111 research outputs found

    Development of a VHF broadband interferometer to investigate unsolved lightning phenomena: high-energy radiation from lightning strikes and multi-stroke positive cloud-to-ground flashes

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    Tesi en modalitat de compendi de publicacions.(English) The aim of this thesis work is the development of a VHF broadband interferometer for lightning research. The observations with this instrument allowed us to investigate some unsolved aspects of lightning related with the high-energy emissions from lightning strikes and the occurrence of multi-stroke positive cloud-to-ground flashes sharing the same channel to ground. This Ph.D. project is contextualized in the European program: Science and Innovation with thunderstorms (SAINT), funded by the Horizon 2020 / Marie Sklodowska Curie Action, grant agreement ID: 722337. A VHF broadband interferometer is an instrument capable of mapping lightning flashes with extremely high temporal resolution. This feature, combined with the omnidirectional field of view and the ability to map lightning within the cloud, makes this instrument really promising for scientific lightning research. Its use is enriched and enhanced by the installation of other detectors or measuring instruments. In this way, it is possible to simultaneously observe with several instruments the same lightning flash and obtain complementary information needed to advance in the understanding of the physical phenomena. The main technical challenge of this work was to build our own version of a broadband VHF digital interferometer. This kind of instrument is still a prototype, off the market, and although some books and scientific papers describe how it works, its hardware and software implementation requires a lot of work and refinements to achieve the high performance required by the scientific research. Once the instrument was built and validated, field campaigns were carried out in Spain, Colombia and Italy. Observations made with our VHF broadband interferometer in coincidence with other instruments such as the high-energy detectors, electric field antenna, and Lightning Mapping Array (LMA) have made it possible to make some progress in understanding some unsolved aspects of lightning physics. The results of our research have been published mainly in the two scientific articles that constitute this Ph.D. thesis. The first article of this compendium presents measurements in coincidence of high-energy radiation with the lightning mapping provided by the VHF broadband interferometer. During a field campaign in north-central Colombia, several intense X-ray bursts were detected from negative stepped leaders and dart leaders. A strong temporal correlation has been observed between the high-energy emissions and the most intense VHF pulses, which suggests the runaway electrons as a shared mechanism. We observed multiple high-energy sources belonging to different leader branches and we speculate that the high-energy emissions could occur more often than expected. We speculated a few possible implications for TGFs. The second article presents the first observation of a multi-stroke positive cloud-to-ground lightning flash sharing the same channel to ground mapped with a VHF broadband interferometer and a Lightning Mapping Array. We propose a mechanism to describe this rare phenomenon and we identify in a fast recoil leader or a fast breakdown a crucial role in reconnecting these previously decayed leader channels and initiating the subsequent positive stroke. We investigated the polarity asymmetry of subsequent strokes.(Español) El objetivo de este trabajo de tesis consistió en desarrollar un interferómetro de banda ancha en Very High Frequency (VHF) para la investigación de rayos. Las observaciones con este instrumento nos permitieron investigar algunos aspectos no resueltos de las descargas eléctricas atmosféricas relacionadas con emisiones de alta energía al igual que ocurrencia de rayos a tierra de polaridad positiva que comparten el mismo canal. Este proyecto de doctorado está en el marco del programa europeo Ciencia e Innovación con tormentas eléctricas (SAINT) financiado por la ayuda Horizonte 2020 / Marie Sklodowska Curie, grant agreement: 722337. Un interferómetro de banda ancha VHF es un instrumento capaz de mapear los rayos con una resolución temporal de alta velocidad. Esta característica, combinada con el campo de visión omnidireccional y la capacidad de mapear los rayos dentro de la nube, hace que este instrumento sea realmente prometedor para la investigación científica de los rayos. Su uso se enriquece y mejora con la instalación de otros detectores o instrumentos de medición. De este modo, es posible observar simultáneamente con varios instrumentos el mismo relámpago y obtener información complementaria necesaria para avanzar en el entendimiento de los fenómenos físicos asociados con los rayos. El principal reto técnico de este trabajo ha sido construir nuestra propia versión de un interferómetro digital VHF de banda ancha. Este tipo de instrumento es todavía un prototipo fuera del mercado, y aunque algunos libros y artículos científicos describen su funcionamiento, su implementación de hardware y software requiere aún mucho más trabajo y perfeccionamiento para lograr su alto rendimiento. Una vez construido y validado el instrumento, se realizaron campañas de observación en España, Colombia e Italia. Las observaciones realizadas con nuestro interferómetro en conjunto con otros instrumentos como los detectores de alta energía, la antena de campo eléctrico y el Lightning Mapping Array (LMA) han permitido avanzar en la comprensión de algunos aspectos no resueltos de la física del rayo. Los resultados de nuestras investigaciones se han publicado principalmente en dos artículos científicos que constituyen esta tesis doctoral. El primer artículo de este compendio presenta las mediciones de rayos proporcionada por el interferómetro en coincidencia con detecciones de alta energía producidas por el mismo fenómeno atmosférico. Durante una campaña de campo en el centro-norte de Colombia, se detectaron varias ráfagas intensas de rayos X producidos por líderes escalonados negativos y líderes dardos. Se ha observado una fuerte correlación temporal entre las emisiones de alta energía y los pulsos VHF más intensos, lo que sugiere un mecanismo compartido con lo runaway electrons. También se observaron múltiples fuentes de alta energía pertenecientes a diferentes ramas de líderes, lo que podría sugerir que las emisiones de alta energía pueden ocurrir con más frecuencia de lo esperado. Especulamos algunas posibles implicaciones para la producción de TGFs. El segundo artículo presenta por primera vez la observación de un rayo de polaridad positiva con múltiples descargas subsecuentes que comparten el mismo canal a tierra y mapeado con el interferómetro y el sistema Lightning Mapping Array. Con esta segunda publicación proponemos un mecanismo para describir este extraño fenómeno en el que identificamos en un líder de retroceso rápido o fast breakdown como un papel crucial en la reconexión de estos canales de líderes rápidos iniciales con en el inicio de la siguiente descarga subsecuente positiva. A su vez, investigamos la asimetría de la polaridad de los líderes de rayos con las descargas subsecuentes a tierra. Por último, en el apéndice se presentan otras mediciones y los documentos del material suplementario de ambos artículos.Postprint (published version

