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The Terrestrial Gamma-ray Flash (TGF) to lightning ratio, computed over the three tropical chimneys, presents a paradox: African thunderstorms produce the most lightning but yield the lowest fraction of TGFs when compared to American and Southeast Asian thunderstorms. To understand the physical insights into this asymmetry, Tropical Rainfall Measuring Mission Precipitation Radar measurements are used to depict the vertical precipitation structure and infer the vertical electrical charge fraction distribution of the observed thunderstorms in the three regions and the thunderstorms during TGF occurrences detected by the AGILE, Fermi-GBM, and RHESSI sensors. Regional differences show that African thunderstorms are taller, smaller, and have a higher concentration of denser ice particles (graupel and or hail) above the freezing level in addition to having more lightning flashes per thunderstorm. The overall TGF-related thunderstorms are taller, more intense (0.5–1.5 dBZ) and present similar radar reflectivity decay with height independent of the region. The two dimensional precipitation vertical distribution diagrams indicate that TGF thunderstorms develop to a mature stage. Independent of the region, thunderstorms show a midlevel negative charge center varying from 4.6 to 8.1 km in height and an upper level positive charge center ranging from 7.4 to 14.6 km. TGF thunderstorms have thicker positive inferred charge layer and present larger vertical distances between the opposite charging layers in comparison with the overall thunderstorm population, here defined as Climatology. African TGF thunderstorms have higher fraction of positive charges near the negative layer, helping to produce more and shorter lightning discharges., Peer Reviewed, Postprint (author's final draft)

The Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station (ISS) provides optical radiances and images of lightning flashes in several spectral bands. This work presents a lightning flash simultaneously observed from space by ASIM, the Geostationary Lightning Mapper (GLM) and the Lightning Imaging Sensor on the International Space Station (ISS-LIS); and from ground by the Colombia-Lightning Mapping Array (Colombia-LMA). Volumetric weather radar provides reflectivity data to help to interpret the effects of the cloud particles on the observed optical features. We found that surges in radiance in the band at 777.4 nm appear to be related mostly with lightning processes involving currents as well with branching of lightning leaders with new leader development. In cloud areas with reflectivity <18 dBZ above the lightning leader channels at altitudes >7 km, these have been imaged by ASIM and GLM. But in the region with reflectivity <23 dBZ above the lightning leader channels, despite its lower cloud tops and similar altitudes of lightning channels, these have been almost undetectable. The calculated relative optical depths are consistent with the observed optical intensity at the cloud top. Despite the effects of the cloud particles and the altitude of the lightning channels on the attenuation of the luminosity, the luminosity of the lightning channels due to different processes is fundamental for the imaging of lightning from space., This work was supported by research grants from the Spanish Ministry of Economy and the European Regional Development Fund (FEDER): ESP2013-48032-C5-3-R, ESP2015-69909-C5-5-R, ESP2017-86263-C4-2-R, and PID2019-109269RB-C42. ASIM is a mission of ESA's SciSpace Program for scientific utilization of the ISS and non-ISS space exploration platforms and space envi-ronment analogs. It is funded by ESA and national contributions through contracts with TERMA and Techni-cal University of Denmark (DTU) in Denmark, University of Bergen (UB) in Norway, and University of Valencia (UV) in Spain. The ASIM Science Data Center (ASDC) at DTU is supported by PRODEX contract PEA 40001 1 5884. The authors are grateful to Keraunos for providing LINET lightning data and the World-Wide Lightning Location Net-work (http://wwlln.net/) for providing WWLLN lightning location data. Also, we would like to thank the Colombian Instituto de Hidrología, Meteorología y Estudios Ambientales (IDEAM) for pro-viding the weather radar information used in this study., Peer Reviewed, Postprint (author's final draft)

