The Influence of Grain Size Distribution on Laboratory‐Generated Volcanic Lightning
Scheu, B.
Manga, M.
Cigala, V.
Cimarelli, C.
Dingwell, D. B.
DOI: https://doi.org/10.1029/2022JB024390
Persistent URL: http://resolver.sub.uni-goettingen.de/purl?gldocs-11858/10420
Cigala, V.; 1 Ludwig‐Maximilians‐Universität München Munich Germany
Cimarelli, C.; 1 Ludwig‐Maximilians‐Universität München Munich Germany
Dingwell, D. B.; 1 Ludwig‐Maximilians‐Universität München Munich Germany
Abstract
Over the last decades, remote observation tools and models have been developed to improve the forecasting of ash‐rich volcanic plumes. One challenge in these forecasts is knowing the properties at the vent, including the mass eruption rate and grain size distribution (GSD). Volcanic lightning is a common feature of explosive eruptions with high mass eruption rates of fine particles. The GSD is expected to play a major role in generating lightning in the gas thrust region via triboelectrification. Here, we experimentally investigate the electrical discharges of volcanic ash as a function of varying GSD. We employ two natural materials, a phonolitic pumice and a tholeiitic basalt (TB), and one synthetic material (soda‐lime glass beads [GB]). For each of the three materials, coarse and fine grain size fractions with known GSDs are mixed, and the particle mixture is subjected to rapid decompression. The experiments are observed using a high‐speed camera to track particle‐gas dispersion dynamics during the experiments. A Faraday cage is used to count the number and measure the magnitude of electrical discharge events. Although quite different in chemical composition, TB and GB show similar vent dynamics and lightning properties. The phonolitic pumice displays significantly different ejection dynamics and a significant reduction in lightning generation. We conclude that particle‐gas coupling during an eruption, which in turn depends on the GSD and bulk density, plays a major role in defining the generation of lightning. The presence of fines, a broad GSD, and dense particles all promote lightning.
Plain Language Summary: Explosive volcanic eruptions are accompanied by volcanic lightning (VL), which are electrical discharges resulting from particles that become electrically charged during eruption. We investigated experimentally the discharge behavior of three different materials by performing shock‐tube experiments. We used different rocks and analog material. We focused on the abundance of particle sizes smaller <10 μm (very fine ash) by testing individual grain size fractions mixed with coarser grains. The jet behavior was recorded by a high‐speed camera. We find that the presence of very fine particles has a major influence on the probability to produce electrical discharges within the particle‐laden jet. Based on our experiments, more VL is expected when (a) fine ash is abundant, (b) there is a wide grain size distribution, and (c) the particles are dense.
Key Points:
Electrical discharges are generated in experimentally decompressed volcanic ash.
The presence of fines (<10 μm), a broad grain size distribution, and dense particles promote laboratory‐generated volcanic lightning.
The coupling of the particles to the jet determines whether an electrical discharge occurs within the jet.