Electrification of Experimental Volcanic Jets with Varying Water Content and Temperature
S. Stern, C. Cimarelli, D. Gaudin, B. Scheu & D.B. Dingwell, 2019
A typical experiment recording (9 MPa initial pressure, 120 °C, dry). Nozzle pressure (a), the deconvoluted signal of the FC (b), and the net charge (c) are all plotted against experimental time. First and second gas arrival, as well as particle arrival, are marked with vertical lines.
Volcanic lightning—a near ubiquitous feature of explosive volcanic eruptions—possesses great potential for the analysis of volcanic plume dynamics. To date, the lack of quantitative knowledge on the relationships between plume characteristics hinders efficient data analysis and application of the resulting parameterizations. We use a shock‐tube apparatus for rapid decompression experiments to produce particle‐ laden jets. We have systematically and independently varied the water content (0–27 wt%) and the temperature (25–320 °C) of the particle‐gas mixture. The addition of a few weight percent of water is sufficient to reduce the observed electrification by an order of magnitude. With increasing temperature, a larger number of smaller discharges are observed, with the overall amount of electrification staying similar. Changes in jet dynamics are proposed as the cause of the temperature‐dependence, while multiple factors (including the higher conductivity of wet ash) can be seen responsible for the decreased electrification in wet experiments.
Plain Language Summary: Volcanic explosive eruptions are accompanied by lightning strikes generated from the volcanic dust cloud. Here we have experimentally studied the effects of atmospheric water and plume temperature on the frequency and intensity of the lightning strikes. The results will feed a model for the use of volcanic lightning strikes to estimate plume contents and intensity.
The experimental setup. The autoclave containing the sample can be surrounded by a furnace. Argon gas is conveyed into the autoclave through the gas inlets until the target pressure is reached. Above, the nozzle is the uppermost grounded part of the setup. The collector tank acts as a Faraday cage. A high‐speed camera is used to record the experiments. All sensors are connected to a data logger.
Volcanic lightning is the result of ash electrification in volcanic plumes. It has been observed at volcanoes all around the world, representing a large variety of magma compositions and eruptive styles (Mather & Harrison, 2006; McNutt & Williams, 2010; Nicoll et al., 2019). It is also associated with several geophysical signals that can be used for volcano monitoring from a safe distance and especially in unfavorable weather conditions (Behnke & McNutt, 2014).
Several observation techniques have been used at active volcanoes to characterize the electrical activity of eruptive plumes, including measurements of electric field variation (R. Anderson et al., 1965; Kikuchi & Endoh, 1982; Miura et al., 2001), direct measurements of fallout‐particles (Gilbert et al., 1991; Miura et al., 2001), and volcanic lightning mapping (Behnke et al., 2013; Thomas et al., 2007). In attempts to correlate variations in the electrical activity with plume dynamics, some of these studies have employed multipara- meter data sets, including infrasound data (J. F. Anderson et al., 2018; Cimarelli et al., 2016; Haney et al., 2018; Smith et al., 2017), high‐speed video recordings (Cimarelli et al., 2016), magnetotelluric measurements (Aizawa et al., 2010; Aizawa et al., 2016), and seismic data (Smith et al., 2018).
Among the noninductive charging mechanisms thought to be operating during volcanic ash emissions, tribo‐electrification (Aplin et al., 2016; Cimarelli et al., 2014; Harrison et al., 2010; Houghton et al., 2013; Méndez Harper & Dufek, 2016) and fracto‐electrification (Aplin et al., 2016; James et al., 2000; James et al., 2008; Méndez Harper et al., 2015) are considered the most relevant in explosive eruptions where magma fragmentation and consequent production of turbulent particle‐laden jets are naturally produced.
Mechanisms more similar to the electrification generated in meteorological thunderclouds (ice‐graupel interaction, e.g., Stolzenburg et al., 1998) have also been proposed, as (1) water vapor is abundant in volcanic plumes and (2) ice formation and riming of ash particles are observed within the plume which can reach the upper troposphere (Arason et al., 2011; McNutt & Williams, 2010; Thomas et al., 2007; Williams & McNutt, 2005). The occurrence of electrical discharges in explosive eruptions triggered or controlled by the presence of external water has drawn attention to the effect of water vapor in the generation of volcanic plume electrification. Documented examples of volcanic lightning produced in water‐rich conditions include the eruptions of Capelinhos, 1957–1958 (Machado et al., 1962), Surtsey, 1963 (R. Anderson et al., 1965), Eyjafjallajökull, 2010 (Arason et al., 2011; Harrison et al., 2010; Petersen et al., 2012), Bogoslof, 2017–2018 (Haney et al., 2018; Van Eaton et al., 2018), and Anak Krakatau, 2018 (Prata et al., 2019).
Due to the complexity of natural systems, field observations alone cannot resolve the effects of the different factors contributing to the electrification of volcanic plumes. Nevertheless, despite the potential first‐order importance of (1) higher temperatures and (2) the ubiquitous presence of water vapor in volcanic plumes, the vast majority of experiments to date have neglected the effects of water and temperature on electrification and discharge mechanisms.
In our study we quantify the effects of water content and temperature of experimentally generated jets on their resulting electrical activity. We consider the effects of both parameters on the charge and discharge modality of volcanic ash and discuss natural occurrences in light of our experimental results.