Estimates of Lightning Activity and Terrestrial Gamma-ray Flash Detectability at Mount Etna for the ESTHER Project

Cimarelli C., Behnke S., Genareau K., Harper J.M., Van Eaton A.R.

The electrification of volcanic plumes has been described intermittently since at least the time of Pliny the Younger and the 79 AD eruption of Vesuvius. Although sometimes disregarded in the past as secondary effects, recent work suggests that the electrical properties of volcanic plumes reveal intrinsic and otherwise inaccessible parameters of explosive eruptions. An increasing number of volcanic lightning studies across the last decade have shown that electrification is ubiquitous in volcanic plumes. Technological advances in engineering and numerical modelling, paired with close observation of recent eruptions and dedicated laboratory studies (shock-tube and current impulse experiments), show that charge generation and electrical activity are related to the physical, chemical, and dynamic processes underpinning the eruption itself. Refining our understanding of volcanic plume electrification will continue advancing the fundamental understanding of eruptive processes to improve volcano monitoring. Realizing this goal, however, requires an interdisciplinary approach at the intersection of volcanology, atmospheric science, atmospheric electricity, and engineering. Our paper summarizes the rapid and steady progress achieved in recent volcanic lightning research and provides a vision for future developments in this growing field.

Tiberia A., Arnone E., Ursi A., Fuschino F., Virgilli E., Preziosi E., Tavani M., Dietrich S.

Typical features of lightning distribution in the mountain area of Mt. Cimone (2165 m a.s.l., Northern-Central Italy) have been studied through detections provided by the ground-based LIghtning NETwork data (LINET) and the Lightning Imaging Sensor (LIS) onboard the International Space Station (ISS-LIS). This study was performed within the context of the Gamma-Flash program, which includes the in situ observation of high-energy radiation (e.g., Terrestrial Gamma-ray Flashes (TGFs), gamma-ray glows) and neutron emissions from thunderstorms at the mountain-top “O. Vittori” climate observatory. LINET VLF/LF radio measurements allowed the characterization of both cloud-to-ground (CG) and intra-cloud (IC) strokes’ geographical distribution and an altitude of occurrence from 2012 through 2020. The lightning distribution showed a remarkable clustering of CGs at the mountain top in contrast to a homogeneous distribution of ICs, highlighting the likely impact of orography. IC strokes peaked around 4 to 6 km altitude, in agreement with the observed typical cloud range. The joint exploitation of ISS-LIS optical observations of LINET detections extended the study to further features of flashes not seen in radio wavelengths and stands as the cross-validation of the two detection methods over such a complex orography. These results gave the quantitative indication of the expected occurrence of lightning and ionizing radiation emissions in the Mt. Cimone area and an example of mountain-driven changes in lightning occurrence.

Briggs M.S., Lesage S., Schultz C., Mailyan B., Holzworth R.H.

Ursi A., Rodriguez Fernandez G., Tiberia A., Virgilli E., Arnone E., Preziosi E., Campana R., Tavani M.

Gamma-Flash is an Italian program devoted to the realization of both a ground-based and an airborne gamma-ray and neutron detection system, for in situ measurements of high-energy phenomena correlated to thunderstorm activity, such as Terrestrial Gamma-ray Flashes (TGFs), gamma-ray glows, and associated neutron emissions. The ground-based Gamma-Flash experiment is currently under installation at the Osservatorio Climatico “Ottavio Vittori” (CNR-ISAC) on Mt. Cimone, in Northern-Central Italy (2165 m a.s.l.), and it will be operational starting in Summer 2022. We studied the detectability of TGFs in the surroundings of the ground-based Gamma-Flash experiment, to identify an investigable spatial region around the detectors from which typical TGFs can survive and be revealed onground. We carried out numerical simulations of gamma-ray propagation in the mid-latitude atmosphere, and we developed a qualitative analytical model to integrate the results. This analysis allows one to identify a spatial region extending up to 4 km distance on ground and up to 10 km altitude a.s.l., considering typical TGFs emitting ∼1018 gamma-ray photons at the source. Lightning sferics data acquired by the LINET network demonstrate that such a region is interested by frequent cloud-to-ground and intra-cloud lightning, pointing out the suitability of the location for the purposes of the Gamma-Flash program.

