Experimental Generation of Volcanic Lightning

Experimental generation of volcanic lightning

C. Cimarelli, M.A. Alatorre-Ibargüengoitia*, U. Kueppers, B. Scheu, and D.B. Dingwell Department of Earth and Environmental Sciences, Ludwig Maximilian University, Theresienstraße 41, 80333 Munich, Germany

ABSTRACT A Transparent collection tank Explosive volcanic eruptions are commonly associated with intense electrical activity and High-speed lightning. Direct measurement of the electric potential at the vent, where the electric activity camera in the volcanic plume is fi rst observed, is severely impeded, limiting progress in its investi- 6-7 cm gation. We have achieved volcanic lightning in the laboratory during rapid decompression 11 cm Antennas and experiments of gas-particle mixtures under controlled conditions, and recorded it using a P-transducer high-speed camera and two antennas. We fi nd that lightning is controlled by the dynamics of 2 cm the particle-laden jet and by the abundance of fi ne particles. The relative movement of clus- Gas inlet ters of charged particles generates the electrical potential, which is necessary for lightning. 10-16.5 cm Sample Diaphragm The experimental generation of volcanic lightning suggests that rapid progress can now be expected in understanding electrical phenomena in volcanic plumes to implement lightning P-transducers monitoring systems and the forecasting of volcanic ash emissions.

INTRODUCTION observed in thunderstorms (Williams and Mc- Lightning discharges are often observed dur- Nutt, 2004; Thomas et al., 2007). As such, the ing explosive volcanic eruptions and are com- presence of hydrometeors within the plume B Nozzle monly associated with the formation of ash has been assigned a decisive role in the genera- plumes (Mather and Harrison, 2006; James et tion of volcanic lightning (Arason et al., 2011). al., 2008; McNutt and Williams, 2010; Rakov Measurements of electrically charged volcanic and Uman, 2003). Their occurrence appears to ash in the fi eld (Miura et al., 2002; Gilbert et be independent of magma composition, eruption al., 1991; Calvari et al., 2012) and in laboratory type, and plume height (McNutt and Williams, experiments (James et al., 2000; Büttner et al., 2010). Increasingly sophisticated lightning 2000) invoke triboelectrifi cation (electrifi cation mapping arrays show that lightning discharges of solids through friction) and fractoemission are ubiquitously produced within three regions (emission of electrons and ions from fresh crack of the plume, each of which is governed by surfaces resulting in a residual charge) as the very distinct dynamics, i.e., (1) the gas-thrust main mechanisms of volcanic particle electrifi - region immediately above the vent, (2) the con- cation (Gilbert et al., 1991; James et al., 2008). P-transducer Antennas vection-driven rising column extending several In previous experiments lightning discharges Figure 1. A: Shock-tube apparatus. B: Close- kilometers above the vent, and (3) the neutrally have not been observed, thus demonstrating that up of nozzle, pressure transducers, and an- buoyant umbrella region (Thomas et al., 2010; particle charging per se is a necessary but insuf- tennas. Nozzle diameter is 2.8 cm, distance Bennett et al., 2010; Behnke et al., 2013). At fi cient condition for lightning generation. Some between antennas is 1 cm. Autoclave is fi lled least two main regimes of electrical discharges important questions remain concerning volcanic with loose particles and equipped with pressure transducers. Gas-particle mixture is de- have been described derived from lightning lightning. How are lightning discharges gener- compressed through diaphragm and ejected mapping array observations (Thomas et al., ated in the near-vent region? What is the domi- into collection tank (atmospheric pressure,

