Optical emissions associated with narrow bipolar events from thunderstorm clouds penetrating into the stratosphere

Narrow bipolar events (NBEs) are signatures in radio signals from thunderstorms observed by ground-based receivers. NBEs may occur at the onset of lightning, but the discharge process is not well understood. Here, we present spectral measurements by the Atmosphere‐Space Interactions Monitor (ASIM) on the International Space Station that are associated with nine negative and three positive NBEs observed by a ground‐based array of receivers. We found that both polarities NBEs are associated with emissions at 337 nm with weak or no detectable emissions at 777.4 nm, suggesting that NBEs are associated with streamer breakdown. The rise times of the emissions for negative NBEs are about 10 μs, consistent with source locations at cloud tops where photons undergo little scattering by cloud particles, and for positive NBEs are ~1 ms, consistent with locations deeper in the clouds. For negative NBEs, the emission strength is almost linearly correlated with the peak current of the associated NBEs. Our findings suggest that ground-based observations of radio signals provide a new means to measure the occurrences and strength of cloud-top discharges near the tropopause.publishedVersio

On Lightning Electromagnetic Field Propagation Along an Irregular Terrain

In this paper, we present a theoretical analysis of the propagation effects of lightning electromagnetic fields over a mountainous terrain. The analysis is supported by experimental observations consisting of simultaneous records of lightning currents and electric fields associated with upward negative lightning flashes to the instrumented Santis tower in Switzerland. The propagation of lightning electromagnetic fields along the mountainous region around the Santis tower is simulated using a full-wave approach based on the finite-difference time-domain method and using the two-dimensional topographic map along the direct path between the tower and the field measurement station located at about 15 km from the tower. We show that, considering the real irregular terrain between the Santis tower and the field measurement station, both the waveshape and amplitude of the simulated electric fields associated with return strokes and fast initial continuous current pulses are in excellent agreement with the measured waveforms. On the other hand, the assumption of a flat ground results in a significant underestimation of the peak electric field. Finally, we discuss the sensitivity of the obtained results to the assumed values for the return stroke speed and the ground conductivity, the adopted return stroke model, as well as the presence of the building on which the sensors were located

Molecular Dynamics Study of Water and Ions Transported during the Nanopore Calcium Silicate Phase: Case Study of Jennite

Durability is an important property that determines the long-term behavior of cement-based materials. Water and ions are transported in nanopores of calcium-silicate-hydrate (C-S-H) gels, the main element in cement-based material, which significantly influences the durability of cement. Because of its structural similarity, jennite, an important mineral analog of C-S-H gel, is first taken to investigate the transport behavior at a molecular level. In this paper, structural and dynamical properties of the water/ions and the jennite interface are studied by the molecular dynamics (MD) simulation method. On the (001) surface of jennite, water molecules diffusing in the channel between silicate chains demonstrate the following structural water features: large density, good orientation preference, ordered interfacial organization, and low diffusion rate. The channel water molecules have more H-bonds connected with the neighboring water molecules and solid surface. As the distance from the channel increases, the structural and dynamical behavior of water molecules varies and gradually translates into bulk water properties at 10-15 angstrom from the liquid-solid interface. With respect to the interaction between jennite and the ions, the surface demonstrates Cl- repulsion and Na+ adsorption. With increased ion concentration, the jennite adsorption capability for Cl- is enhanced because Na+ and Cl- aggregate to form a cluster in the interfacial region. The simulation results, matching well with the Cl35 Nuclear Magnetic Resonance (NMR) studies and isotherm adsorption tests, give a molecular-scale interpretation of experimental studies

Two-Scale Modeling of Transport Properties of Cement Paste: Formation Factor, Electrical Conductivity and Chloride Diffusivity

Predicting transport properties of cement-based materials directly from the microstructure is very challenging, due to the problems of bridging length scales and the difficulties of realistically representing the microstructure. Based on a two-scale representation of the microstructure, a scheme is proposed in this paper to model the transport properties of cement paste through two-scale random walk simulation. A random walk algorithm is firstly applied at the sub-micro-scale to determine the diffusion tortuosity of the outer C-S-H layer. This is then up-scaled to the micro-scale to compute the diffusion tortuosity of cement paste. Based on physical laws, the diffusion tortuosity is transformed into the formation factor, and further into the electrical conductivity and the chloride diffusion coefficient of cement paste, and subsequently validated. It is proven that a more realistic representation of the microstructure makes it possible to derive transport properties of cement paste, directly and accurately, from the microstructure

Morphology of Calcium Silicate Hydrate (C-S-H) Gel: A Molecular Dynamic Study

Due to its complexity at nanoscale, calcium silicate hydrate (C-S-H), the dominant binding phase in cement hydrates, is not yet completely understood. In this study, molecular dynamics was employed to simulate the hydration products at low and high calcium/silicon ratios. It was found that two morphologies of calcium silicate hydrate gels can be distinguished - a branched structure at low calcium/silicon ratios and an ellipsoid particle structure at high calcium/silicon ratios. Using virtual X-ray diffraction (XRD), nuclear magnetic resonance (NMR) and small-angle neutron scattering (SANS) techniques, the simulated structures were characterised, confirming that they show features of calcium silicate hydrate as revealed by experimental approaches. The short-range structures of calcium and silicon atoms and the distorted calcium tetrahedrons resemble the features of silicate glasses obtained from experiments, implying the amorphous nature of the local structure in calcium silicate hydrate gel. Furthermore, formation mechanisms for the two morphologies are proposed. In the hydration process, calcium ions play roles in depolymerising the silicate structure and preventing the amorphous network formation. Therefore, at low calcium/silicon ratios, the reaction is governed by silicate skeleton growth, but at high calcium/silicon ratio, aggregations of calcium ions and short silicate chains dominate

FDTD Modeling of Lightning Electromagnetic Field Propagation over Mountainous Terrain

The Full-Wave Finite-Difference-Time-Domain (FDTD) electromagnetic (EM) simulation method can simulate large domains and produce time-marching solutions for arbitrary time-domain sources, making it ideal for many lightning applications. In this paper, a two-dimensional (2D) axial-symmetric and a three-dimensional (3D) FDTD model are developed that include irregular terrain based on real topographic data. The models open the door to a wide variety of advanced modeling applications in the analysis of the lightning EM field propagation over mountainous terrain and the location error of lightning location systems (LLSs) in the presence of in mountainous regions. © 2019 ACES