Belz J.W., Krehbiel P.R., Remington J., Stanley M.A., Abbasi R.U., LeVon R., Rison W., Rodeheffer D., Abu‐Zayyad T., Allen M., Barcikowski E., Bergman D.R., Blake S.A., Byrne M., Cady R., et. al.

In this paper we report the first close, high-resolution observations of downward-directed terrestrial gamma-ray flashes (TGFs) detected by the large-area Telescope Array cosmic ray observatory, obtained in conjunction with broadband VHF interferometer and fast electric field change measurements of the parent discharge. The results show that the TGFs occur during strong initial breakdown pulses (IBPs) in the first few milliseconds of negative cloud-to-ground and low-altitude intracloud flashes, and that the IBPs are produced by a newly-identified streamer-based discharge process called fast negative breakdown. The observations indicate the relativistic runaway electron avalanches (RREAs) responsible for producing the TGFs are initiated by embedded spark-like transient conducting events (TCEs) within the fast streamer system, and potentially also by individual fast streamers themselves. The TCEs are inferred to be the cause of impulsive sub-pulses that are characteristic features of classic IBP sferics. Additional development of the avalanches would be facilitated by the enhanced electric field ahead of the advancing front of the fast negative breakdown. In addition to showing the nature of IBPs and their enigmatic sub-pulses, the observations also provide a possible explanation for the unsolved question of how the streamer to leader transition occurs during the initial negative breakdown, namely as a result of strong currents flowing in the final stage of successive IBPs, extending backward through both the IBP itself and the negative streamer breakdown preceding the IBP.

Pu Y., Cummer S.A., Huang A., Briggs M., Mailyan B., Lesage S.

Maiorana C., Marisaldi M., Lindanger A., Østgaard N., Ursi A., Sarria D., Galli M., Labanti C., Tavani M., Pittori C., Verrecchia F.

Lindanger A., Marisaldi M., Maiorana C., Sarria D., Albrechtsen K., Østgaard N., Galli M., Ursi A., Labanti C., Tavani M., Pittori C., Verrecchia F.

Ortberg J., Smith D.M., Li J., Dwyer J., Bowers G.

In 2015, Bowers et al. (2018, https://doi.org/10.1029/2017JD027771) detected a terrestrial gamma ray flash (TGF) in Hurricane Patricia from an aircraft flying at 2.6 km through what they argued to be a beam of downward gamma radiation produced by the positron component of the TGF. This paper uses the energy spectrum for gamma rays produced by the positrons of a relativistic runaway electron avalanche as simulated by the REAM code, propagated through a model of the Earth's atmosphere in Geant4, to examine the feasibility of detecting a typical upward TGF through its reverse positron beam at various altitudes on the ground. We find that, with patience, modest-sized scintillators on mountains as low as 1 km should be able to observe the same TGFs seen from spacecraft.

Pleshinger D.J., Alnussirat S.T., Arias J., Bai S., Banadaki Y., Cherry M.L., Hoffman J.H., Khosravi E., Legault M.D., Rodriguez R., Smith D., Smith D., del Toro E., Trepanier J.C., Sunda‐Meya A.

AbstractIn its first 2 years of operation, the ground‐based Terrestrial gamma ray flash and Energetic Thunderstorm Rooftop Array (TETRA)‐II array of gamma ray detectors has recorded 22 bursts of gamma rays of millisecond‐scale duration associated with lightning. In this study, we present the TETRA‐II observations detected at the three TETRA‐II ground‐level sites in Louisiana, Puerto Rico, and Panama together with the simultaneous radio frequency signals from the lightning data sets VAISALA Global Lightning Dataset, VAISALA National Lightning Detection Network, Earth Networks Total Lightning Network, and World Wide Lightning Location Network. The relative timing between the gamma ray events and the lightning activity is a key parameter for understanding the production mechanism(s) of the bursts. The gamma ray time profiles and their correlation with radio sferics suggest that the gamma ray events are initiated by lightning leader activity and are produced near the last stage of lightning leader channel development prior to the lightning return stroke.

