Optical emission spectra of high current and high voltage generated arcs representing lightning

A recently developed spectrographic technique was used for the comparison of observed optical emission spectra from self-breakdown in air of high current and high voltage impulse arcs which are often used to represent the respective components of a natural lightning strike for a variety of lightning studies. These components are separated as it is practically impossible to simultaneously generate both in a single arc within the laboratory. The high current aspect is typically related to the amount of damage a lightning arc does to an impacted object and can be used to characterize conductivity properties, whereas the high voltage aspect is typically related to the attachment mechanism of lightning to an object when creating a path to ground and can be used to characterize insulation properties. The spectra of a range of high current arcs up to 100 kA and high voltage arcs up to 120 kV across a 14 mm air-gap between a pair of hemispherical tungsten electrodes were investigated and compared with the natural lightning reference spectrum reconstructed from the literature. All three spectra were found to present similar trends, but the high voltage arc had notably lower blackbody radiation, indicating a less energetic and cooler arc. Further, whereas atomic lines from oxygen and nitrogen were clearly present, an atomic line from argon was not prominent, indicating that the bulk energy delivered into the air was in the range of 1400–1520 kJ/mol. The high current arc, much like natural lightning, delivered a bulk energy greater than 1520 kJ/mol

Investigation of lightning direct effects on aircraft materials

Lightning is one of the most unpredictable and destructive forces in nature, and ensuring the protection and safety of aircraft in flight, as well as other modes of airborne transport, when struck by lightning presents a unique set of engineering challenges. In order to progress into a new generation of conductive carbon composites and/or embedded light-weight lightning protection systems, further scientific understanding is required on the mechanisms and impact of lightning on carbon composite aircraft. This paper recalls how modern lightning research has progressed from deducing information from natural lightning events to reproducible and instrumented lightning generators capable of accurate and repeatable experimentation. Results of investigations on how aerospace materials, with and without lightning protection, react when struck by lightning are presented illustrating the extent of damage that could be caused. Furthermore, methods of studying lightning effects, such as mechanical deflection, chemical element interaction, and temperature measurements, are then presented to illustrate the role of lightning experimentation as a means to help support aspects of modelling such as material behavior, aircraft design and potentially flight performance

Analysis of light emission and Schlieren from short gap high voltage streamers representing lightning impulses

Light emissions and Schlieren structures were simultaneously observed from streamers produced by tens of kilovolts 1.2/50 μs impulses, representing the high voltage component of lightning, applied across a 4 cm air gap between a variety of electrode geometries and a ground plane in an unconfned environment. The results demonstrated that the light emissions and Schlieren structures coincide along the same streamer flaments but on diferent timescales; the light existing only during the microsecond timeframe impulse whereas the Schlieren continued to develop into the millisecond timeframe, moving towards the centre of the air gap whilst difusing into the surrounding air within 100 ms. If an electrical breakdown did occur, the Schlieren structures outside the arc remained visible. Streamer formation theory for high voltage impulses is subsequently refned to include the observed Schlieren mechanism

Topographic roughness as a signature of the emergence of bedrock in eroding landscapes

Rock is exposed at the Earth surface when rates of erosion locally exceed rates of soil production. The thinning of soils and emergence of bedrock has implications spanning geomorphology, ecology and hydrology. Soil-mantled hillslopes are typically shaped by diffusion-like sediment transport processes that act to smooth topography through time, generating the familiar smooth, convex hillslope profiles that are common in low relief landscapes. Other processes, however, can roughen the landscape. Bedrock emergence can produce rough terrain; in this contribution we exploit the contrast between rough patches of bedrock outcrop and smooth, diffusion-dominated soil to detect bedrock outcrops. Specifically, we demonstrate that the local variability of surface normal vectors, measured from 1 m resolution airborne LiDAR data, can be used as a topographic signature to identify areas within landscapes where rock exposure is present. We then use this roughness metric to investigate the transition from soil-mantled to bedrock hillslopes as erosion rates increase in two transient landscapes, Bald Rock Basin, which drains into the Middle Fork Feather River, California, and Harrington Creek, a tributary of the Salmon River, Idaho. Rather than being abrupt, as predicted by traditional soil production models, in both cases the transition from fully soil-mantled to bedrock hillslopes is gradual and spatially heterogeneous, with rapidly eroding hillslopes supporting a patchwork of bedrock and soil that is well documented by changes in topographic roughness, highlighting the utility of this metric for testing hypotheses concerning the emergence of bedrock and adding to a growing body of evidence that indicates the persistence of partial soil mantles in steep, rapidly eroding landscapes

