  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.
