Electrification of Experimental Volcanic Jets with Varying Water Content and Temperature

Abstract

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 particleladen 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. Key Points:• Natural observations on the influence of temperature and water content on volcanic lightning have been modelled in laboratory experiments. • The magnitude of electrification and discharging in plumes decreases significantly with added water. • At higher temperatures a decrease in magnitude of individual discharges is observed, while the total neutralized charge stays similar.

Radio Frequency Characteristics of Volcanic Lightning and Vent Discharges

“…Such global lightning detection data was used during the response to the eruption of the remote, and under-instrumented Bogoslof volcano in Alaska (Coombs et al, 2018), where lightning was detected from only roughly half of the ash-producing eruptions. Van Eaton et al (2020) showed that secondary ice-charging processes (Arason et al, 2011;Behnke et al, 2013;Van Eaton et al, 2016;Woodhouse & Behnke, 2014), rather than ash-charging processes (e.g., Houghton et al, 2013;James et al, 2000;Mendez Harper & Dufek, 2016;Mendez Harper et al, 2020;Stern et al, 2019), were likely responsible for the lightning detected by the global networks, and speculated that it was very likely that there was much more lightning and electrical activity that occurred that wasn't detected by the long-range VLF sensors. During this eruption, the global lightning data was a boon for the volcano response effort, though the latency between eruption and first lightning detection varied from one minute to one hour.…”

Microphysical Effects of Water Content and Temperature on the Triboelectrification of Volcanic Ash on Long Time Scales

“…All rights reserved. Stern et al [2019], the collision energies produced in the tumbler are more representative of conditions in convective columns transitioning to mature plumes than those in the gas-thrust region. The low energy interactions between particles did not produce obvious mechanical modification to the grains (as quantified by optical microscopy analysis performed before and after an experiment).…”

“…%) in a 300 o C jet. Stern et al [2019] also explored the role of temperature within the range of 25 -320 o C and report a moderate increase in electrical activity with increasing temperature.…”

“…James et al (2000), for instance, reported that particles electrified via fragmentation and frictional processes were possibly influenced by relative humidity (RH), but those authors indicate that their results may reflect anomalous behaviors of their measurement equipment rather than changes in the amount of charge collected by the ash. More recently, Stern et al (2019) used a laboratory-scale shock tube to approximate the eruption of a wet (i.e., completely saturated), hot granular flow. These investigators found that the number of spark discharges decreases with increasing water content (from 0 to 27 wt.%) in a 300°C jet.…”

“…These investigators found that the number of spark discharges decreases with increasing water content (from 0 to 27 wt.%) in a 300°C jet. Stern et al (2019) also explored the role of temperature within the range of 25-320°C and report a moderate increase in electrical activity with increasing temperature. The authors hypothesize that the increase in electrical activity in variable temperature experiments was likely due to changes in jet dynamics, not to intrinsic changes in the ash's ability to collect charge.…”