes of electrostatic field, potential association of infrasound pulses with IC discharges, and high elevation angles of arrival for near infrasound sources indicate that an electrostatic mechanism is probably responsible for their generation. It is discussed that distinguishing of the relative role of thermal and electrostatic mechanism is difficult, and that none of published models of electrostatic production of infrasound thunder can explain the presented observations precisely. A modification of the current models, based on consideration of at least two charged layers is suggested. Further theoretical and experimental investigations are however needed to get a better description of the generation mechanism of those infrasound pulses.

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Diendorfer G., H. Pichler, W. Schulz:
EUCLID Located Strokes to the Gaisberg Tower – Accuracy of Location and its assigned Confidence Ellipse

23rd International Lightning Detection Conference and 5th International Lightning Meteorology Conference (ILDC/ILMC), Tucson, Arizona, 2014

In this paper we analyze LLS located discharges (return strokes and superimposed ICC pulses) to the Gaisberg Tower (GBT) in terms of their location accuracy and assigned confidence ellipse (often also called error-ellipse). From 2000 to 2013 EUCLID (ALDIS) located 681 return strokes and 779 ICC pulses in upward initiated flashes from the GBT. We found that for 49 % of the return strokes and for 48 % of the ICC pulses the true tower location was inside the 50% confidence ellipse assigned by the LLS to the LLS estimated striking point. After implementation of several improvements in the location algorithm median location accuracy for strokes to the GBT is in the range of 100 m. For the most recent years (GBT data since 2010) we observe a significantly higher than expected percentage of GBT location being included in the assigned confidence ellipse. Most likely this is a result of the discretization of the length of the semi-major axis of confidence ellipse in 100 m steps, which is in the same range as the median location accuracy.

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Diendorfer G., H. Zhou, H. Pichler, R. Thottappillil, M. Mair:
Currents and radiated E-fields of upward initiated lightning from the Gaisberg Tower in Austria

European Geosciences Union, General Assembly (EGU), 2014

Parameters of upward initiated lightning from tall objects are gaining increasing interest in recent years. This is a result of the installation of tall objects (wind turbines, radio towers, etc.) experiencing a high number of lightning flashes. Lightning current waveforms of flashes initiated from the Gaisberg Tower (GBT) in Austria are continuously measured since 1998. On average this radio tower (tower height 100 m) located on a small mountain (1287 m ASL) next to the city of Salzburg triggers about 60-70 flashes per year. More than 50% of the triggered flashes occurred during cold season (similar to so-called winter lightning in Japan) and more or less independent of the overall thunderstorm activity in Austria. Up to now, more than 800 flashes have been recorded by employing a 0.25 m shunt at the tower top. Compared to inductive sensors, with their limited lower bandwidth, a shunt allows correct measurement of the slowly varying, low amplitude initial continuing current (ICC). ICCs are lasting for several hundreds of milliseconds and they are the main contributors to the transferred charge by a flash. Maximum transferred charge in a single flash was 783 C and this negative flash was recorded during cold season on October15th, 2012. Contrary to observations in winter lightning in Japan, where upward initiated flashes with very large charge transfer are predominantly bipolar, at the GBT 7 out of the 10 flashes exceeding a charge transfer of 300 C were negative, 2 were positive and 1 bipolar.

Most of the flashes (93%) triggered by the GBT are of negative polarity, initiated by a positive upward propagating leader. But also positive (4%) and bipolar (3%) flashes are observed at the GBT. In case of negative lightning flashes, 45% of current records exhibit an ICC only, 23% of flashes has pulses of peak current exceeding 2 kA superimposed on the ICC, and return strokes following the ICC after a period of “no current” is observed in 32% of the events. Median peak current of return strokes (N=913) is 9.2 kA and similar to values observed in triggered lightning and to lightning location system peak current estimates for subsequent strokes in cloud-to-ground lightning. Detectability of upward flashes by typical lightning location systems strongly depends on the presence of fast rising current pulses, either as return strokes or superimposed pulses on the ICC. In addition to the current records, corresponding vertical electric fields at close distance (170m) and far distance (about 100 km) are measured with flat-plate antennas. Upward initiated lightning often shows extensive branching and this is assumed to be the reason for the observation of rather complex overall current waveforms. In the presentation we will provide a review of the statistical analyses of the lightning parameters. Records of the E-fields at a distance of about 100 km and radiated by the return strokes to the tower show a significantly shorter peak-to-zero time (10 µs) than typically observed in cloud-to-ground lightning (30 – 40 μs).