We analyze a nighttime negative cloud-to-ground lightning flash in Colombia observed from the ground with a high-speed camera at 5,000 images per second and from space by the Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station (ISS), the Lightning Imaging Sensor also on the ISS (ISS-LIS), and the Geostationary Lightning Mapper (GLM) on GOES-16. The space instruments measure the oxygen band at 777.4 nm, allowing for direct comparisons of measurements, and the ground-based camera observes in a wide visible band. After conversion to energy emitted at the cloud top, we find a good linear correspondence of the optical energies measured by the three space instruments, except that GLM values were 3 times higher. We attribute this mainly to the difference in viewing angles between spacecraft and the cloud. Over the entirety of the ASIM observed flash, optical pulses were detected by GLM and LIS, only when the energy reported by ASIM was greater than 332 J and 949 J, respectively. Their detection rate corresponds to 14% and 2.5%, respectively, of the flash duration observed by ASIM. The temporal variation of the high-speed camera luminosity matched well the features observed by ASIM around the time of the stroke but reached ~3.9 times higher peak intensity during the return stroke, attributed to its broader spectral sensitivity band and a viewing angle advantage., Peer Reviewed, Postprint (published version)

Africa is one of the most productive lightning regions on Earth, yet it has a lower TGF-to-lightning ratio especially compared with Central America. In this paper we have analyzed the global distribution of different meteorological parameters in order to explain the TGF/lightning ratio asymmetry. We show here that a drier surface and larger CAPE in Africa may produce thunderstorms with intense electric charge regions but elevated in the atmosphere and closer to each other, which allows for higher flash rates and less energetic, shorter and smaller flashes. The results we present here suggest that continental thunderstorms in Africa more rarely fulfill the lightning and thundercloud requirements for TGF production inferred from observations and models., Peer Reviewed, Postprint (published version)

On 29–30 October 2013, a low-light video camera installed at Pic du Midi (2877¿m), recorded transient luminous events above a very active storm over the Mediterranean Sea. The minimum cloud top temperature reached -73°C, while its cloud to ground (CG) flash rate exceeded 30¿fl¿min-1. Some sprite events have long duration and resemble to dancing sprites. We analyze in detail the temporal evolution and estimated location of two series of sprite sequences, as well as the cloud structure, the lightning activity, the electric field radiated in a broad range of low frequencies, and the current moment waveform of the lightning strokes. (i) In each series, successive sprite sequences reflect time and location of corresponding positive lightning strokes across the stratiform region. (ii) The longer time-delayed (>20¿ms) sprite elements correspond to the lower impulsive charge moment changes (iCMC) of the parent strokes (<200¿C¿km), and they are shifted few tens of kilometers from their SP¿+¿CG stroke. However, both short and long time-delayed sprite elements also occur after strokes that produce a large iCMC and that are followed by a continuing current. (iii) The long time-delayed sprite elements during the continuing current correspond to surges in the current moment waveform. They occur sometimes at an altitude apparently lower than the previous short time-delayed sprite elements, possibly because of changes in the local conductivity. (iv) The largest and brightest sprite elements produce significant current signatures, visible when their delay is not too short (~3–5¿ms)., Peer Reviewed, Preprint

3D mapping system like the LMA - Lightning Mapping Array - are a leap forward in lightning observation. LMA measurements has lead to an improvement on the analysis of the fine structure of lightning, allowing to characterize the duration and maximum extension of the cloud fraction of a lightning flash. During several years of operation, the first LMA deployed in Europe has been providing a large amount of data which now allows a statistical approach to compute the full duration and horizontal extension of the in-cloud phase of a lightning flash. The “Ebro Lightning Mapping Array” (ELMA) is used in the present study. Summer and winter lighting were analyzed for seasonal periods (Dec–Feb and Jun–Aug). A simple method based on an ellipse fitting technique (EFT) has been used to characterize the spatio-temporal dimensions from a set of about 29,000 lightning flashes including both summer and winter events. Results show an average lightning flash duration of 440 ms (450 ms in winter) and a horizontal maximum length of 15.0 km (18.4 km in winter). The uncertainties for summer lightning lengths were about ± 1.2 km and ± 0.7 km for the mean and median values respectively. In case of winter lightning, the level of uncertainty reaches up to 1 km and 0.7 km of mean and median value. The results of the successful correlation of CG discharges with the EFT method, represent 6.9% and 35.5% of the total LMA flashes detected in summer and winter respectively. Additionally, the median value of lightning lengths calculated through this correlative method was approximately 17 km for both seasons. On the other hand, the highest median ratios of lightning length to CG discharges in both summer and winter were reported for positive CG discharges., Peer Reviewed, Preprint

