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.

火山喷发与广泛的静电效应有关。越来越多的证据表明，成熟羽流中的高空放电（闪电）是由需要形成冰的电气化过程驱动的（类似于支撑气象雷暴的过程）。然而，在火山喷口处或附近放电也很常见。已经提出了许多“无冰”电气化机制来解释这种活动：压裂充电、摩擦充电、放射性充电和通过感应充电。然而，每种机制对喷气机总带电的贡献程度以及气体推力区域的带电如何影响高空静电过程仍然缺乏约束。这里，我们使用冲击管来模拟超压火山喷流，能够在没有冰的情况下产生火花放电。这些排放可能代表在多次喷发中观察到的连续射频 (CRF) 排放。使用一套静电传感器，我们首次证明了在承载放电的流动中存在与尺寸相关的双极充电 (SDBC)。SDBC 在其他情况下很容易与摩擦带电联系起来，并提供直接证据表明接触和摩擦带电在喷发的喷发区和近喷发区的静电过程中起着重要作用。此外，我们发现离开发生放电区域的粒子保持适度带电。这种程度的电气化可能足以驱动柱中更高的近排气孔闪电。因此，尽管涉及更大程度的电荷分离，但近排放口放电可能受到驱动 CRF 的相同电气化机制的支持。