    Modeling of time of arrival method for lightning locating systems

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    Various methods are used to locate cloud-to-ground lightning flashes. Even though a higher cost is incurred, a network of sensor stations is preferable to a single station due to the improved accuracy. For a single station measuring system, the accuracy of its analyses is mostly based on the chosen mathematical equations which can be solved in either linear or nonlinear mode. The sensitivity of the measuring equipment used is also particularly significant. This paper concentrates on the modelling of time of arrival (TOA) technique for locating a lightning flash by utilizing three broadband antennas. Consequently, by employing the developed model, the influences of geometric parameters on the accuracy of the model are evaluated. Therefore, a Matlab based simulation of the measuring system is developed. In the developed codes, randomly located lightning flash with its corresponding electromagnetic radiation was modelled. Results show that parameters such as lightning path shape, distance of the leader, and leader location can directly affect the accuracy of the TOA technique for extracting the azimuth and elevation

    Lightning generated electric field over land and sea at Northern Region of Peninsular Malaysia: Measuring Setup

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    Lightning is the transfer of significant charge between two charged object, it can appear between cloud-to-cloud, cloud-to-air and cloud-to-ground. All lightning activities are correlated with charge movement and therefore, it can be studied using measurement of electrostatic field (slow field changes), radiation field (fast field changes) and magnetic field affiliated with charge movement. The measuring equipment was a parallel flat plate and vertical whip antenna with an analog filter buffer circuit, connected to a digital high speed oscilloscope. In principle, both antenna have a similar measuring operation, with the exclusion of the antennas dimension, mode of transient recoder and the association of circuitry characteristic (decay time constant). This measurement setup are commonly used to perform electric field characterization related to various lightning events such as as preliminary breakdown, stepped leader, return stroke, subsequent return stroke, dart leader, dart-stepped leader, narrow bipolar pulses and cloud activity between strokes, such as regular pulse trains, K and M changes and chaotic pulse trains. The measurement systems are located very close to the sea, with respect in obtaining a perspicuous explaination of lightning propagation effects on sea instead on land. Noise-distorted effects in data captured usually introduce in a noisy environment or when the lightning strike far from the measurement station, thus the data gathered from the measurement need to be filter using MATLAB computer simulation before conducting further analysis. A promising wavelet 1-D technique, then, were used in signal denoising process rather using conventional filtration. In this works, the state of art in measuring vertical component of electric field focalized sea area during the thunderstorm days been demonstrate and explained

