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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).

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.

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.

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.

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 VHF lightning mapping network, consistent with the currently most accepted production theories. We were able to identify associated thunderstorms in 32 of 90 correlation cases. We have analysed the thunderstorm development stage at the moment of TGF production, the vertical extent and areal extent. It was found that analysed thunderstorms shows preferences for TGF production during developing or mature stages and great vertical extension what agrees with situations with strong updrafts and high CAPE values determined in the first part of the study. The last part presents an analysis of the association between X-ray events and lightning registered at the Eagle Nest Tower in the Pyrenees. We have determined that downward negative lightning seems to be the only candidate for x-ray emissions. The theories that support this hypothesis could also explain TGF production, specifically, as in examples of upward negative leaders shown in the previous section. We have also studied whether the high energy background radiation increased in association with thunderstorm electrification. However, in the cases analysed this increase has been associated with radon-ion daughters., 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 agrupado en distintas categorías. Para cada una de ellas se ha intentado justificar la relación entre el TGF y el rayo en base a ejemplos de rayos detectados por una red VHF y a las teorías de producción más aceptadas hasta la fecha. De 32 de estos casos de correlación se ha podido identificar y analizar la tormenta asociada. Se ha analizado la fase de desarrollo de la tormenta en el momento de la producción del TGF, la extensión vertical y la extensión horizontal. Se ha visto que las tormentas analizadas muestran una preferencia para la producción de TGFs en las fases de desarrollo o madurez y de gran extensión vertical que concuerda con situaciones de fuertes corrientes ascendentes y altos valores de CAPE determinados en el estudio previo. En la última parte se ha analizado eventos registrados en la torre instrumentalizada del Nido del Águila en los Pirineos. Estos eventos corresponden a rayos naturales y radiación X. Se ha determinado que los líderes negativos parecen ser los únicos candidatos para la emisión de rayos X. Las teorías que soportan esta hipótesis podrías explicar la producción de TGFs y, en concreto, los casos reportados en el estudio anterior. Se ha estudiado también si el aumento de la radiación de fondo de alta energía está asociado a la posible electrificación de las tormentas. En el caso analizado se ha asociado este aumento con los descendientes radiactivos de la cadena de desintegración del radón

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.

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.

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.

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.