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Diendorfer G., H. Pichler, G. Achleitner, M. Broneder:
Lightning caused outages in the Austrian Power Grid transmission line network

32nd International Conference on Lightning Protection (ICLP), Shanghai, China, 2014

Austria is one of the European countries with relatively high lightning activity [1] and lightning is a main cause of transmission line outages. In this presentation we have analyzed in detail lightning caused outages in the grid of the Austrian transmission system operator Austrian Power Grid (APG). 222 outage related lightning flashes were identified in the database of the Austrian lightning location system ALDIS as these flashes were located within a corridor of ±1 km along the considered lines and within a time window of ±1 second of the recorded outage time. These 222 flashes (203 of negative and 19 of positive polarity) are analyzed in terms of their local occurrence along the transmission lines and their peak currents. Tower footing resistances, which are affecting the occurrence of back flashover failures, were found to be log-normal distributed with a median of 4.4 Ω and a σln = 1.1. Median peak current of negative and positive flashes causing transmission line outages is -11.3 kA and 37.9 kA, respectively, and median distance of flash locations to the line is 322 m.

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Lugrin G., N. M. Parra, F. Rachidi, M. Rubinstein, S. Member, G. Diendorfer:
On the Location of Lightning Discharges Using Time Reversal of Electromagnetic Fields

IEEE Transaction on Electromagnetic Compatibility, Vol. 56, pp. 149–158, 2014

In this paper, we discuss the use of the electromagnetic time reversal (EMTR) method to locate lightning strikes. After a brief description of the EMTR and its application to lightning location, we mathematically demonstrate that the time-of-arrival method can be seen as a subset of EMTR. We propose three different models of backpropagation to address the issue of EMTR not being invariant for lossy media. Two sets of simulations are carried out to evaluate the accuracy of the proposed methods. The first set of simulations is performed using numerically generated fields and the proposed algorithm is shown to give very good results even if the soil is not perfectly conducting. In particular, we show that considering a model in which losses are inverted in the back propagation yields theoretically exact results for the source location. We show also that a lack of access to the complete recorded waveforms may lead to higher location errors, even though the computed errors are found to be within the range of performance of current lightning location systems (LLS). A second set of simulations is performed using the sensor data reported by the Austrian LLS. The locations obtained by way of the proposed EMTR method using only the available sensor data (amplitude, arrival time, and time-to-peak), are observed to be within a few kilometers of the locations supplied by the LLS.

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Poelman D. R., W. Schulz, G. Diendorfer, M. Bernardi:
European cloud-to-ground lightning characteristics

32nd International Conference on Lightning Protection (ICLP), Shanghai, China, 2014

Cloud-to-ground (CG) lightning data from the European Cooperation for Lightning Detection (EUCLID) network over the period 2006-2012 are explored. Mean CG flash densities vary over the European continent, with the highest density of about 7 km-2yr-1 found at the triple point between Austria, Italy and Slovenia. The majority of lightning activity takes place between May and September, accounting for 85% of the total observed CG activity. Furthermore, the thunderstorm season reaches its highest activity in July, while the diurnal cycle peaks around 1500 UTC. A difference between CG flashes over land and sea becomes apparent when looking at the peak current estimates. It is found that flashes with higher peak currents occur in greater numbers over sea than over land

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Schulz W., D. R. Poelman, S. Pedeboy, C. Vergeiner, H. Pichler, G. Diendorfer, S. Pack:
Performance Validation of the European Lightning Location System EUCLID

CIGRE International Colloquium on Lightning and Power systems, Lyon, France, 2014

In this paper we present a performance analysis of the European lightning location system EUCLID in terms of location accuracy (LA), detection efficiency (DE) and peak current estimation. The performance analysis is based on ground truth data from direct lightning current measurements at the Gaisberg Tower (GBT) and data from E-field and video recordings. The E-field and video recordings were taken in three different regions in Europe, in Austria, in Belgium and in France. The analysis shows a significant improvement of the LA over the past seven years. Currently the median LA is in the range of 100 m. The observed DE in Austria and Belgium is similar yet a slightly lower value is found in France because during the measurement period in France a nearby lightning location sensor was out of order. The accuracy of the lightning location system (LLS) peak current estimation for subsequent strokes is reasonable keeping in mind that the LLS estimated peak currents are determined from the radiated electromagnetic fields assuming a constant return stroke speed.