2007, 2010; Behnke et al., 2013): (1) the vent nating mechanism for particle charging and Pa ~ 0.1 MPa, atmospheric temperature, Ta ~ discharges (sparks) and near-vent lightning, as- electrical discharge at the inception of an ex- 24 °C, relative humidity ~60%). All sensors are synchronized with high-speed camera sociated with the fragmentation of magma and plosive eruption? Does this mechanism depend recording at as much as 50,000 frames/s. collision of particles occurring during the explo- on particle size distribution? Finally, if charging sion, and (2) the plume lightning, dominated by mechanism and charge distribution are key pa- gravitational separation of the ejecta, occurring rameters for lightning generation, to what extent through a nozzle of 2.8 cm diameter (D) into a in the convective plume (Thomas et al., 2010; is the charging mechanism and charge distribu- large tank fi lled with air at atmospheric condi- Behnke et al., 2013). Field studies of electric tion model proposed for thunderclouds (Rakov tions. Because of their impulsive character, our fi eld variations induced by volcanic plumes and Uman, 2003) valid for volcanic plumes? experiments most closely represent the condi- have focused mainly on the convective and um- tions encountered in the gas-thrust region of the brella regions (Anderson et al., 1965; Lane and METHOD plume, when pyroclasts are fi rst ejected from Gilbert, 1992; James et al., 1998; McNutt and We generated lightning in rapid decompres- the crater. We used sieved natural ash with dif- Davis, 2000; Miura et al., 2002). Current mod- sion experiments in a shock tube (Alatorre- ferent grain sizes (Table DR1 in the GSA Data els of electrical charging within the convective Ibargüengoitia et al., 2011) (Fig. 1A). Upon Repository1) from Popocatépetl (Mexico), column propose that volcanic plumes may be- decompression (from ~10 MPa argon pressure Eyjafjallajökull (Iceland), and Soufrière Hills

have as “dirty thunderstorms,” thus being able to atmospheric pressure Pa = 0.1 MPa), loose (Montserrat) volcanoes, as well as micromet- to produce lightning discharges as commonly particles are vertically accelerated and ejected ric glass beads to constrain the inﬂ uence of

*Current address: Centro de Investigación en Gestión de Riesgos y Cambio Climático, Universidad de Ciencias y Artes de Chiapas, Tuxtla Gutiérrez, Chiapas 29039, Mexico. 1GSA Data Repository item 2014018, Table DR1 (experimental conditions), Figure DR1 (electronic schematics of the antennas), Figures DR2–DR5 (video frames of lightning ﬂ ashes), and Videos DR1 and DR2 (high-speed videos of two experiments with different camera exposure time), is available online at www.geosociety.org /pubs/ft2014.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

GEOLOGY, January 2014; v. 42; no. 1; p. 79–82; Data Repository item 2014018 | doi:10.1130/G34802.1 | Published online 6 December 2013 GEOLOGY© 2013 Geological | January Society 2014 of America.| www.gsapubs.org Gold Open Access: This paper is published under the terms of the CC-BY license. 79

Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/42/1/79/3545975/79.pdf by guest on 30 September 2021 material properties on lightning. We monitored with the overpressure at the nozzle and the therefore will infl uence the fl uid dynamic prop- the dynamics of the particle-laden jets with a presence of the turbulent shell (Fig. 2A). Fre- erties of the particles, especially with regard to high-speed camera and the pressure and electric quent smaller potential changes characterize the the coupling to the gas phase, and thereby con- potential at the nozzle using a pressure trans- beginning of this phase, which evolves to less trol the mean distribution of particles in the jet ducer and two copper ring antennas connected frequent, higher voltage discharges. During the near-fi eld. These factors are likely to infl uence to a high-impedance data acquisition system, third phase, particles are ejected for a further the charging mechanism of particles in the ex- respectively (Fig. 1B; Fig. DR1). 200–250 ms: the pressure at the nozzle rapidly periments and in nature.

decreases to Pa, the electric potential recovers EXPERIMENTAL RESULTS the initial value, no turbulent shell is observed, Effect of Grain-Size Distribution on and no further discharges are recorded. Discharge Number and Flow Structure High Speed Imaging and Electric In one experiment, the antennas can record The spherical glass beads, used in this study Measurements hundreds of electrical discharges (amplitude as a standard material, are highly homoge- We distinguish three temporal phases within >0.2 V, duration 0.6 μs) associated with ﬂ ashes neous in density, chemistry, and shape, inde- each experiment (Fig. 2): (1) the escape of the to 5 cm in length (Figs. DR2–DR5). High-speed pendent of size. The response of a particle in a

