The electrification of volcanic jets and controlling parameters: A laboratory study
Damien Gaudin, Corrado Cimarelli, 2019
Composite images of high-speed video still frames, showing the same flash seen from a) the side of the setup and b) the top orthogonal view of the setup. Red arrow and circles mark the location of the inner part of the autoclave (dAin Fig.2). The flash displays a main branch starting from the top of the nozzle and have some branches at the limit of the barrel shock structure (Mendez-Harper et al., 2018). In order to limit the effect of the particle jet obscuring the flash, the experiment was done in the dark, so that only the flash is visible. For spatial reference, still frames of the flash are superimposed to the pictures of the setup before the experimental run.
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.
Summary of all the experiments showing the relationship between the total magnitude of the positive discharge and the forcing parameters (initial pressure, proportion of fines and mass erupted). Note that the vertical spacing between the bottom of the Faraday cage and the nozzle rim (hFC) induces a small, but noticeable variation, experiments with 20mm spacing having a total magnitude of discharges decreased by ∼20% compared to 5mm.
Ash electrification is a common feature in ash-rich volcanic plumes (McNutt and Williams, 2010). It may produce volcanic lightning at different stages of the plume development: (i) during the jet phase, when the plume motion is driven by inertia (vent discharges according to Thomas et al., 2010 classification) (ii) during the buoyant rise of the plume (near vent discharges) or (iii) within the umbrella of the plume (plume lightning).
The emission of radio-frequency impulses by electrical dis- charges makes them easily detectable by instrumental networks with different detection ranges. Local Lightning Monitoring Arrays (LMA) (Behnke and McNutt, 2014; Thomas et al., 2007, 2004) or global antenna networks such as the World Wide Lightning Lo- cation Network (WWLLN) and the ATDnet system (Bennett et al., 2010; Van Eaton et al., 2016, 2017), enable instrumental detection of volcanic explosive eruptions at safe distance and in all weather conditions (Behnke and McNutt, 2014). Several studies of recent eruptions have highlighted the link between electrical activity and the spatial and temporal evolution of volcanic plumes (Aizawa et al., 2016; Behnke et al., 2013; Thomas et al., 2010). Indeed, eruptions such as Augustine (2006) (Thomas et al., 2007, 2010, Redoubt (2009) (Behnke et al., 2013), Eyjafjallajökull (2010) (Arason et al., 2011; Bennet et al., 2010; Méndez Harper and Dufek, 2016), and more recently Bogoslof (2017–2018) were monitored from an electrical point of view. Meanwhile, the frequent eruptions of Sakurajima (Japan) have led to numerous studies using local arrays trying to link volcanic processes and electrification (Miura et al., 2002; Cimarelli et al., 2016). So far, no model exists to link the dynamics and the electrical activity of a volcanic plume, thus hindering the derivation of quantitative real-time information on the eruption from instrumental detection of electrical signal. Field studies have suggested that, al- though magma composition seems to have no effect (McNutt and Williams, 2010), the electrical activity may correlate with the magnitude of the eruptions.
From an experimental point of view, studies have been focusing on the charging mechanism of solid particles, especially aiming at disentangling the effects of tribo-electrification (exchange of electrical charges between two bodies rubbed together, see Harrison et al., 2010; Houghton et al., 2013; Méndez Harper and Dufek, 2016) from those of fractoelectrification (caused by the fragmentation of the erupting magma and pyroclasts, see James et al., 2000, 2008; Méndez Harper et al., 2015). However, these experiments only focus on the charging mechanism of volcanic particles and neglect the equally important mechanisms of discharge. In addition, they do not provide scalable numbers to transpose the results in the context of volcanic explosive eruptions. Finally, they mainly focus on the near-vent and plume lightning, neglecting the specificities of vent lightning which is the earliest detectable electrical signal and is thus crucial for real-time monitoring.
Focusing on the jet phase and vent lightning, Cimarelli et al. (2013) managed to produce electric discharges by rapidly de- compressing a dry gas-particle mixtures in a shock tube apparatus. Their experiments succeed in reproducing the whole charge- discharge process, from the electrification of the particles, through the particle and charge separation, to the generation of the dis- charges. Electrical discharges were observed both visually using high-speed video recordings and through variations of the electrical potential using small antennas.
In this study, we further implement the experimental setup described by Cimarelli et al. (2013) and setup a data processing method in order to more precisely quantify the charging of the particles and the discharges within the jet (referred as “vent lightning” in Thomas et al., 2010). By using a Faraday cage, we compute the net electric charge associated with the gas-particle flow, and the number and magnitude of the electrical discharges generated within. We focus our analysis on the effect of three main parameters, recognized to be crucial for the generation of volcanic lightning: (i) the mass of ejected particles (eruption magnitude), (ii) the pressure within the conduit and in the jet flow (eruption intensity) and (iii) the proportion of fine particles over the total solid load in the flow (grainsize distribution). Others factors that may play a significant role on electrification, (such as atmospheric conditions, water content, ash composition and crystallinity or temperature) have been deliberately excluded from the analysis for simplicity; however, the method developed here will be easily applicable to explore the effects of these parameters.