Wada Y., Enoto T., Nakazawa K., Furuta Y., Yuasa T., Nakamura Y., Morimoto T., Matsumoto T., Makishima K., Tsuchiya H.

During a winter thunderstorm on 24 November 2017, a strong burst of gamma rays with energies up to $\ensuremath{\sim}10\text{ }\text{ }\mathrm{MeV}$ was detected coincident with a lightning discharge, by scintillation detectors installed at the Kashiwazaki-Kariwa Nuclear Power Station at sea level in Japan. The burst had a subsecond duration, which is suggestive of photoneutron production. The leading part of the burst was resolved into four intense gamma-ray bunches, each coincident with a low-frequency radio pulse. These bunches were separated by 0.7--1.5 ms, with a duration of $\ensuremath{\ll}1\text{ }\text{ }\mathrm{ms}$ each. Thus, the present burst may be considered as a ``downward'' terrestrial gamma-ray flash (TGF), which is analogous to upgoing TGFs observed from space. Although the scintillation detectors were heavily saturated by these bunches, the total dose associated with them was successfully measured by ionization chambers, employed by nine monitoring posts surrounding the power plant. From this information and Monte Carlo simulations, the present downward TGF is suggested to have taken place at an altitude of $2500\ifmmode\pm\else\textpm\fi{}500\text{ }\text{ }\mathrm{m}$, involving ${8}_{\ensuremath{-}4}^{+8}\ifmmode\times\else\texttimes\fi{}{10}^{18}$ avalanche electrons with energies above 1 MeV. This number is comparable to those in upgoing TGFs.

Smith D.M., Bowers G.S., Kamogawa M., Wang D., Ushio T., Ortberg J., Dwyer J.R., Stock M.

We compare two observations of gamma-rays before, during, and after lightning flashes initiated by upward leaders from a tower during low-altitude winter thunderstorms on the western coast of Honshu, Japan. While the two leaders appear similar, one produced a terrestrial gamma-ray flash (TGF) so bright that it paralyzed the gamma-ray detectors while it was occurring, and could be observed only via the weaker flux of neutrons created in its wake, while the other produced no detectable TGF gamma-rays at all. The ratio between the indirectly derived gamma-ray fluence for the TGF and the 95% confidence gamma-ray upper limit for the gamma-ray quiet flash is a factor of $1\times10^7$. With the only two observations of this type providing such dramatically different results -- a TGF probably as bright as those seen from space and a powerful upper limit -- we recognize that weak, sub-luminous TGFs in this situation are probably not common, and we quantify this conclusion. While the gamma-ray quiet flash appeared to have a faster leader and more powerful initial continuous current pulse than the flash that produced a TGF, the TGF-producing flash occurred during a weak gamma-ray "glow", while the gamma-ray quiet flash did not, implying a higher electric field aloft when the TGF was produced. We suggest that the field in the high-field region approached by a leader may be more important for whether a TGF is produced than the characteristics of the leader itself.

Abbasi R.U., Abu‐Zayyad T., Allen M., Barcikowski E., Belz J.W., Bergman D.R., Blake S.A., Byrne M., Cady R., Cheon B.G., Chiba J., Chikawa M., Fujii T., Fukushima M., Furlich G., et. al.

