Experimental generation of fulgurite under realistic lightning discharge conditions

Fulgurites have been documented in geological deposits from throughout Earth's history. They have also been assigned a potential role in prebiotic chemistry as a source of reactants. Fulgurites are generated in nature by cloud-to-ground lightning strikes. The unpredictability in space and time of the occurrence of lightning events has limited the investigation of both the mechanisms by, and the conditions under, which fulgurites form. A laboratory-based approach can mitigate these limitations. Here, we describe experimentally generated fulgurites generated from Laacher See volcanic ash. We employ a DC source with a trigger-pulse setup in a high voltage laboratory, whose capabilities enable experimental conditions that correspond closely to the electrical characteristics of natural lightning strikes. The experimentally generated fulgurites closely resemble naturally-occurring fulgurites in both state and texture. These experimental investigations yield a high reproducibility of the characteristic of fulgurites generated under well-constrained conditions, enabling some inferences to be made regarding the processes involved in the generation of fulgurites in nature. This work provides a basis for a systematic characterization of experimental fulgurites and the characteristic of lightning discharges.

Spatial, Temporal, and Electrical Characteristics of Lightning in Reported Lightning-Initiated Wildfire Events

Analysis was performed to determine if a lightning flash could be associated with every reported lightning-initiated wildfire that grew to at least 4 km². In total, 905 lightning-initiated wildfires within CONUS between 2012 and 2015 were analyzed. Fixed and fire radius search methods showed that 81-88% of wildfires had a corresponding lightning flash within a 14 day period prior to the report date. The two methods showed that 52-60% of lightning-initiated wildfire were reported on the same day as the closest lightning flash. The fire radius method indicated the most promising spatial results, where the median distance between the closest lightning and the wildfire start location was 0.83 km, followed by a 75^th percentile of 1.6 km, and a 95^th percentile of 5.86 km. Ninety percent of the closest lightning flashes to wildfires were negative polarity. Maximum flash densities were less than 0.41 flashes km2 for the 24 hour period at the fire start location. The majority of lightning-initiated holdover events were observed in the Western CONUS, with a peak density in north-central Idaho. A twelve day holdover event from New Mexico was also discussed; outlining the opportunities and limitations of using lightning data to characterize wildfires.

Natural and artificially initiated lightning

Recent research on lightning has been motivated, in part, by the desire to prevent spectacular accidents, such as occurred in 1969 during the launch of Apollo 12 and in 1987 during the launch of Atlas-Centaur 67, and by the need to protect advanced ground-based and airborne systems that utilize low voltage, solid-state electronics. The present understanding of both natural and artificially initiated (triggered) lightning is reviewed, and suggestions are given for future research that can improve our understanding both of the physics of lightning and the parameters that are important for protection.

Lightning spectroscopy-photographic techniques

Some spectroscopic studies of lightning from 1901 to the present are reviewed, with special emphasis on slitless techniques-time-integrated and time-resolved. Some of the best examples of the authors' time-integrated spectra are reproduced to display and identify emission and absorption features from 3000 A to almost 9000 A on a nearly uniform scale approximating 20 A/mm. Time-resolved spectra in the visible region are reproduced in the range of milliseconds for a continuing-current luminosity, and microseconds for a return stroke and a stepped leader.