The longevity of electrostatic charge on airborne pyroclasts

Abstract:

Volcanic columns are often electrically charged as evidenced by the dramatic electrical storms that accompany eruptions. Ash can charge through a number of electrification mechanisms (tribofractocharging, radioactive decay, or water based charging). Additionally, there is mounting evidence that electrostatic processes in plumes can be leveraged to monitor eruptions remotely. However, we need a better understanding about how charge is coupled to the dynamics of the ash flow, the presence of volatiles, and environmental conditions in the column. The amount of charge on any ash particle at any given time reflects the balance between the rate of charge acquisition and the rate at which charge is lost from particle surfaces. While research over the last 20 years has revealed much about how particles gain charge (i.e. electrification mechanisms), our understanding about how particles lose charge is extremely limited. This gap in knowledge cripples our capacity to develop complete electrostatic models of volcanic columns and effective remote sensing tools. Here, we present a set of experiments designed to fill these gaps. Using acoustic levitation, we suspend natural ash grains in air on time scales of days to weeks and measure how charge decreases on the particle over time. We find that charge is lost through at least two mechanisms and is extremely sensitive to water at relative humidities higher than 50%. At low relative humidities, particles may remain charged for more than a month, while at higher relative humidities charge may be lost in a matter of minutes. Overall, our experiments suggest that water in volcanic columns (either magmatic or entrained) may effectively shut off "dry" electrification mechanisms (e.g. tribofractoelectrification) as the rate of charge loss becomes much greater than that of charge accumulation. These results lend support to the idea that water-based electrification mechanisms (such as the interaction of ice and graupel), rather than tribo- or fragmentation charging, dominate electrostatic processes in maturing volcanic columns.