Electrical activity during the 2015 eruption of Sakurajima and the structure of an underexpanded jet. (a) Data collected with a Lightning Mapping Array during a 15 June eruption at Sakurajima (Behnke et al., 2018) showing the spectral differences between vent and plume lightning. While plume lightning manifests itself as discrete, impulsive signals (red, vertical lines), vent activity appears as a “continual” signal rising from the noise floor. As can be seen, continual radio frequency correlates temporally with the duration of an explosive event (gray shaded area). Figure modified from Behnke et al., 2018. (b) Schematic structure of an underexpanded jet, showing the region of rarefaction between the vent and the normal shock, contained within a barrel shock.

Volcanic eruptions often display a range of electrostatic processes, including anomalous electric fields and spectacular lightning displays (Aizawa et al., 2016; Behnke et al., 2013; Behnke & Bruning, 2015; Cimarelli et al., 2016; Gilbert et al., 1991; Lane & Gilbert, 1992; Miura et al., 2002; Woodhouse & Behnke, 2014). Although the underlying microphysics of ash electrification are not fully understood, recent field campaigns have revealed that electrical phenomena are intimately related to eruptive hydrodynamics (Behnke & Bruning, 2015; Cimarelli et al., 2016, 2016; Van Eaton et al., 2016). Understanding these connections remains a prominent goal in volcanology because electrical activity emits radiation that can be monitored remotely. Thus, if well characterized, these signals may serve to probe the internal, obscured dynamics of eruptive events in manners that have not been achieved previously. For instance, observations during the Augustine (2006) and Redoubt (2009) eruptions suggest that electrostatic processes coevolve with changes in flow behavior (Behnke & Bruning, 2015; Behnke et al., 2018, 2013; Smith et al., 2018; Thomas et al., 2007). In particular, both Alaskan volcanoes produced two distinct electrical behaviors during their eruptions:

(1) initial, quasi-continual radio frequency (RF) outbursts associated with individual explosions followed by

(2) more intermittent, although more spatially extensive, spark discharges in maturing plumes. The latter modality, comprising near-vent and plume lightning, typically generates brilliant flashes several hundreds of meters to kilometers in length and emits impulsive RF signals (Figure 1a; Thomas et al., 2007). These spark discharges occur anywhere between a few hundreds of meters above the vent to kilometers away in the distal plume. Because of their conspicuous nature and similarity to thundercloud lightning, both near-vent and plume lightning have been the focus of the most recent studies of electrostatic phenomena at volcanoes (Aizawa et al., 2016; Cimarelli et al., 2016; Nicora et al., 2013; Van Eaton et al., 2016).

Contrastingly, the first discharge modality, referred to as vent discharges (sometimes, vent lightning), is more obscure. Revealing themselves en masse as a continual RF (CRF) or persistent RF “hum” at a distance, vent discharges have been inferred to be small, visually inappreciable sparks occurring between individual ash particles or small clusters of particles in the flow (Behnke et al., 2013, 2018; Thomas et al., 2007). Such discharges occur at or just above the volcanic vent and occur in tempo with discrete explosions (Figure 1a). Interestingly, the RF energy emitted by vent discharges has been shown to increase with the magnitude of overpressure at the vent for a given explosion (Behnke et al., 2013; Behnke & Bruning, 2015).

The small length scales (likely, centimeters to a few meters) and high rates (up to several dozen events per millisecond) associated with vent discharges hint that the volumetric charge density in proximal volcanic jets may be as much as an order of magnitude larger than that found in thunderclouds or distal plume regions (Aizawa et al., 2016; McNutt & Williams, 2010). This is an unsurprising observation given that explosions likely generate large amounts of charge per unit surface area through elevated rates of fractoelectric and triboelectric charging (James et al., 2000; Méndez Harper et al., 2015; Méndez Harper & Dufek, 2016). Indeed, proximal discharges could reflect conventional or dielectric breakdown processes (Aizawa et al., 2016), rather than the more complex processes thought to operate in thunderclouds (Dwyer, 2005; Dwyer & Uman, 2014; Gurevich et al., 1992). Furthermore, unlike near-vent and plume discharges, vent discharges do not seem to require large-scale charge separation and do not importantly modify ambient electric fields as measured from the ground (Behnke et al., 2018). Together, these characteristics—the timing of discharges, their small length scales, and their confinement to regions directly adjacent to the vent—may suggest that this electrical activity directly reflects explosive processes at the volcanic source. However, a physical mechanism linking explosive processes to electrostatic phenomena has not been described until now. Here we show that vent discharges are driven by supersonic expansion processes associated with powerful explosions. Indeed, our works demonstrate that the compressible fluid dynamics describing overpressured jets has the ability to finely tune the breakdown strength of a gas carrying charged particles, placing strict temporal and spatial limits on the occurrence of proximal discharges.