argon originally above the loaded particles; videos (Videos DR1 and DR2 in the Data Re- fl ow is characterized by its Stokes number Sk = τ τ τ (2) the ejection of the front of the gas-particle pository) show that most of the fl ashes originate p/ f, where p is the time required for a particle mixture and the generation of a more turbulent and propagate within a region defi ned by the to obtain a velocity of 63% of the fl uid veloc- τ conical region surrounding the main jet (turbu- turbulent shell and a 2 D vertical distance above ity, and f is a characteristic fl ow time scale. It

lent shell); and (3) the ejection of the remain- the nozzle. This distance is consistent with the has been shown that particles with Sk >> 1 are ing particles in a well-collimated jet. In the fi rst theoretical Mach disk height for a pressure of ~6 unresponsive to fl uctuations within the fl ow

phase (Fig. 2B), the shock wave and the argon Pa at the vent (Ogden et al., 2008). As is the case and that preferential clustering of particles fi rst ≈ escape produce a sharp pressure increase, the for thundercloud lightning, we also observe in occurs when Sk 1 in regions of relatively high gas condensation, and a negative transient in the our experiments downward- and upward-propa- strain and low velocity (Longmire and Eaton, τ electric potential relative to Earth. Experimental gating fl ashes associated with both positive and 1992). As a fi rst-order approximation, p can runs without particles only exhibit this nega- negative discharges, as recorded by shape and be expressed in terms of the particle diameter tive electrical transient associated with the gas polarity of spikes in the potential signal. using the Ergun (1952) equation. In our experi- τ escape, and no electrical discharge is observed. We observe that more discharges are gener- ments, f ~ 0.6 ms, and therefore Sk = 1 corre- Due to partial decoupling of gas and particles, a ated for fi ner starting material (Table DR1) and sponds to particles between 60 μm for volca- gas fraction escapes ahead of the mixture front, that there is no correlation between number of nic ash and 100 μm for glass beads. For time generating an additional pressure peak and a discharges and ash chemistry. We also observe scales of a few seconds, expected in volcanic

second negative electric transient. In the sec- that ﬁ ner ashes produce higher number of dis- eruptions, Sk = 1 corresponds to clasts between ond phase (Fig. 2C), corresponding to the ﬁ rst charges and that nonwashed samples generate 500 and 1000 μm, i.e., in the size range of vol- arrival of particles, the pressure at the nozzle more discharges than their washed counter- canic ash. Notably, almost no discharges are increases. The overpressure at the nozzle leads parts. This is likely due to the presence of very produced during experiments with monodis-

to an unconﬁ ned expansion of the gas-particle ﬁ ne ash shards on the surface of nonwashed perse coarse beads (500 µm, Sk >> 1). With

mixture and generates a turbulent shell around coarser particles. Nevertheless, the compo- increasing weight percent of fi nes (50 µm, Sk the core of the fl ow (Ogden et al., 2008). Ob- nentry (glass ± crystals ± lithics), density, and ≈ 1), the number of discharges increases pro- served fl ashes and related electrical discharges shape of ash particles from a single eruptive portionally (Fig. 3A). We also observe that the are generated exclusively during this second event can vary widely with grain size. Shape structure of the jet changes accordingly with phase (lasting 6–9 ms) and clearly correlate and density changes affect the drag force and the addition of fi nes.

Gas Gas + Particles Figure 2. Decompression Lightning D E experiment with 250 µm A 2 cm Popocatépetl ash. A: Elec- tric potential recorded by antennas. B: Pressure at nozzle. C: Angle of core ﬂ ow (β) and turbulent shell α Upper antenna ( ) to vertical. Shaded area

Electric potential (V) Lower antenna shows time of ﬂ ash occur- B rence. D–F: Consecutive phases of experiment. D: Condensing argon before t = -0.64 ms t = 3.16 ms particle ejection (t = time). E: Turbulent shell sur- F G rounds particle-laden jet

Vent pressure (MPa) Vent and ﬂ ashes are recorded. F: Turbulent shell is no longer C turbulent shell α visible, discharges stop, main flow β β gas-particle mix is further ejected in collimated ﬂ ow. G: Schematic section of jet,

