Investigating the Origin of Continual Radio Frequency Impulses During Explosive Volcanic Eruptions

Abstract

Volcanic lightning studies have revealed that there is a relatively long‐lasting source of very high frequency radiation associated with the onset of explosive volcanic eruptions that is distinct from radiation produced by lightning. This very high frequency signal is referred to as “continual radio frequency (CRF)” due to its long‐lasting nature. The discharge mechanism producing this signal was previously hypothesized to be caused by numerous, small (10–100 m) leader‐forming discharges near the vent of the volcano. To test this hypothesis, a multiparametric data set of electrical and volcanic activity occurring during explosive eruptions of Sakurajima Volcano in Japan was collected from May to June 2015. Our observations show that a single CRF impulse has a duration on the order of 160 ns (giving an upper limit on discharge length of 10 m) and is distinct from near‐vent lightning discharges that were on the order of 30 m in length. CRF impulses did not produce discernible electric field changes and occurred in the absence of a net static electric field. Lightning mapping data and infrared video observations of the eruption column showed that CRF impulses originated from the gas thrust region of the column. These observations indicate that CRF impulses are not produced by small, leader‐forming discharges but rather are more similar to a streamer discharge, likely on the order of a few meters in length.

The Effect of Electrostatic Charge on the Propagation of GPS (L‐band) Signals Through Volcanic Plumes

“…Similarly, at Etna, only the satellite tracks (i.e., the signal paths linking transmitter and receiver) that fell within 3 km of the vent were associated with signal reductions. Additionally, studies at Augustine, Redoubt, and Sakurajima employing LMA sensors and high‐speed cameras indicate that flows exiting the vent may have volumetric charge densities significantly larger than those found in conventional thunderstorms (Aizawa et al, ; Behnke et al, ; Cimarelli et al, ). This combination of high mass loading and elevated charge densities in proximity to the vent likely contributed significantly to the anomalous GPS attenuations observed at Redoubt, Etna, and other volcanoes.…”

Inferring Compressible Fluid Dynamics From Vent Discharges During Volcanic Eruptions

“…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, ; Behnke et al, , ; Smith et al, ; Thomas et al, ). 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.…”

“…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, ) 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, . (b) Schematic structure of an underexpanded jet, showing the region of rarefaction between the vent and the normal shock, contained within a barrel shock.…”

“…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, , ; Thomas et al, ). Such discharges occur at or just above the volcanic vent and occur in tempo with discrete explosions (Figure a).…”