During the night of October 29-30, 2013, a low-light video camera at Pic du Midi (2877 m) in the French Pyrénées, recorded TLEs above a very active storm over the Mediterranean Sea. The minimum cloud top temperature reached -73 °C at ~1600 UTC while its cloud to ground (CG) flash rate reached ~30 fl min-1. Some sprite events with long duration are classified as dancing sprites. We analyze in detail the temporal evolution and estimated location of sprite elements for two cases of these events. They consist in series of sprite sequences with a duration that exceeds 1 second. By associating the cloud structure, the lightning activity, the electric field radiated in a broad range of low frequencies and the current moment waveform of the lightning strokes, some findings are highlighted: (i) In each series, successive sprite sequences reflect the occurrence time and location of individual positive lightning strokes across the stratiform region. (ii) The longer time-delayed (> 20 ms) sprite elements correspond to the lower impulsive charge moment changes (iCMC) of the parent stroke (< 200 C km) and they are shifted few tens of kilometres from their SP+CG stroke. However, both short and long time-delayed sprite elements also occur after strokes that produce a large iCMC and that are followed by a continuing current. (iii) The long time-delayed sprite elements produced during the continuing current correspond to surges in the current moment waveform. They occur sometimes at an altitude apparently lower than the previous short time-delayed sprite elements, possibly because of the lowered altitude of the ionosphere potential. (iv) The largest and brightest sprite elements produce significant current signatures, visible when their delay is not too short (~3-5 ms)., Preprint

Electromagnetic field from simulated double-headed mid-gap streamers is computed. The effect of the applied electric field is studied by considering electric fields higher than the conventional breakdown threshold (‘overvoltage’). The obtained results show a 20 dB decay at 3.8 GHz for conventional breakdown electric field, 35 GHz for ambient electric fields five times the conventional breakdown and 170 GHz for fields ten times the conventional breakdown electric field. The limitation in nature of the intense electric fields suggest further experiments using a radio receiver at frequencies of more than 10 GHz in order to investigate the origin of the detected x-rays in laboratory sparks., Preprint

The paper presents winter lightning maps on specific regions in the northern hemisphere. Four different degrees of winter lightning activity are defined based on information derived from Japanese case. Based on this reference case it is possible to determine regions where winter lightning can be a threat to specific structures. Guidance on risk assessment to tall structures and wind turbines are described as well., Preprint

On April 2015 a Lightning Mapping Array network (COL-LMA) was installed at the north of Colombia. This network provides 3D mapping of the development of lightning leaders. For the first time such network has been setup in the tropics. That allows us to investigate the high altitude development of lightning leaders. Here we present the results of the first measurements to confirm that negative leaders at altitudes of 15 km are common in the observed thunderstorms., Postprint (published version)

Earth’s atmospheric potential rapidly increases up to few tens of kilovolts below 200 m altitude. This potential drop will induce charge to tall objects at ground by virtue of electrostatic induction. In this work we investigate the induced electric charges in fair weather to a 1.5 MW and 5 MW wind turbines. The effect of rotation is included and the current calculated result in currents of few micro-amps. The production of point discharge and corona is investigated and some experiments are conducted by means of instrumented kites., Preprint

The analysis of high-energy background radiation (0.1 – 2 MeV) enhancements during eight winter thunderstorms and ¿ve summer storms in the Ebro delta region in the northeast of Spain is presented. For the ¿rst time, high-energy radiation counts, precipitation, radar re¿ectivity, and very high frequency lightning detections to infer charge regions altitude have been analyzed in order to ¿nd out what produces the measured background radiation increments associated with storms. The good agreement between radar re¿ectivity and precipitation with increases in background radiation counts coupled with the spectrum analysis comparing rain/no rain periods suggests that radon-ion daughters play a major role in the radiation increments reported. No evidence has been found supporting that measured background radiation enhancements can be produced by storm electric ¿elds. Finally, a single case of a high-energy radiation increase was prior to a cloud-to-ground lightning stroke, which reinforces the theory that a lower positive charge layer’s existence is important for the production of Terrestrial Ground Enhancements., Peer Reviewed, Preprint

