Charge injection into the atmosphere by explosive volcanic eruptions through triboelectrification and fragmentation charging
Joshua Méndez Harper, Corrado Cimarelli, Valeria Cigala, Ulrich Kueppers & Josef Dufek, 2021
a) Schematic showing two kinds of discharge in the experiments. 1) Intra-jet discharges occur between grains or clusters of grains of opposite polarity. 2) Jet to nozzle discharges occur between the charged grains and grounded metallic nozzle. b) Typical intra-jet discharges in an experiment with the 90-300μm particle size distribution. c) Jet to nozzle discharge in the same experiment as b).
Volcanic eruptions are associated with a wide range of electrostatic effects. Increasing evidence suggests that high-altitude discharges (lightning) in maturing plumes are driven by electrification processes that require the formation of ice (analogous to processes underpinning meteorological thunderstorms). However, electrical discharges are also common at or near the volcanic vent. A number of “ice- free” electrification mechanisms have been proposed to account for this activity: fractocharging, triboelectric charging, radioactive charging, and charging through induction. Yet, the degree to which each mechanism contributes to a jet’s total electrification and how electrification in the gas-thrust region influences electrostatic processes aloft remains poorly constrained. Here, we use a shock-tube to simulate overpressured volcanic jets capable of producing spark discharges in the absence of ice. These discharges may be representative of the continual radio frequency (CRF) emissions observed at a number of eruptions. Using a suite of electrostatic sensors, we demonstrate the presence of size- dependent bipolar charging (SDBC) in a discharge-bearing flow for the first time. SDBC has been readily associated with triboelectric charging in other contexts and provides direct evidence that contact and frictional electrification play significant roles in electrostatic processes in the vent and near-vent regions of an eruption. Additionally, we find that particles leaving the region where discharges occur remain moderately electrified. This degree of electrification may be sufficient to drive near-vent lightning higher in the column. Thus, near-vent discharges may be underpinned by the same electrification mechanisms driving CRF, albeit involving greater degrees of charge separation.
Experimental setup. a) We generate granular jets with the shock-tube setup described previously in Cimarelli et al. (2014). The jet’s dynamics approximate those in the gas-thrust region of volcanic columns and produce spark discharges. Venting into free air, the “column” is then diverted away from the shock-tube by an air stream. Particles falling out of the “plume” are then characterized by two electrostatic measurement systems: 1) an electrostatic separator and 2) a through-type Faraday sensor (described in Fig.2a andb, respectively). b) Size distributions for the three samples used in these experiments as measured by a diffraction analyzer with the following nominal ranges: 1) 90-300μm, 2) 50%90-300 and 50% 300-1000 mixtureμm (by volume), and 3) 300-1000μm.
1.1. Plume lightning and proximal discharges
Investigations over the last two decades reveal that electri- cal activity in volcanic columns may be broadly characterized into plume lightning and vent/near-vent discharges (Thomas et al., 2007; Behnke et al., 2013; Cimarelli et al., 2016; Aizawa et al., 2016; Van Eaton et al., 2020). The first modality comprises large-scale discharges at elevation in maturing plumes and, in many regards, is analogous to meteorological lightning (Prata et al., 2020; Van Eaton et al., 2020). Because of the large energies involved, plume lightning can often be detected with wide-range lightning networks (Van Eaton et al. (2020); Prata et al. (2020)). The second category, vent and near-vent discharges, are electrical events that neutralize lower amounts of charge per event and, as their names suggest, occur closer to the volcanic vent (Thomas et al., 2007; Behnke et al., 2013, 2018). Although often lumped together, Behnke et al. (2018) showed that vent and near-vent discharges originate from fundamentally distinct breakdown processes. Vent discharges are innumerable streamer discharges that occur within or directly above the vent and are no more than a few tens of meters in length (Thomas et al., 2007; Behnke et al., 2013, 2018). With current measurement techniques, these minute discharges cannot be detected individually (either at optical or RF wavelengths) or at very great distances. Collectively, how- ever, vent discharges produce a continuous electromagnetic “hum” (commonly referred to as continual radio frequency or CRF) that can be observed with instruments like the lightning mapping ar- ray (Thomas et al., 2007; Behnke et al., 2013; Behnke and Bruning, 2015; Behnke et al., 2018). CRF is often detected together with seismic and acoustic signals implying a relationship with explosions and over-pressure conditions at the vent (Note: we will use vent-discharges and CRF sources interchangeably) (Smith et al., 2020). Occurring somewhat higher in the column and later in the eruption, near-vent lightning involves leader discharges that can have lengths between a few hundred meters to several kilometers and can be detected individually (Aizawa et al., 2016; Cimarelli et al., 2016; Behnke et al., 2018). Although larger than vent dis- charges, near-vent lightning still moves smaller amounts of charge per event than meteorological lightning and, thus, may be invisible to global detection networks (Vossen et al., 2021). Locally, however, it may produce changes to the ambient electric field (Behnke et al., 2018). Aizawa et al. (2016) notes that meteorological/plume light- ning shares many characteristics with near-vent lightning, hinting that separating both into two categories may be unnecessary. Nonetheless, an explicit distinction between the two (which we make in the present work) may be warranted given the likely differences in electrification mechanisms underlying near-vent and plume/meteorological lightning.
