Ferran Fabró

Observation of intrinsically bright terrestrial gamma ray flashes from the Mediterranean basin

Abstract We present three terrestrial gamma ray flashes (TGFs) observed over the Mediterranean basin by the Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI) satellite. Since the occurrence of these events in the Mediterranean region is quite rare, the characterization of the events was optimized by combining different approaches in order to better define the cloud of origin. The TGFs on 7 November 2004 and 16 October 2006 came from clouds with cloud top higher than 10–12 km where often a strong penetration into the stratosphere is found. This kind of cloud is usually associated with heavy precipitation and intense lightning activity. Nevertheless, the analysis of the cloud type based on satellite retrievals shows that the TGF on 27 May 2004 was produced by an unusual shallow convection. This result appears to be supported by the model simulation of the particle distribution and phase in the upper troposphere. The TGF on 7 November 2004 is among the brightest ever measured by RHESSI. The analysis of the energy spectrum of this event is consistent with a production altitude ≤12 km, which is in the upper part of the cloud, as found by the meteorological analysis of the TGF‐producing thunderstorm. This event must be unusually bright at the source in order to produce such a strong signal in RHESSI. We estimate that this TGF must contain ∼3 × 1018 initial photons with energy >1 MeV. This is 1 order of magnitude brighter than earlier estimations of an average RHESSI TGF.

Registration of X-rays at 2500 m altitude in association with lightning flashes and thunderstorms

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

Dancing sprites: Detailed analysis of two case studies

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).

Winter lightning activity in specific global regions and implications to wind turbines and tall structures

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.

Analysis of energetic radiation associated with thunderstorms in the Ebro delta region in Spain

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.

Fair weather induced charges and currents on tall wind turbines and experiments with kites

Earths 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.