Experimental generation of volcanic lightning
C. Cimarelli, M.A. Alatorre-Ibargüengoitia, U. Kueppers, B. Scheu & D.B. Dingwell, 2013
A: Shock-tube apparatus. B: Closeup of nozzle, pressure transducers, and antennas. Nozzle diameter is 2.8 cm, distance between antennas is 1 cm. Autoclave is filled with loose particles and equipped with pressure transducers. Gas-particle mixture is decompressed through diaphragm and ejected into collection tank (atmospheric pressure, Pa ~ 0.1 MPa, atmospheric temperature, Ta ~ 24 °C, relative humidity ~60%). All sensors are synchronized with high-speed camera recording at as much as 50,000 frames/s.
Explosive volcanic eruptions are commonly associated with intense electrical activity and lightning. Direct measurement of the electric potential at the vent, where the electric activity in the volcanic plume is fi rst observed, is severely impeded, limiting progress in its investigation. We have achieved volcanic lightning in the laboratory during rapid decompression experiments of gas-particle mixtures under controlled conditions, and recorded it using a high-speed camera and two antennas. We fi nd that lightning is controlled by the dynamics of the particle-laden jet and by the abundance of fi ne particles. The relative movement of clusters of charged particles generates the electrical potential, which is necessary for lightning. The experimental generation of volcanic lightning suggests that rapid progress can now be expected in understanding electrical phenomena in volcanic plumes to implement lightning monitoring systems and the forecasting of volcanic ash emissions.
Decompression experiment with 250 μm Popocatépetl ash. A: Electric potential recorded by antennas. B: Pressure at nozzle. C: Angle of core fl ow (β) and turbulent shell (α) to vertical. Shaded area shows time of fl ash occurrence. D–F: Consecutive phases of experiment. D: Condensing argon before particle ejection (t = time). E: Turbulent shell surrounds particle-laden jet and fl ashes are recorded. F: Turbulent shell is no longer visible, discharges stop, gas-particle mix is further ejected in collimated flow. G: Schematic section of jet, showing fl ow core (coarse particles, dark gray), turbulent shell (fi ne particles, light gray), and respective opening angles β and α.
Lightning discharges are often observed during explosive volcanic eruptions and are commonly associated with the formation of ash plumes (Mather and Harrison, 2006; James et al., 2008; McNutt and Williams, 2010; Rakov and Uman, 2003). Their occurrence appears to be independent of magma composition, eruption type, and plume height (McNutt and Williams, 2010). Increasingly sophisticated lightning mapping arrays show that lightning discharges are ubiquitously produced within three regions of the plume, each of which is governed by very distinct dynamics, i.e., (1) the gas-thrust region immediately above the vent, (2) the convection-driven rising column extending several kilometers above the vent, and (3) the neutrally buoyant umbrella region (Thomas et al., 2010; Bennett et al., 2010; Behnke et al., 2013). At least two main regimes of electrical discharges have been described derived from lightning mapping array observations (Thomas et al., 2007, 2010; Behnke et al., 2013): (1) the vent discharges (sparks) and near-vent lightning, associated with the fragmentation of magma and collision of particles occurring during the explosion, and (2) the plume lightning, dominated by gravitational separation of the ejecta, occurring in the convective plume (Thomas et al., 2010; Behnke et al., 2013). Field studies of electric fi eld variations induced by volcanic plumes have focused mainly on the convective and umbrella regions (Anderson et al., 1965; Lane and Gilbert, 1992; James et al., 1998; McNutt and Davis, 2000; Miura et al., 2002). Current models of electrical charging within the convective column propose that volcanic plumes may behave as “dirty thunderstorms,” thus being able to produce lightning discharges as commonly observed in thunderstorms (Williams and Mc- Nutt, 2004; Thomas et al., 2007). As such, the presence of hydrometeors within the plume has been assigned a decisive role in the generation of volcanic lightning (Arason et al., 2011). Measurements of electrically charged volcanic ash in the fi eld (Miura et al., 2002; Gilbert et al., 1991; Calvari et al., 2012) and in laboratory experiments (James et al., 2000; Büttner et al., 2000) invoke triboelectrifi cation (electrifi cation of solids through friction) and fractoemission (emission of electrons and ions from fresh crack surfaces resulting in a residual charge) as the main mechanisms of volcanic particle electrification (Gilbert et al., 1991; James et al., 2008). In previous experiments lightning discharges have not been observed, thus demonstrating that particle charging per se is a necessary but insuffi cient condition for lightning generation. Some important questions remain concerning volcanic lightning. How are lightning discharges generated in the near-vent region? What is the dominating mechanism for particle charging and electrical discharge at the inception of an explosive eruption? Does this mechanism depend on particle size distribution? Finally, if charging mechanism and charge distribution are key parameters for lightning generation, to what extent is the charging mechanism and charge distribution model proposed for thunderclouds (Rakov and Uman, 2003) valid for volcanic plumes?