    Evaluation and optimization of wavelet technique to enhance the mapping accuracy of lightning VHF interferometry

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    Despite the significant progress in the understanding of the phenomenon of lightning and the physics behind it, locating and mapping its occurrence remain a challenge. Such localization and mapping of very high frequency (VHF) lightning radiation sources provide a foundation for the subsequent research on predicting lightning, saving lives, and protecting valuable assets. A major technical challenge in attempting to map the sources of lightning is mapping accuracy. Several methods have been proposed for estimating the real pattern of the temporal location and spatial map of the lightning strikes. However, due to the complexity of lightning signals and the noise accompanying its recording, providing accurate lightning maps estimation remains a challenging task. To advance the lightning mapping it is vital to improve how lightning signals are pre-processed and how noise is filtered. Most existing studies of lightning mapping make use of the VHF interferometer (ITF) alongside crosscorrelation in time and frequency domain and phase difference of arrival techniques. These methods involve selecting a set of parameters which usually fail to accommodate all types of lightning flashes, discarding information that could be beneficial for further improvement of lightning mapping accuracy. In this thesis, a wavelet-based cross-correlation (CCWD) is proposed for a reliable lightning mapping estimation through means of signal enhancement and noise reduction, providing a better time- frequency resolution. Interpolation techniques were introduced to smoothen the correlation peaks for more accurate lightning localization. To confirm the effectiveness of the proposed method, a simulation of lightning signals was created, and the mapping results were verified. Moreover, a comparative study to investigate the effectiveness of different processing techniques was carried out. The benchmark environment involved the use of different filtering and cross-correlation techniques, introducing new processing methods such as Kalman filter and wavelet-based crosscorrelation. In addition, a particle swarm optimization technique is used to optimize the trajectory of the CCWD-based lightning maps by finding the optimal sliding window of the cross-correlation. The CCWD-PSO technique was further enhanced through the introduction of a novel lightning event extraction method that enables faster processing of the lightning mapping. Six positive narrow bipolar events were analyzed, and the results indicate that a good estimation of the lightning radiation sources was achieved using wavelet de-noising and CCWD with a minimal error of 3.46°. The results were further improved with the use of CCWD-PSO technique with Euclidean distance of 0.6243 at 300 iterations. The investigations carried out in this study confirm that the ITF mapping system could effectively map the lightning VHF radiation source, which makes the combination of ITF and the CCWD a potential candidate for lightning mapping technology

    Compact Intracloud Discharges

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    Investigation of narrow bipolar events in Mississippi thunderstorms

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    In the first part of the study FA and Log-RF data were used to examine low-altitude (\u3c8.0 km) negative NBEs (NNBEs) to see how many -CG flashes were initiated by NNBEs. Out of 686 -CG flashes only 33 (5%) flashes were initiated by an NNBE. These NNBEs occurred at an average altitude of 6.2 km had average amplitude (range-normalized amplitude to 100 km) of 0.4 V/m and had average VHF (Log-RF) power of 130 W. Since the low-altitude NNBEs were substantially weaker than positive NBEs that initiate intracloud (IC) flashes it is hypothesized that -CG flashes are easier to initiate than IC flashes. The second part of the study investigated the properties of 201 positive NBEs (+NBEs). The +NBEs were classified in two ways: into Types A-D [Karunarathne et al. 2015] and into three different groups: Isolated Not-isolated and IC flash-initiator or INBE [Wu et al. 2014]. The average VHF (Log-RF) power of NBEs within the A-D categories were 1.9 4 9.8 and 13.2 kW respectively while the powers for the Isolated Not-isolated and INBE categories were 10.1 4 and 10.1 kW respectively. The third study considered 34 NBEs that occurred close in space and time to each other: 13 pairs of NBEs one group of three +NBEs and one group of five +NBEs. The NBEs were overlaid on radar data of the parent thunderstorms. The data indicate that the individual NBEs in each group initiated in separate intense electric field regions of small extent rather than in a single large-scale electric field region. This dissertation investigates Narrow Bipolar Events (NBEs) which are a type of short-duration (10- 30 µs) lightning discharge. The study primarily used data from NBEs collected in 2016 at seven sensor sites within 50 km of Oxford Mississippi USA. Each sensor site had three electromagnetic antennas called Fast Antenna (FA) dE⁄dt and Log-RF with bandwidths of 0-2.5 MHz 0-1.0 MHz and 186-192 MHz respectively. NBEs are often isolated from other lightning events in thunderclouds but they sometimes initiate negative cloud-to-ground (-CG) and intracloud (IC) lightning flashes or they can occur during these flashes

