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We observed two Terrestrial Gamma‐ray Flashes (TGFs) in Uchinada, Japan associated with negative cloud‐to‐ground lightning strokes exactly 1 year apart on 18 December 2020 and 2021. The events were remarkable for their lateral distance from the associated strokes—each about 5 km away from the detector site. Not only was that lateral distance remarkable on its own for a ground based detection, but the low‐altitude profile of winter thunderstorms in Japan would suggest the detections occurred at unprecedented nadir angles—73.3° off axis for the 2020 event with the standard assumption of a vertically oriented TGF. Unsurprisingly, Monte Carlo simulations of the straightforward interpretation of these events yield fluences 2 orders of magnitude lower than observed data. We investigate a variety of ways to attempt to resolve the contradiction between expected and observed behavior.

 

Brief bursts of high‐frequency (HF) and very high frequency (VHF) radio emissions unaccompanied by strong low‐frequency radiation have been observed during initiation and propagation of lightning or thunderstorm electrical breakdown without leading to fully fledged lightning. This paper investigates a physical mechanism to generate such radio bursts by electrical discharge activity inside a thundercloud. When a discharge consists of many high‐frequency emission sources, such as streamers, that generate currents in random directions, its radiation spectrum peaks in the HF and VHF bands, and the spectral magnitudes in low frequencies are much smaller or even negligible. Combined with recent observational findings, the present study suggests that lightning initiation may begin with a short burst of many randomly occurring small‐scale discharges in a localized thundercloud region.

 

We present observations of X-rays from laboratory sparks created in the air at atmospheric pressure by applying an impulse voltage with long (250 µs) rise-time. X-ray production in 35 and 46 cm gaps for three different electrode configurations was studied. The results demonstrate, for the first time, the production of X-rays in gaps subjected to switching impulses. The low rate of rise of the voltage in switching impulses does not significantly reduce the production of X-rays. Additionally, the timing of the X-ray occurrence suggests the possibility that the mechanism of X-ray production by sparks is related to the collision of streamers of opposite polarity. 

Prior research into the conceptual underpinnings of the public's institutional trust in zoos and aquariums has suggested a range of ethical dimensions that set these types of cultural institutions apart from others in the museum sector. As the recognized holders, care‐takers, and nurturers of wild animals, zoos and aquariums are sustained at least in part by the public's perception that these activities are legitimate pursuits and essential to the long‐term conservation of the natural world. This paper builds on recent research that identified the ethical dimensions of trust in zoos and aquariums and assessed their distribution among the U.S. public by analyzing survey responses with respect to the importance of trust criteria. We hypothesized that distinct clusters of individuals, as defined by their response to trust criteria items, would emerge and that these clusters would prioritize different dimensions in their trust of zoos and aquariums. Usingk‐means clustering, we identified four relevant clusters of individuals on seven dimensions of institutional trust in zoos and aquariums. Based on these clusters, we suggest strategies for addressing what may be necessary for zoos and aquariums to claim authority as agents promoting conservation behaviors in society.

 

Terrestrial gamma‐ray flashes (TGFs) are bright bursts of gamma rays produced by thunderstorms, typically observed by spacecraft in the low‐Earth orbit. Unfortunately, it has been difficult to disentangle the source altitude and the width and direction of the gamma‐ray beam using single point spacecraft measurements, which has hampered attempts to constrain TGF models. Polarimetry of astrophysical sources has been of interest for many decades, which raises the question: Do TGFs and X‐rays from lightning have observable polarization, and if so, what would this polarization tell us about their source? REAM Monte Carlo code has been modified to record the linear polarization of X‐rays and gamma rays as a function of source altitude and beam geometry. It is found that polarization degree of a 20‐km narrow beam of TGF is substantially different from a 15‐km‐wide beam, which could be used to constrain the source geometry of TGFs. However, due to the low fluence of these events in space, detecting this level of polarization would be challenging. It is also found that low‐altitude TGFs (source at 3.5 km) produce polarizations up to about 8%; however, detectors need to be very close to the source region. Furthermore, very low altitude ground‐level TGFs and X‐rays showed a maximum polarization of 13% on the ground, of which the TGF's fluence was large enough for polarimetry. In addition, polarization reached its maximum further away from thezaxis as the TGF's beam broadened. The dominant mechanism of the polarization was found to be Compton scattering.

 

Plasma discharge fluid simulations indicate a negative streamer can be initiated from the narrow tip of a cone‐shaped, realistically sized hydrometeor in a uniform electric field as low as 0.65E_k. A negative streamer, however, cannot be formed from a cone‐shaped hydrometeor with a slightly smaller base radius or a column hydrometeor under otherwise the same conditions. The simulation also shows the negative streamer is accompanied by a positive streamer. The results present implications for understanding lightning initiation and the physical mechanism of the recently discovered thunderstorm‐related phenomenon called fast negative breakdown. Finally, the initiated negative streamer develops with a relatively constant head radius and velocity in contrast with the expanding and accelerating positive streamer. It appears that the differences between the properties of each polarity of streamer are more pronounced in an electric field belowE_k.

 

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.

 

In this letter, we report simulation results of streamer propagation and collision that produce electromagnetic radiation in the very high frequency (VHF) and ultra high frequency (UHF) bands. The streamers are initiated in overbreakdown field conditions, 1.5E_kand2E_k, respectively, which may be found during the corona flash stage of negative leader stepping processes. We find that while streamer propagation produces stronger VHF radiation, the head‐on collision of streamers dominates UHF, and even higher‐frequency radiation. Analysis of the energy spectral densities obtained from different simulation cases shows that the total length and radii of colliding streamers, as well as the ambient field, are important parameters for the UHF radiation produced by streamer collisions. The larger those parameters are, the stronger UHF radiation produced. Finally, by comparing with the measured spectral magnitude of lightning field in the VHF range, it is found that there are probably 10⁵–10⁷streamers involved during the lightning corona flash stage.