The electrification of volcanic jets and controlling parameters: A laboratory study.

Abstract Lightning is ubiquitous in large ash-rich eruptions. However, the quantification of the link between volcanic and electrical activities is still missing, hindering its potential for monitoring explosive eruptions. Here, we focus on vent lightning, i.e. discharges occurring within the ash-laden jet. We use a shock tube apparatus generating jets with variable mass of ash, grain size distribution and initial overpressure. The experimental jet is directed inside a Faraday cage, where the current (flux of electrical charges) is measured, allowing to estimate the total charge of the plume, and the number and magnitude of jet-to-ground discharges. Three mechanisms control the electrical structure of the jet: (i) the tribocharging of the ash particles against the shock tube walls; (ii) the particle–particle tribocharging in the jet flow; (iii) the particle and charge separation according to the particle size, leading to the formation of clusters of electrical charges. The number and magnitude of discharges mainly rely on the two latter mechanisms: while the particle–particle interactions define the total charge in all the clusters, which is linked to the observed cumulative magnitude of the discharges, the jet structure defines the size of the individual clusters, and, in turn, how the cumulative magnitude is partitioned on a number of discharges. Finally, our experimental relationships between eruptive and electrical parameters are compatible with field observations, suggesting that the pattern of discharges recorded by electrical monitoring systems can be used to interpret the structure of volcanic jets and their dynamic evolution. Highlights • Electrification of ash-laden jets is quantified by shock-tube experiments. • Charge partition between fine and coarse ash modulates the discharges. • Mass of fines and burst overpressure control total discharge magnitude. • Jet structure determines the number and size of the discharges. • Experimental observation can be scaled to natural eruptions. [ABSTRACT FROM AUTHOR]