The results presented in this paper can be used to estimate the performance of the EUCLID network for regions with similar sensor baseline and sensor technology.

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Smorgonskiy A., A. Tajalli, F. Rachidi, M. Rubinstein, G. Diendorfer, H. Pichler:
Analysis of Lightning Events Preceding Upward Flashes from Gaisberg and Säntis Towers

32nd International Conference on Lightning Protection (ICLP), Shanghai, China, 2014

In this paper, we present an analysis of the initiation of upward lightning flashes for the Gaisberg and the Säntis Towers. The results are compared with previous studies. It is found that the majority of upward lightning discharges from both towers are initiated without any preceding lightning activity. We show also that the results of the presented studies on the initiation of upward flashes from tall structures might be affected by the selected parameters of the study.

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Zhou H., R. Thottappillil, G. Diendorfer:
A new approach to calculate electric fields and charge density distribution when lightning strikes a tall object

Electric Power Systems Research, Vol. 113, pp. 15–24, 2014

We derive electric field expressions, associated with lightning strikes to a tall object, using the monopole (Continuity Equation) technique which is distinctly different from the traditional dipole (Lorentz Condition) technique. Expressions to calculate the charge density along the tall object and lightning channel based on the assumptions of the transmission line model of the lightning strikes to a tall object and a series point current source placed at the object top, are also derived. These expressions are used to calculate the very close-range electric fields in the monopole (Continuity Equation) technique in terms of the retarded current and charge density along the tower and lightning channel and their results are compared with those calculated from the traditional dipole (Lorentz Condition) technique in terms of the retarded current along the tower and lightning channel. Alternative explanations are provided to the inversion of polarity of the vertical electric field at very close range based on distribution of charge density along the tower and lightning channel.

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Zhou H., G. Diendorfer, R. Thottappillil, H. Pichler, M. Mair:
The Influence of Meteorological Conditions on Upward Lightning Initiation at the Gaisberg Tower

32nd International Conference on Lightning Protection (ICLP), Shanghai, China, 2014

Meteorological parameters associated with the initiation of upward lightning discharges from a tall tower were investigated. The parameters, including temperature, wind speed, air pressure, and relative humidity, were measured at the Gaisberg Tower top and at a distance of 170 m from the tower. A comparison of parameters associated with self-initiated upward flashes (initiated without any nearby preceding lightning activity) and nearby-lightning-triggered upward flashes (triggered by nearby preceding lightning activity) was made. It shows that at the Gaisberg Tower (GBT), relatively low ambient temperature facilitates to initiate upward flashes without any nearby preceding lightning activity. Relatively high ambient temperature (8.5 °C to 15.5 °C) facilitates to initiate upward positive stepped leaders, rather than the common continuous leaders, in the self-initiated upward negative flashes. No reliable correlation between wind speed and upward initiation of lightning was found.

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Nag A., M. J. Murphy, W. Schulz, K. L. Cummins:
Lightning Locating Systems : Characteristics and Validation Techniques

32nd International Conference on Lightning Protection (ICLP), Shanghai, China, 2014

Ground-based or satellite-based lightning locating systems are the most common way to geolocate lightning. Depending upon the frequency range of operation, such systems can also report a variety of characteristics associated with lightning events (channel formation processes, leader pulses, cloud-to-ground return strokes, M-components, ICC pulses, and cloud lightning pulses). In this paper, we summarize the various methods to geolocate lightning, both ground-based and satellite-based, and discuss the characteristics of lightning data available from various sources. The performance characteristics of lightning locating systems are determined by their ability to geolocate lightning events accurately and report various features such as lightning type and peak current. We examine the various methods used to validate the performance characteristics of different types of lightning locating systems.