Lightning is one of the major threats to multi-megawatt wind turbines and a concern for modern aircraft due to the use of lightweight composite materials. Both wind turbines and aircraft can initiate lightning, and very favorable conditions for lightning initiation occur in winter thunderstorms. Moreover, winter thunderstorms are characterized by a relatively high production of very energetic lightning. This paper reviews the different types of lightning interactions and summarizes the well-known winter thunderstorm areas. Until now comprehensive maps of global distribution of winter lightning prevalence to be used for risk assessment have been unavailable. In this paper we present the global winter lightning activity for a period of 5 years. Using lightning location data and meteorological re-analysis data, six maps are created: annual winter lightning stroke density, seasonal variation of the winter lightning and the annual number of winter thunderstorm days. In the Northern Hemisphere, the maps confirmed Japan to be one of the most active regions but other areas such as the Mediterranean and the USA are active as well. In the Southern Hemisphere, Uruguay and surrounding area, the southwestern Indian Ocean and the Tasman Sea experience the highest activity. The maps provided here can be used in the development of a risk assessment., Peer Reviewed, Postprint (published version)

The size and duration of lightning flashes are examined. Data from the Ebro Valley Laboratory Lightning Mapping Array is used as reference. Additional data from the VLF/LF LINET network is included. In the analysis, each flash is simplified by a confidence ellipse fitting most of the detected sources. The major axis of the ellipse is adopted as the flash length. Flash durations are computed too. The analysis of 778 flashes results in a median flash length of ¿14 km with a median duration of ¿0.3 s. The results presented, besides characterizing the storm activity, they can be useful to define stroke grouping criteria, lightning flash density calculations and lightning warning purposes., Postprint (published version)

Terrestrial Gamma ray Flashes (TGFs) have been known since 20 years ago, but since the first moment of the discovery it was clear that they were related to lightning and thunderstorms. The latest studies revealed that TGFs could be produced during the first stages of the lightning within thunderclouds by a process which is very similar to the X-ray emissions from natural lightning observed from ground or discharges in high voltage laboratory. The study of such phenomena will help to extend and complete the knowledge about lightning and electrical discharges. This study compares geographic, monthly and diurnal cycle global distributions of TGFs and lightning. A fourth region with enhanced production of TGFs has been identified apart from the other three regions previously identified. This analysis further investigates the relation between TGFs, lightning activity and thunderstorms. Moreover, we have seen that the TGF/lightning ratio is different in each region which could be explained by meteorological differences between regions and not by differences in regional lightning detection efficiency. For the first time, meteorological conditions and lightning activity have been compared among situations where a satellite with capability to detect TGFs did or did not detect them. The objective is to identify the best atmospheric conditions for TGF production. We have established that electrification processes associated with strong updrafts and high CAPE values are important to create the best atmospheric conditions that favours TGF generation. In the second part of the study we defined criteria for the TGF/lightning correlation in time. Using these criteria we have identified 90 correlations in South America. Depending on the time difference between TGF and lightning the correlations have been grouped into different categories. For each category the relation between TGF and lightning was justified based on examples of upward propagation of negative leaders detected by a V, Las emisiones de rayos gamma terrestres (TGFs) se conocen desde hace apenas 20 años, pero desde el primer momento se vio claramente su relación con los rayos y tormentas. Los últimos estudios revelan que se podrían producir durante las primeras fases de los rayos en el interior de las nubes por un proceso que sería muy similar a la emisión de rayos X por rayos naturales observados desde suelo o por descargas en el laboratorio de alta tensión. El estudio de estos fenómenos ayudará a ampliar y completar los conocimientos sobre el fenómeno del rayo y de las descargas eléctricas. En primer lugar se han comparado las distribuciones globales geográficas, mensuales y el ciclo diurno de TGFs y rayos. Se ha identificado una cuarta región de producción de TGFs a parte de las tres ya identificadas previamente. Con este análisis se ha contribuido principalmente a la confirmación de las correlaciones de los TGFs con la actividad de rayos y tormentas. Además, se ha visto que las ratios de TGF/rayo varían en las distintas regiones con lo que es probable que se pueda explicar por las diferencias meteorológicas entre ellas y no por diferencias regionales en la eficiencia de detección de rayos. Por primera vez se han analizado las condiciones meteorológicas y actividad de rayos en aquellas situaciones en que un satélite con capacidad para detectar TGFs no los detectaba y se han comparado con aquellas situaciones en que si se detectaron. El objetivo es el de distinguir las mejores condiciones atmosféricas para su producción. Se ha determinado que los procesos de electrificación asociados a fuertes corrientes ascendentes y altos valores de CAPE son importantes para crear las mejores condiciones atmosféricas que favorecen la generación de TGFs. En la segunda parte se definido un criterio para la correlación temporal entre rayos y TGFs. Con este criterio se han identificado un total de 90 correlaciones en Sudamérica. En función de la diferencia temporal entre el rayo y el TGF se han agru, Postprint (published version)