Angle (°) showing ﬂ ow core (coarse α particles, dark gray), turbulent shell (ﬁ ne particles, light gray), and respective time (ms) t = 6.96 ms opening angles β and α.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/42/1/79/3545975/79.pdf by guest on 30 September 2021 Figure 3. Discharge gen- are observed tens of seconds after (Aizawa et eration and dependence A 1200 on grain size. A: Num- al., 2010), when the plume is hundreds of me- 800 ber of discharges >0.2 ters high and expanding by convective intake V recorded by lower 400 of air (e.g., 8 February 2010 eruption; Smith-

antenna in experiments # discharges lower antenna 0 sonian Institution National Museum of Natural with bimodal glass beads 0 20 40 60 80 100 History Global Volcanism Program, 2010). We (500 and 50 μm) as func- wt% fine particles tion of weight percent of observed these two modes of lightning occur- fi nes. Stokes number (S ) k BC D rence during recent vigorous eruptive episodes differs by ~2 orders of (14–24 July 2013) at Sakurajima. It should be magnitude between two particle types. B: Mono- noted that, even during the 19 July episode, disperse coarse beads where the column reached a maximum height form collimated fl ow and of 6100 m above sea level (a.s.l.) (according to no lightning occurs. C: In the Tokyo Volcanic Ash Advisory Center), the bimodal blends, coarse ash never reached the isotherm –20 °C, which beads (S >> 1) are at k during those days was measured well above core of fl ow and fi nes (Sk ≈ 1) form turbulent shell. 8000 m a.s.l. (Japan Meteorological Agency, D: Monodisperse fi ne http://www.jma.go.jp/jma/indexe.html), thus --+ -- --++ +-- EF+ + + + beads move according - -- + - - ++ - excluding the presence of ice in the column. - - + + ------+ - - - to local fl ow turbulence. - - - + + - -- - - + - + + - - + -- + + E: For bimodal blends, - - - +- - - +++ + - - - - - + ------+ - + + coarser particles tend - -- + + - - - + + - + - -- - SUMMARY - - + + ------to have relative positive - - - -- + + ------++ ------Ash-rich volcanic plumes, e.g., 2010 Ey------+ + - - - ++ - - charge with respect to - - + - - - + + - - - - - + + ------jafjallajökull (Iceland) eruption (Bennett et Earth, whereas smaller - + + - + - - + + - - - +- al., 2010; Taddeucci et al., 2011) and the ash- particles tend to charge - - + - - - - - +++ - - - - + + - - + - negatively. Their differ- - + + - - + + rich Vulcanian explosions at Stromboli (Italy) ent responses to fl uid B (Calvari et al., 2012) and Sakurajima (Aizawa dynamics provide mech- et al., 2010), often produce lightning. Our ex- anism for charging and charge separation according to grain size. F: For monodisperse periments are consistent with this observation fi ne particles, transient clustering with different relative charge density provides necessary gradient for discharges (in E and F, positive and negative symbols represent relative charge and further reveal a direct relation between the density, not necessarily different polarity). number of electrical discharges and the abundance of ejected fi ne particles. We propose that clustering of particles provides an effi cient MECHANISM OF LIGHTNING by the expanding gas to the periphery to form mechanism for both charge generation and GENERATION the turbulent shell (Fig. 3C), providing an ef- lightning discharge within volcanic plumes. High-speed videos show that for monodis- fi cient mechanism for charge separation and Clustering can be particularly effective in the