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Schulz W., S. Pedeboy, C. Vergeiner, E. Defer, W. Rison:
Validation of the EUCLID LLS during HyMeX SOP1

23rd International Lightning Detection Conference and 5th International Lightning Meteorology Conference (ILDC/ILMC), Tucson, Arizona, 2014

This paper deals with performance evaluation of the European lightning location system EUCLID in France during the HyMeX [1] Special Observation Period 1 (SOP1) in 2012. Beside other instruments a Lightning Mapping Array (HyLMA) and a mobile Video and Field Recording System (VFRS) was deployed in the south of France. The data of those independent systems are used to determine the performance of the EUCLID lightning location system (LLS) in terms of detection efficiency (DE) and location accuracy (LA) for both CG and IC flashes.

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Pédeboy S. and W. Schulz:
Validation of a ground strike point identification algorithm based on ground truth data

23rd International Lightning Detection Conference and 5th International Lightning Meteorology Conference (ILDC/ILMC), Tucson, Arizona, 2014

Lightning locating systems (LLS) can provide flash data derived from individual return stroke based on a flash grouping algorithm. However the latter considers negative cloud-to-ground (CG) flashes striking the ground in a unique point represented by the location of the first return stroke. According to video observations flashes have often different ground strike points. This can be a limitation in some engineering applications like the lightning risk assessment where the actual number of ground contacts is an important parameter. To get around this limitation Météorage has developed an algorithm allowing the identification of the location of the ground strike points (GSP) based on a statistical clustering (‘k-means’) method. The effectiveness of this algorithm, using operational LLS data, is tested on a total of 227 negative CG flashes observed with high speed video cameras in Austria and in France, in 2012 and 2013 respectively. The comparison between GSP computation and video observations reveals a GSP detection efficiency (DE) of about 95%. In addition the algorithm is able to discriminate between strokes creating a new ground contact (NGC) or using a pre-existing channel (PEC) in 83% out of the 767 observed strokes. The analysis shows that the limitation of the model is highly depending on the DE and location accuracy (LA) of the LLS collecting the data. Nevertheless, the fairly good results obtained with the GSP identification algorithm permits to build from existing VLF/LF LLS lightning data a hierarchical interlocked data structure composed of chronological events starting with the flash as the root event which is composed of GSPs being containing themselves strokes. This new dataset describes in a more complete way some lightning parameters related to a flash (e.g. flash multiplicity and number of ground strike points per flash) and their individual relationship, giving room to the improvement of engineering and research applications.

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Marshall T., W. Schulz, N. Karunarathna, S. Karunarathne, M. Stolzenburg, C. Vergeiner, T. Warner:
On the percentage of lightning flashes that begin with initial breakdown pulses

Journal of Geophysical Research: Atmospheres, Vol. 119, No. 2, p. 2013JD020854, 2014

The initial breakdown (IB) stage of lightning flashes typically occurs in the first 20ms of a flash and
includes a series of IB pulses often detected with electric field change sensors. There is disagreement about the percentage of negative cloud-to-ground (CG) flashes that begin with IB pulses. This study includes new data on IB pulses in 198 CG flashes in Austria (latitude ~48°N), Florida, USA (~29°N) and South Dakota, USA (~44°N) with, respectively, 100%, 100%, and 95% of the flashes having IB pulses. The data indicate that the amplitude of the largest IB pulse, range normalized to 100 km, is often weak,< 0.5 Vm1,with the lower latitude having a greater percentage (36%) of these weak maximumIB pulses than the higher latitude (11%). Since sensor noise levels are often larger than this value, detection of smaller amplitude IB pulses may be difficult. A similar result is seen in the amplitude ratio of the largest IB pulse to the first return stroke: at the lower latitude, 50% of flashes had a ratio<0.1 versus 8% of flashes at the higher latitude. However, comparisons of the amplitude ratios from
Austria (~48°) and South Dakota (~44°) do not support a simple latitude dependence. The data also show that 5–10% of IB pulses occur more than 100ms before the first return stroke. These findingsmay explain why some previous studies found percentages <100%. Overall, the results indicate that all negative CG flashes probably begin with IB pulses.

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