Woody encroachment and forest degradation in sub-Saharan Africa's woodlands and savannas 1982-2006

We review the literature and find 16 studies from across Africa's savannas and woodlands where woody encroachment dominates. These small-scale studies are supplemented by an analysis of long-term continent-wide satellite data, specifically the Normalized Difference Vegetation Index (NDVI) time series from the Global Inventory Modeling and Mapping Studies (GIMMS) dataset. Using dry-season data to separate the tree and grass signals, we find 4.0% of non-rainforest woody vegetation in sub-Saharan Africa (excluding West Africa) significantly increased in NDVI from 1982 to 2006, whereas 3.52% decreased. The increases in NDVI were found predominantly to the north of the Congo Basin, with decreases concentrated in the Miombo woodland belt. We hypothesize that areas of increasing dry-season NDVI are undergoing woody encroachment, but the coarse resolution of the study and uncertain relationship between NDVI and woody cover mean that the results should be interpreted with caution; certainly, these results do not contradict studies finding widespread deforestation throughout the continent. However, woody encroachment could be widespread, and warrants further investigation as it has important consequences for the global carbon cycle and land–climate interactions

25 years of satellite InSAR monitoring of ground instability and coastal geohazards in the archaeological site of Capo Colonna, Italy

For centuries the promontory of Capo Colonna in Calabria region, southern Italy, experienced land subsidence and coastline retreat to an extent that the archaeological ruins of the ancient Greek sanctuary are currently under threat of cliff failure, toppling and irreversible loss. Gas extraction in nearby wells is a further anthropogenic element to account for at the regional scale. Exploiting an unprecedented satellite Synthetic Aperture Radar (SAR) time series including ERS-1/2, ENVISAT, TerraSAR-X, COSMO-SkyMed and Sentinel-1A data stacks acquired between 1992 and 2016, this paper presents the first and most complete Interferometric SAR (InSAR) baseline assessment of land subsidence and coastal processes affecting Capo Colonna. We analyse the regional displacement trends, the correlation between vertical displacements with gas extraction volumes, the impact on stability of the archaeological heritage, and the coastal geohazard susceptibility. In the last 25 years, the land has subsided uninterruptedly, with highest annual line-of-sight deformation rates ranging between -15 and -20 mm/year in 2011-2014. The installation of 40 pairs of corner reflectors along the northern coastline and within the archaeological park resulted in an improved imaging capability and higher density of measurement points. This proved to be beneficial for the ground stability assessment of recent archaeological excavations, in an area where field surveying in November 2015 highlighted new events of cliff failure. The conceptual model developed suggests that combining InSAR results, geomorphological assessments and inventorying of wave-storms will contribute to unveil the complexity of coastal geohazards in Capo Colonna. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Mechanical Forces Due to Lightning Strikes to Aircraft A Pseudo-Stereo DIC Technique for Measuring Full-Field Displacement

Abstract. One of the major considerations currently affecting the design of composite aircraft structures is the damage resulting from lightning attachment. Full-field measurements of the displacement of materials under lightning attachment would provide a greater understanding of the forces induced by the high current waveform. Furthermore the understanding of the forces involved would allow for the validation of finite element models to simulate the effects of lightning attachment, therefore aiding in the design of solutions to reduce damage to aircraft structures. The study aimed to develop a pseudo-stereo high speed digital image correlation technique in order to obtain full-field information during lightning attachment based on a 100kA initial strike over a 500μs duration, the most severe waveform experienced. The technique that was developed gave full-field measurements for a 550x550x2mm 6082-T6 aluminium panel under a 100kA lightning attachment. Two correlation measurements were recorded at 3000 and 5000 frames per second. The displacement results are comparative with the theory of a cylindrical pressure expansion arising from the acoustic shockwave on attachment to the material. Further developments to this system could allow for more reliable results and higher frame rates which can be used to develop finite element simulations based on measured physical data