perse 500 µm beads (Sk >> 1), particle motion electrical discharge (Fig. 3E). presence of prevalently fi ne ash–laden jets ex- is dominated by inertia so that the fl ow is well We thus propose that the formation of tran- iting the volcanic conduit. Further charging by collimated above the nozzle and particle- sient clusters is crucial for electrical discharges magma fragmentation, convection, and buoy- particle interaction (hence charging) is neg- in monodisperse fi ne particle jets (Fig. 3F). ancy of particles in the upper regions of the ligible (Fig. 3B). On the contrary, for mono- Clusters form and break up by densifi cation plume, along with the formation of hydrome- ≈ disperse 50 µm beads (Sk 1), particles are and rarefaction of the particle-laden jet. A teors, may provide additional mechanisms of small enough to be coupled with the gas and cluster’s lifetime is regulated by the turbulence plume electrifi cation. are affected by local turbulence. In turbulent time scale and its modifi cation during the evo- Our experiments open a new perspective in regions, particles form clusters (Ogden et al., lution of the fl ow (Burton and Eaton, 2005). In the investigation of volcanic lightning genera- 2008), thus promoting collision and triboelec- addition to the radial acceleration of particles tion with emphasis on the plume’s gas-thrust trical charging (Fig. 3D). Glass spheres did not by the expanding gas, cluster generation and region, where electrical discharges are fi rst ob- macroscopically fragment during particle-par- disruption provide the necessary conditions served. We anticipate that high-speed camera ticle collisions, but since fractoemission acts for particle electrifi cation by collision, lo- observations synchronized with magnetotellu- at the molecular scale, we cannot exclude that cal concentration of charges, and consequent ric, Doppler radar, and lightning mapping array microscale spalling might contribute to particle separation, thus creating the electric potential measurements will ensure further advances in charging. For bimodal blends the fl ow structure gradient necessary to generate electrostatic our understanding of electrifi cation processes is transitional between the two monodisperse discharges. Our experiments show that the at active volcanoes. We believe that such im- end members. Relative motion of particles ac- frequency and amplitude of the discharges are proved lightning monitoring has the potential to

cording to their Sk enhances charging by colli- inversely related (Fig. 2A), meaning that the provide fi rst-hand information not only on the sion. Self-charging of glass beads in fl uidized potential between clusters increases with time location of the eruption and the structure of the beds is well documented (Lowell and Truscott, according to the changing length scale of the plume but, more important, on the presence and 1986; Pähtz et al., 2010), and tribological stud- fl ow (with expansion collisions become less amount of fi ne ash ejected, a fundamental input ies with bimodal populations demonstrate the frequent and clusters are progressively more in ash-dispersion forecast models. Furthermore, tendency of relatively smaller beads to charge distant from each other). our experiments are signifi cant for the investi- negatively while relatively larger ones charge In a very similar fashion, during impulsive gation of self-charging mechanism of particles positively (Lacks and Levandovsky, 2007). In explosion at Sakurajima volcano in Japan, fre- that are relevant for atmospheric phenomena on bimodal blends inertia forces the coarse parti- quent and shorter discharges are observed near Earth (such as dust storms and mesocyclones) cles in a well-collimated fl ow at the core of the the crater concomitant to the explosion, while and other planetary bodies, and industrial pro- jet while fi ne particles are radially accelerated longer and more luminous lightning discharges cesses involving granular materials.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/42/1/79/3545975/79.pdf by guest on 30 September 2021 ACKNOWLEDGMENTS terization of the event and quantifi cation of Miura, T., Koyaguchi, T., and Tanaka, Y., 2002, This work has been supported by the AXA Re- the ejecta: Journal of Geophysical Research, Measurements of electric charge distribution search Fund via the grant “Risk from Volcanic Ash v. 117, B05201, doi:10.1029/2011JB009048. in volcanic plumes at Sakurajima Volcano, Ja- in the Earth System”. We appreciate the constructive Ergun, S., 1952, Fluid fl ow through packed col- pan: Bulletin of Volcanology, v. 64, p. 75–93, reviews from three anonymous reviewers. umns: Chemical Engineering Progress, v. 48, doi:10.1007/s00445-001-0182-1. p. 89–94. 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ERRATUM

Stress-controlled ﬂ uid ﬂ ow in fractures at the site of a potential nuclear waste repository, Finland Jussi Mattila and Eveliina Tammisto (Geology, v. 40, p. 299–302, doi:10.1130/G32832.1 Equation (3) for shear traction was published in incorrect form. The right-hand side of the equation should have been a square-root of the given version. The correct form is presented here: τσσ=−⎡ 22 2 +− σσ222 +− σσ22 2⎤12 ⎣()12lm ()23 mn ()31 ln⎦

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