The Effects of Ice on the Frictional Electrification of Plumes

Abstract:

Studies of the Redoubt (2009), Eyjafjallajökull (2010), and the Cordón Caulle (2011) eruptions and associated electrical activity suggest that ice is a catalyst for the generation of plume lightning [Behnke et al., 2012; Arason et al., 2011; Nicora et al., 2013]. Indeed, the number lightning flashes in the umbrella and convective regions of the plume seems to correlate with lower, ice-forming temperatures. As in conventional thunderstorms, electric charge accumulation and separation in cold plumes may arise from the interplay between ice grains and graupel. However, charging may also be driven by triboelectrification (frictional charging), resulting from collisions between ice and ash grains. Decades of research have shown that fluidized granular materials comprising species of distinct composition often produce more efficient electrification than the interaction of chemically identical particles under similar hydrodynamic regime [e.g. Forward et al., 2009]. Thus, tribocharging in a combined ice and silicate granular flow is likely to yield grains with higher charges than those encountered in a flow composed solely of silicate grains, facilitating the generation of lightning [Méndez Harper and Dufek, 2015, submitted]. To indagate this hypothesis, we have developed a novel methodology to measure the time-dependent charging of individual micron-sized particles in both dry, silicate flows and silicate plus ice flows. Pure ash runs are conducted in a dry (<1 % humidity) environment with a temperature variation of -40^o to 25^o C. Runs involving ice are performed between -40^o and 2^o C, permitting us to quantify charging in the mixture as water transmogrifies from solid ice to a more mush-like substance. Additionally, we report on how collisional energy and rates influence electrification in these granular materials. We show that, although the variations in ash composition (basalt to rhyolite) tend to be of little consequence for charging, the presence of a water phase can importantly tune the electrostatic behavior of the flow.