    Interferometric Imaging of Lightning Initiation through LOFAR: Uncovering the Spontaneous and Not-So-Spontaneous Nature of Lightning Initiation in 3D

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    With recent advances in instrumentation and the continued development and refinement of analytical methods, the hindrances that previously existed in uncovering the physical processes governing the behavior of lightning are diminishing. The focus of this dissertation research, interferometric imaging of lightning initiation through beamforming via the Low Frequency Array (LOFAR), will describe in detail how both the instrumentation and methods cooperate to enable the detection of lightning processes in which are below the level of the galactic and thermal very high frequency (VHF) background on individual antennas within the array. These conditions have proven to be integral in uncovering of two novel methods of lightning initiation. For one event, a broad discharge is observed propagate with a velocity of 4.8 +/- 0.1 x 10^6 m/s while increasing in intensity from below the LOFAR noise level. For the second mode of initiation, a negative discharge was observed to propagate with a velocity of 1.5 x 10^3 m/s, which is three orders of magnitude slower than normal negative leaders. The first shares features with previously conceptualized ideas of how lightning initiates. This is supported by other researchers, but the findings have unique features that are not explained by the current theories how lightning initiates. Furthermore, the second initiation method is new and unlike any other known lightning process. Lastly, it should be noted that the results we present these use true 3D interferometric imaging techniques. Without the development and implementation of these methods the results reported within this work would not be possible. This thesis will briefly discuss the current understanding of lightning and related phenomena to give an overview the topic and context for why the study of lightning is important. This will the be followed by current theories of how lightning initiates, and then by discussion of the development of the 3D interferometric techniques and their implementation. Next, the thesis will present two recently observed processes by which lightning leaders form, after which is a discussion of the implications of these findings and how they are distinct from known lightning processes. Additionally, we will discuss results from the possible detection of gamma ray glows from the thunderstorm balloon campaign. These findings are the result of the updated methodology and instrumentation when this project was transferred to the University of New Hampshire from the Florida Institute of Technology. Lastly, the thesis concludes with a review of the implications of these discoveries and a discussion of future investigations, as well as, propose methods to further uncover additional details of the physical processes behind lightning initiation and where the results of the observations reported in this thesis fit within the current understanding of how lightning initiates

    Effect of water on electrical properties of Refined, Bleached, and Deodorized Palm Oil (RBDPO) as electrical insulating material

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    This paper describes the properties of refined, bleached, deodorized palm oil (RBDPO) as having the potential to be used as insulating liquid. There are several important properties such as electrical breakdown, dielectric dissipation factor, specific gravity, flash point, viscosity and pour point of RBDPO that was measured and compared to commercial mineral oil which is largely in current use as insulating liquid in power transformers. Experimental results of the electrical properties revealed that the average breakdown voltage of the RBDPO sample, without the addition of water at room temperature, is 13.368 kV. The result also revealed that due to effect of water, the breakdown voltage is lower than that of commercial mineral oil (Hyrax). However, the flash point and the pour point of RBDPO is very high compared to mineral oil thus giving it advantageous possibility to be used safely as insulating liquid. The results showed that RBDPO is greatly influenced by water, causing the breakdown voltage to decrease and the dissipation factor to increase; this is attributable to the high amounts of dissolved water