Bursts of gamma ray showers have been observed in coincidence with downward propagating negative leaders in lightning flashes by the Telescope Array Surface Detector (TASD). The TASD is a 700~square kilometer cosmic ray observatory located in southwestern Utah, U.S.A. In data collected between 2014 and 2016, correlated observations showing the structure and temporal development of three shower-producing flashes were obtained with a 3D lightning mapping array, and electric field change measurements were obtained for an additional seven flashes, in both cases co-located with the TASD. National Lightning Detection Network (NLDN) information was also used throughout. The showers arrived in a sequence of 2--5 short-duration ($\le$10~$\mu$s) bursts over time intervals of several hundred microseconds, and originated at an altitude of $\simeq$3--5 kilometers above ground level during the first 1--2 ms of downward negative leader breakdown at the beginning of cloud-to-ground lightning flashes. The shower footprints, associated waveforms and the effect of atmospheric propagation indicate that the showers consist primarily of downward-beamed gamma radiation. This has been supported by GEANT simulation studies, which indicate primary source fluxes of $\simeq$$10^{12}$--$10^{14}$ photons for $16^{\circ}$ half-angle beams. We conclude that the showers are terrestrial gamma-ray flashes (TGFs), similar to those observed by satellites, but that the ground-based observations are more representative of the temporal source activity and are also more sensitive than satellite observations, which detect only the most powerful TGFs.

Enoto T., Wada Y., Furuta Y., Nakazawa K., Yuasa T., Okuda K., Makishima K., Sato M., Sato Y., Nakano T., Umemoto D., Tsuchiya H.

Ground-based observations during a thunderstorm provide conclusive evidence of positrons being produced after lightning, confirming that lightning can trigger photonuclear reactions. Lightning, particularly the very energetic γ-ray flashes, can theoretically generate radioactive isotopes through the interaction of relativistic electrons with atoms and molecules in the air. Some weak observational evidence for this was recently claimed. Teruaki Enoto and collaborators report observations of a coastal thunderstorm in Japan on 6 February 2017, in which they see a clear signature of positron annihilation associated with γ-ray flashes, combined with γ-rays arising in the de-excitation of nuclei excited by neutron capture. They conclude that the positrons arise from the decay of neutrons after the lightning. Lightning and thunderclouds are natural particle accelerators1. Avalanches of relativistic runaway electrons, which develop in electric fields within thunderclouds2,3, emit bremsstrahlung γ-rays. These γ-rays have been detected by ground-based observatories4,5,6,7,8,9, by airborne detectors10 and as terrestrial γ-ray flashes from space10,11,12,13,14. The energy of the γ-rays is sufficiently high that they can trigger atmospheric photonuclear reactions10,15,16,17,18,19 that produce neutrons and eventually positrons via β+ decay of the unstable radioactive isotopes, most notably 13N, which is generated via 14N + γ → 13N + n, where γ denotes a photon and n a neutron. However, this reaction has hitherto not been observed conclusively, despite increasing observational evidence of neutrons7,20,21 and positrons10,22 that are presumably derived from such reactions. Here we report ground-based observations of neutron and positron signals after lightning. During a thunderstorm on 6 February 2017 in Japan, a γ-ray flash with a duration of less than one millisecond was detected at our monitoring sites 0.5–1.7 kilometres away from the lightning. The subsequent γ-ray afterglow subsided quickly, with an exponential decay constant of 40–60 milliseconds, and was followed by prolonged line emission at about 0.511 megaelectronvolts, which lasted for a minute. The observed decay timescale and spectral cutoff at about 10 megaelectronvolts of the γ-ray afterglow are well explained by de-excitation γ-rays from nuclei excited by neutron capture. The centre energy of the prolonged line emission corresponds to electron–positron annihilation, providing conclusive evidence of positrons being produced after the lightning.

Bowers G.S., Smith D.M., Martinez‐McKinney G.F., Kamogawa M., Cummer S.A., Dwyer J.R., Wang D., Stock M., Kawasaki Z.

Following a lightning strike to a wind turbine in Japan, we have observed a large burst of neutrons lasting 100 ms with a ground fluence of ~1,000 n cm−2, thousands of times greater than the peak neutron flux associated with the largest ground level solar particle event ever observed. This is the first detection of an unequivocal signature of neutrons from a terrestrial gamma ray flash, consisting of a 2.223 MeV gamma-ray spectral line from a neutron-capture on hydrogen reaction occurring in our detector, and is shown to be consistent with the production of 1012–1013 photoneutrons from a downward terrestrial gamma ray flash (TGF) at 1.0 km, with a gamma ray brightness typical of upward TGFs observed by satellites.