Global distribution of the Terrestrial Gamma-ray Flashes (TGFs) detected by AGILE and RHESSI for the period from March 2009 to July 2012 has been analysed. A fourth TGF production region has been distinguished over the Pacific. It is confirmed that TGF occurrence follows the Intertropical Convergence Zone (ITCZ) seasonal migration and prefers afternoons. TGF/lightning ratio appears to be lower over America than other regions suggesting that meteorological regional differences are important for the TGF production. Diurnal cycle of TGFs peaks in the afternoon suggesting that Convective Available Potential Energy (CAPE) and convection are important for TGF production. Moreover all AGILE passages over South America in the same period have been analysed to find meteorological preferences for TGF occurrence. In each passage the analysis of Cloud Top Altitude (CfA), CAPE, number of strokes, number of storms and coverage area of clouds with temperatures below 70 degrees C (Cloud Top Coverage area, CTC) are computed. On average, AGILE has been exposed to 19,100 strokes between each TGF representing similar to 68 h of exposure over active storms. High CAPE values, high cloud tops and high stroke occurrence suggest that meteorological conditions conducive to vigorous and electrically active storms are important for TGF production. It is shown that a high number of thunderstorms is preferable for TGF production which may be explained by a greater chance of the presence of a thunderstorm in the best development stage for TGF production. High tropopause altitude seems to be important but not primordial for TGF production. (C) 2015 Elsevier Ltd. All rights reserved., Peer Reviewed, Preprint

Lightning flashes involve high energy processes that still are not well understood. In the laboratory, high voltage pulses are used to produce long sparks in open air allowing the production of energetic radiation. In this paper X-rays emitted by long sparks in air are simultaneously measured with the RF power radiation at 2.4 GHz. The experiment showed that the measured RF power systematically peaks at the time of the X-rays generation (in the microsecond time scale). All of the triggered sparks present peaks of RF radiation before the breakdown of the gap. The RF peaks are related to the applied voltage to the gap. RF peaks are also detected in discharges without breakdown. Cases where X-rays are detected presented higher RF power. The results indicate that at some stage of the discharge, before the breakdown, electrons are very fast accelerated letting in some cases to produce X-rays. Microwave radiation and X-rays may come from the same process., Preprint

Postprint (published version)

Electric fields and high-energy radiation of natural lightning measured at close range from a mountaintop tower are discussed. In none of the 12 negative cloud-to-ground upward flashes were X-rays observed. Also no energetic radiation was found in one negative upward leader at close range (20¿m). In the first of two consecutive negative cloud-to-ground flashes, X-rays were detected during the last ~1.75¿ms of the leader. During the time of energetic radiation in the flash an intense burst of intracloud VHF sources was located by the interferometers. The X-ray production is attributed to the high electric field runaway electron mechanism during leader stepping. Even though the second flash struck closer than the previous one, no X-rays were detected. The absence of energetic radiation is attributed to being outside of the beam of X-ray photons from the leader tip or to the stepping process not allowing sufficiently intense electric fields ahead of the leader tip. High-speed video of downward negative leaders at the time when X-rays are commonly detected on the ground revealed the increase of speed and luminosity of the leader. Both phenomena allow higher electric fields at the leader front favoring energetic radiation. Background radiation was also measured during thunderstorms. The count rate of a particular day is presented and discussed. The increases in the radiation count rate are more coincident with radar reflectivity levels above ~30 dBZ than with the total lightning activity close to the site. The increases of dose are attributed to radon daughter-ion precipitation, Postprint (published version)