    Broadband radio mapping and imaging of lightning processes

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    Though thunderstorms and lightning are commonplace on Earth, it is still unclear how lightning initiates, propagates, and how it is involved in generating intense bursts of gamma-rays that can be detected by spacecraft. Lightning is a hot, highly-ionized plasma channel, capable of carrying up to hundreds of kiloamperes electric current, and extending many kilometers in length for hundreds of milliseconds at a time. Despite its immensity, lightning can be difficult to observe, as it primarily initiates and propagates deep within thunderclouds, optically obscured by thousands of cubic kilometers of cloud water and ice. Broadband radio interferometry has been developed to study lightning at radio frequencies, offering us a way to “see” inside the clouds. The technique, which is still in its infancy for lightning research, allows for lightning radio emissions to be mapped and/or imaged with extremely fine time resolution. In this dissertation, a newly-developed three-element, broadband VHF (~14-88 MHz), 16-bit radio interferometer (INTF) is used to investigate extremely transient thunderstorm electrical phenomena involved in lightning initiation, propagation, and high-energy photon production. The investigations demonstrate the novel science that can be done with the INTF system, and reveal previously unforeseen dynamics of lightning formation. Specifically, we image and map the VHF emissions of narrow bipolar events (NBEs), initial breakdown pulses (IBPs), and an energetic in-cloud pulse (EIP) with sub-microsecond resolution. NBEs have long been of interest to the lightning community because they are the most powerful natural emitters of high-frequency and very-high-frequency radio waves on Earth. Moreover, NBEs are readily identifiable by their narrow (~10 µs wide), bipolar sferics (~3 kHz-3 MHz radio emissions). NBEs are not lightning, but appear to be a precursor to lightning, occurring either in complete isolation, or at the beginning of a lightning flash. IBPs, in contrast, never occur in isolation, but rather are the hallmark of lightning channel formation. IBPs typically occur in long trains of sferic pulses, and indicate the imminence of lightning during the first milliseconds after lightning initiation. An IBP is also identified by its sferic, having a bipolar waveform some tens of microseconds wide, the initial pulse of which is superimposed by ~1 µs-wide subpulses. Lastly, EIPs are high-peak-current (\u3e200 kA) events that are involved in the generation of terrestrial gamma-ray flashes (TGFs), which are intense bursts of gamma-rays that radiate out the tops of thunderclouds and are detected in space. EIPs have a signature high-amplitude, ~50 µs-wide sferic, which is time-aligned with satellite-borne gamma-ray detections. EIPs can thus serve as a proxy for TGFs, offering a way to investigate TGFs using ground-based radio sensors, without necessarily needing satellite data. The physical natures of NBEs, IBPs, and EIPs have been active areas of research over the last decade. For over half a century, the role that IBPs play in initial hot channel formation has been under debate. More recently, intense investigation has been focused on exactly how NBEs are involved in lightning initiation. Just in the last few years, EIPs were discovered, offering a new way to investigate the role that lightning plays in TGF generation. By investigating NBEs with the INTF, we discovered a newly-identified form of streamer-based breakdown, termed fast negative breakdown, that does not fit with our current understanding of lightning initiation. Streamers are cold filamentary plasma channels, and based on conventional dielectric theory, it was hypothesized that lightning should be initiated by positive streamers, which carry electric current in their propagation direction. However, fast negative breakdown carries electric current opposite its propagation direction, propagating ~500 m through virgin air with an unusually fast speed of ~10^7 m/s. Aside from breakdown polarity, fast negative breakdown is in many ways similar to recently reported fast positive breakdown that generates the majority of NBEs, and that is expected from conventional dielectric theory. We additionally show that similarly fast breakdown is involved in the production of both IBPs and EIPs. Using the INTF, we show that the IBP process is dominated by a fast-propagating ∼10^7 m/s streamer-based negative breakdown that propagates the channel about ~100 m into virgin air, similar to the fast negative breakdown associated with NBEs. We show that the streamer-based channel extension leads to a sustained electric current, indicating the existence of a hot conductive lightning channel. Fast-propagating ~10^7-10^8 m/s breakdown of both polarities is also a prominent feature during the EIP, but occurs over a larger (\u3e1-km altitude) volume than during NBEs or IBPs. We show that repeated downward- and upward-propagating fast positive and negative breakdown are somehow coupled to the generation of relativistic electrons and associated ionization. We conclude that the electric current that produces the EIP sferic is generated by a newly discovered type of self-sustaining discharge termed a relativistic feedback discharge (RFD), which involves multiple generations of relativistic electron avalanches and back-scattered positrons and X-rays. Our study further demonstrates that TGFs can be produced by RFDs. The INTF was developed by New Mexico Tech, and deployed and operated at Kennedy Space Center (KSC) in Florida during summer 2016 to obtain the data used herein
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