An ever growing number of observations suggests that vent and near-vent discharges –what we will collectively call proximal discharges– are common during explosive eruptions (Thomas et al., 2007; Behnke et al., 2013; Aizawa et al., 2016; Cimarelli et al., 2016; Behnke et al., 2018; Smith et al., 2020; Vossen et al., 2021). These observations imply that erupted material charged had efficiently within the conduit and in the jet-thrust region. Further- more, there is evidence that proximal discharges contain valuable information about the source of the eruption. For instance, CRF is only detected with forcing at the vent and occurs within the gas-thrust region (Behnke et al., 2018; Smith et al., 2020). Smith et al. (2020) demonstrated this fact by showing that CRF can be correlated with the acoustic and seismic signals associated with active fragmentation. Experimentally, Méndez Harper et al. (2018a) showed that the location and timing CRF emissions reflect the geometry and temporal evolution of barrel shock structures in supersonic jets. These spatiotemporal constraints suggest CRF is a valuable tool to detect incipient eruptions (Behnke et al., 2018). Regarding near-vent lightning, Cimarelli et al. (2016) indicate that the number of discharges is proportional to the over-pressure at the vent. These authors conclude that the intensity of near-vent electrical activity scales with the energy of eruptions. Furthermore, Aizawa et al. (2016) argue that the volumetric charge density in proximal jets may be much larger than that in thunderstorms. Be- cause of these elevated charged loadings, the proximal regions of the volcanic system may also be interrogated using active methods such as GNSS occultation (Méndez Harper et al., 2019). Using an array of electrostatic instruments, Behnke et al. (2018) report complex feedback mechanisms between CRF sources and larger near-vent discharges, suggesting that both forms of discharge may depend on a shared charge budget.
Nonetheless, the physical, chemical, and dynamical processes that charge pyroclasts within the conduit and the gas-thrust region remain poorly constrained. Ice and graupel are generally absent in any large quantities (Cimarelli et al., 2016; Vossen et al., 2021). Thus, in contrast to volcanic lightning at altitude (Van Eaton et al., 2020; Prata et al., 2020), electrification mechanisms comparable to those in thunderclouds cannot account for electrical activity near the vent. Instead, proximal discharges likely reflect “dry” charging processes operating with varying degrees of efficiency within the conduit and an expanding jet.
Starting at depth, material possibly charges during the brittle failure of the magmatic column and subsequent disruptive clast- clast collisions (James et al., 2000). Fractocharging may involve a number of pathways, including piezoelectricity, pyroelectricity, atomic dislocations, positive-hole activation, and the release and capture of positive and negative ions as new surfaces are created (Dickinson et al., 1981; Xie and Li, 2018). James et al. (2000) frac- tured pumice through repeated impacts and abrasion and found that fragments carried elevated surface charge densities. Quite recently, Smith et al. (2018) found that eruptions producing more equant grains were associated with CRF, perhaps suggesting that milling (secondary fragmentation) plays a role in vent-discharges. It is worth noting that, although the fracture mechanism is often invoked to account for electrification in the near-vent region, not a single experimental follow up work has been conducted on the matter using natural materials (pumice) in the last 20 years. As such, fractocharging is perhaps the least-well understood “major” charging mechanism in the volcanic context.