RHESSI and AGILE TGFs distributions in the period 2009-2012 are compared. Analysis of differences in geographical, hourly, monthly distributions of the three main TGF production regions in Earth due to differences in lightning production and ITCZ movement is presented. A fourth region producing TGFs over Pacific not reported before is also analysed. Second analysis has been performed in order to identify preferable meteorological conditions for TGF production. Some publications suggested that regional differences in lightning/TGF ratio may be due to differences in meteorological thunderstorm and lightning production. All AGILE passes over South America (SA) in 2009-2012 has been studied. No special meteorological features characteristic of SA has been identified. TGFs are detected in situations with many lightning, thunderstorms, high CAPEs and high Top Clouds. TGFs are more probable to be detected in active situations., Preprint

Lightning involves high energy processes that still are not well understood. In the laboratory voltages pulses are used to produce long sparks allowing the production of energetic radiation. In this paper X-rays emitted by long sparks in air are simultaneously measured with the RF power radiation at ¿2.4 GHz. The experiment showed that RF power systematically peaks at the time of the X-rays generation (in the measurement time scale). Sparks presents peaks of RF radiation before the breakdown of the gap. The peaks are related to the applied voltage to the gap. RF peaks are also detected in discharges without breakdown. Cases where X-rays are detected present higher RF peak power. The results indicate that at some stage of the discharge before the breakdown electrons are very fast accelerating letting in some cases to produce X-rays. Microwave radiation and X-rays may come from the same process., Postprint (published version)

in this paper size and duration of lightning flashes are examined. Data from the Ebro Valley Laboratory Lightning Mapping Array is used as reference. Additional data from the VLF/LF LINET network and meteorological radar is included. In order to simplify the complex geometry of a lightning flash, each flash is described by a confidence ellipse fitting most of the detected sources. The flash length is represented by the major axis of the ellipse. The analysis of 778 flashes results in a median flash length of 14 km with a median duration of 0.3 s. The results presented here, besides characterizing the storm activity, provide valuable information in order to improve the stroke grouping criteria in the flash algorithm, lightning flash density calculations and lightning warning procedures., Peer Reviewed, Postprint (published version)

Sprites and Elves are commonly observed in Europe, especially also in winter maritime storms. It is well known that sprites are related to large electric charge positive flashes and elves are related with high amplitude in either negative and positive flashes. The analysis of the occurrence of sprites and elves revealed that sprites can occur inland and over the sea whereas elves prefer to occur over the sea. The distribution of positive and negative flashes during the warm period (April-September) and the cold period (October-March) depicts that flashes with intense amplitudes present a particular pattern. Intense negative flashes occur mostly over the sea and coastal areas during summer and winter whereas intense positive tend to occur in the mountains during the warm season and over sea during the cold period. Also, sprite and elves can indicate regions where energetic flashes occur. This can be a valuable information related to the risk assesment of wind turbines., Postprint (published version)

Electric fields and high-energy radiation of natural lightning measured at close range from a mountaintop tower are discussed. In none of the 12 negative cloud-to-ground upward flashes were X-rays observed. Also no energetic radiation was found in one negative upward leader at close range (20¿m). In the first of two consecutive negative cloud-to-ground flashes, X-rays were detected during the last ~1.75¿ms of the leader. During the time of energetic radiation in the flash an intense burst of intracloud VHF sources was located by the interferometers. The X-ray production is attributed to the high electric field runaway electron mechanism during leader stepping. Even though the second flash struck closer than the previous one, no X-rays were detected. The absence of energetic radiation is attributed to being outside of the beam of X-ray photons from the leader tip or to the stepping process not allowing sufficiently intense electric fields ahead of the leader tip. High-speed video of downward negative leaders at the time when X-rays are commonly detected on the ground revealed the increase of speed and luminosity of the leader. Both phenomena allow higher electric fields at the leader front favoring energetic radiation. Background radiation was also measured during thunderstorms. The count rate of a particular day is presented and discussed. The increases in the radiation count rate are more coincident with radar reflectivity levels above ~30 dBZ than with the total lightning activity close to the site. The increases of dose are attributed to radon daughter-ion precipitation, Postprint (published version)