First In Situ Observations of Gaseous Volcanic Plume Electrification
Keri Nicoll, Martin Airey, Corrado Cimarelli, Alec Bennett, Giles Harrison, Damien Gaudin, Karen Aplin, Kuang Liang Koh, Marco Knueve and Graeme Marlton, 2019
Vertical profiles through the volcanic plume at Stromboli at 11:55:15 UT on 4 October 2017 (flight 2) measured by a Vaisala RS92 radiosonde and the VOLCLAB sensor package. (a) temperature (±0.5°C), (b) relative humidity (±5%), (c) space charge density (the cyan “+” and light gray “×” denote errors in descent and ascent space charge data, respectively). The initial descent through the plume is shown in blue, and ascent in clear air in gray. For references to color in this figure, please see the online version of the article.
Volcanic plumes become electrically charged, often producing spectacular displays of lightning. Previous research has focused on understanding volcanic lightning, primarily the large electric fields produced by charging of ash particles. Here we report on the previously overlooked phenomenon of volcanic plume electrification in the absence of detectable ash. We present the first in situ vertical profile measurements of charge, thermodynamic, and microphysical properties inside predominantly gaseous plumes directly above an erupting volcano. Our measurements demonstrate that substantial charge (at least ±8,000 pC/m3) is present in gaseous volcanic clouds without detectable ash. We suggest that plume charging may be enhanced by the emission of radon gas from the volcano, which causes ionization. This presents a hitherto unrecognized, but likely to be common, mechanism for charge generation in volcanic plumes, which is expected to modulate plume characteristics and lifetime. This process is currently neglected in recognized mechanisms of volcanic plume electrification.
Time series around launch of flight 2 of (a) voltage measured by the primary antenna of the Biral BTD electrostatic sensor (±120 μV) and (b) SO2 concentration (±3 ppm) measured by the VOLCLAB sensor package. Balloon launch time was at 11:55:15 and shown by the gray vertical dotted line.
Volcanic plumes become charged through a variety of mechanisms including fractoemission, triboelectrifi- cation, and hydrometeor‐ash particle interactions (Aplin et al., 2016; Mather & Harrison, 2006). Fractoemission typically occurs close to the vent, where explosive activity causes fragmentation of magma (James et al., 2000), leading to the emission of photons, electrons, positive ions, and charged particulates (Lane et al., 2011). Triboelectrification is a type of contact electrification caused by charge transfer at the sur- faces of particles (Houghton et al., 2013; Lacks & Levandovsky, 2007; Méndez‐Harper & Dufek, 2016). Ice‐ ash particle interactions have been considered analogous to the ice‐graupel interactions generating charge in thunderstorms (Arason et al., 2011; Williams & McNutt, 2005).
Understanding of the charging mechanisms above comes from laboratory experiments (Cimarelli et al., 2014; Méndez‐Harper et al., 2018) detection of volcanic lightning from (1) lightning mapping arrays (Behnke et al., 2013), (2) global lightning detection networks (Bennett et al., 2010), or (3) high‐speed ima- ging techniques (Aizawa et al., 2016; Cimarelli et al., 2016) and measurements of plume charge overhead from ground based electric field mills (James et al., 1998). The only direct measurements of volcanic plume charge have been made by collecting fallout ash particles (Gilbert et al., 1991; Miura et al., 2002) in a Faraday pail situated a few kilometers away from the eruptive vent. In situ plume charge mea- surements have, so far, been lacking due to the intrinsic difficulties of working in proximity to active volcanoes.
Renewed interest in understanding the behavior of charged aerosol clouds (including volcanic plumes and dust clouds) has arisen due to realization that existing long‐range particle transport models do not accurately predict the transport of large particles (Ryder et al., 2013; van der Does et al., 2018; Weinzierl et al., 2017). Charging modifies the fall speeds of small particles in the atmospheric electric field, changes aggregation rates, and enhances the washout of particles by rainfall (Harrison & Carslaw, 2003). Charge may also act to prolong the transport of particles in substantial electric fields (Ulanowski et al., 2007). Despite all of the aforementioned potential effects of charge on the behavior of aerosols, the lack of in situ charge measure- ments in volcanic plumes means that the magnitude of particle charging is unquantified, and therefore the importance of such mechanisms, for example, long‐range transport and particle fallout, is as yet unknown.
A major challenge in understanding the electrification of volcanic plumes is the separation of different charging mechanisms, due to the multiple ash processes simultaneously in action. The plumes studied here present the opportunity to study simplified volcanic plumes of gaseous vapor and liquid droplets in the absence of observable ash. Previous remotely sensed (using surface‐based potential gradient [PG] measurements) gaseous plumes were observed to be charged only when substantial ash concentrations were present (Miura et al., 2002). The generally repeatable behavior of the PG perturbations as the var- ious components of the volcanic plume pass overhead has led investigators to suggest that the gaseous component adopts a net positive charge, and the ash particles a net negative charge (e.g., Hatekayama & Uchikawa, 1952), with separation suggested to result from gravitational settling. The opposite charging of gas and ash particles is thought to originate from fractoemission, which occurs due to magma fragmen- tation, with lab experiments supporting the concept of positively charged gas and negatively charged ash particles (James et al., 2000). The only previous estimate of the charge in the gaseous region of such plumes is reported by Miura et al. (2002), who assumed a point charge geometry and derived 0.2 C from their surface PG measurements, which were made 2–5 km from the crater, when the plume was at an altitude of ~2 km above the sensor.
Here we used newly developed disposable sensors, which can be safely deployed to measure charge directly within a volcanic plume. These represent the first vertical profile measurements of thermodynamic, electri- cal, and microphysical properties inside a volcanic plume close to its source and provide new information about the magnitude, polarity, and vertical distribution of charge within prevalently gaseous volcanic clouds. Such measurements are relevant to refine our understanding of the electrical structure of volcanic plumes. Further, by focusing only on the gaseous component of volcanic plumes, we demonstrate that volcanic plumes do not require solid ash particles to become electrified, thereby providing evidence for an additional and hitherto unrecognized charging mechanism, which does not involve ash.