Non-disruptive collisions may too lead to electrification through the well-known (but imperfectly understood) triboelectric effect (Hatakeyama and Uchikawa, 1951; Kikuchi and Endoh, 1982; Aplin et al., 2014; Méndez Harper and Dufek, 2016; Méndez Harper et al., 2017, 2020, 2021). Importantly, not only does triboelectricity have the ability to produce efficient charging in a granular material, but may separate charges of opposite polarity based on particle size (Hatakeyama and Uchikawa, 1951; Kikuchi and En- doh, 1982; Zhao et al., 2003; Forward et al., 2009; Waitukaitis et al., 2014). Indeed, triboelectric charging often results in smaller, negatively-charged grains and larger grains with generally positive charges. This phenomenological feature, size-dependent bipolar charging (SDBC), may be critical in the production of discharges in proximal volcanic jets (and other dusty planetary environments) as particles of different sizes and opposite charge become separated through hydrodynamics (Cimarelli et al., 2014) or sedimentation (Harrison et al., 2016). A handful of studies have been explicitly designed to investigate triboelectric SDBC using volcanic materials. Forward et al. (2009) employed a fluidized bed to electrify basalt particles under partial vacuum. This study, however, used heavily altered materi- als to approximate Martian regoliths rather than recently erupted ash. Nonetheless, serendipitous reports of size-dependent bipolar charging in chemically-unmodified volcanic ash exist in the literature. Many of these observations were not placed within the modern framework of triboelectrification simply because the models had not yet been formulated. Hatakeyama and Uchikawa (1951) studied the frictional electrification of Aso and Asama ash samples. Those investigators reported standard SDBC –that is positive large grains, negative small grains– in Aso ash. However, the Asama ash samples displayed inverse SDBC (negative large grains, positive small grains). In these experiments, particles were allowed to contact foreign objects (an aluminum plate, for example), possibly biasing the polarity of the charge in manners that would not be encountered in natural systems. Thirty years later, Kikuchi and Endoh (1982) conducted similar experiments and found standard SDBC in ash particles from the 1977 Usu eruption. At Sakurajima, Miura et al. (2002) measured changes in the atmospheric potential gradient associated with small explosive events and estimated the surface charge density and polarity of ash falling out of plumes using an electrostatic separator (a method similar to the one we describe below). Those authors report particles with surface charge densities approaching the ionization limit (10−6 –10−5 Cm−2 ) and standard SDBC.
In addition to tribo- and fractoelectric processes, other mechanisms have been proposed to account for proximal discharges. Pahtz et al. (2010) suggests that materials like volcanic ash and mineral dust could charge through the polarizing effects of an ambient electric field. Aplin et al. (2014) provide evidence that the decay of radioactive elements in the magma may lead to “self- charging” of ash. Very recently, Nicoll et al. (2019) deployed sensors into a plume at Stromboli, finding that the gas phase itself is charged.
Building monitoring tools that effectively leverage proximal electrical effects requires a better understanding of the mechanisms that charge pyroclasts. Accomplishing such a feat, however, is complicated by the fact that much uncertainty remains regard- ing proposed electrification mechanisms themselves. For instance, while triboelectrification has been described since the time of the ancient Greeks, we have yet to unequivocally identify the charge carriers being exchanged during frictional interactions (Lacks and Sankaran, 2011; Lacks and Shinbrot, 2019). These charge carriers could be electrons, ions, or both. Similarly, some authors have presented evidence that triboelectrification arises from surface dam- age at minute spatial scales, implying that contact and frictional electrification are ultimately forms of fragmentation charging (Pan and Zhang, 2019; Lacks and Shinbrot, 2019).
Beyond questions surrounding the charging mechanisms that putatively drive vent and near-vent discharges, little is known about how proximal electrification influences the long term electrostatic evolution of the eruptive column. One possibility is that pyroclasts advected high into the atmosphere retain charge generated in the conduit and the gas-thrust region. This “pre-charging” may have important consequences for subsequent electrical effects, as some work indicates that charging in a granular material de- pends on pre-existing electric fields (e.g. Pahtz et al. (2010)). A second possibility is that the abundance of proximal discharges effectively neutralizes charge gained at or near the vent. Recom- bination in the gas-thrust region would imply that “downstream” lightning storms in mature plumes generally necessitate additional cycles of electrification (perhaps driven by ice) and reflect little about eruption dynamics at the source. Evidence for this second hypothesis exists in field data collected at Augustine (Thomas et al., 2007) and Redoubt (Behnke et al., 2013), which show that electrical activity waned after the initial explosive phases. These periods of electrical inactivity could signify that volcanic columns emerge from the gas-thrust region with weak degrees of charging. The resumption of electrical activity in mature plumes could indi- cate activation of water-based electrification mechanisms (Prata et al., 2020; Van Eaton et al., 2020).
Here, we use a shock-tube to simulate explosive, overpressured volcanic jets and address a subset of the questions posed above. Our setup allows us to investigate the charge mechanisms that drive CRF sources and make inferences regarding subsequent near-vent lightning. For the first time, we identify nominal size- dependent bipolar charging in a simulated volcanic jet bearing streamer discharges. SDBC in our shock-tube experiments provides direct evidence that tribocharging is a dominant electrification mechanism in the gas-thrust region. Additionally, we find that particles emerging from the supersonic flow carry charge densities comparable to those measured on grains falling out of proximal volcanic columns (e.g. Gilbert et al. (1991); Miura et al. (2002)). Further analysis shows that this amount of charge may be sufficient to drive near-vent lightning. As such, our results indicate that near-vent lightning is likely underpinned by the same electrification mechanisms as CRF sources, but reflects larger scale charge separation in columns.