Abstract
Volcanic jet flows in explosive eruptions emit radio frequency signatures, indicative of their fluid dynamic and electrostatic conditions. The emissions originate from sparks supported by an electric field built up by the ejected charged volcanic particles. When shock-defined, low-pressure regions confine the sparks, the signatures may be limited to high-frequency content corresponding to the early components of the avalanche-streamer-leader hierarchy. Here, we image sparks and a standing shock together in a transient supersonic jet of micro-diamonds entrained in argon. Fluid dynamic and kinetic simulations of the experiment demonstrate that the observed sparks originate upstream of the standing shock. The sparks are initiated in the rarefaction region, and cut off at the shock, which would limit their radio frequency emissions to a tell-tale high-frequency regime. We show that sparks transmit an impression of the explosive flow, and open the way for novel instrumentation to diagnose currently inaccessible explosive phenomena.
Highlights
Volcanic jet flows in explosive eruptions emit radio frequency signatures, indicative of their fluid dynamic and electrostatic conditions
In contrast to previous shock tube experiments designed to investigate the role of grain size distribution[20], mass eruption rate[20], and water content[21] in the electrification of volcanic jet analogs, here we focus on the time-space relationship of the discharges and the standing shock, with the objective of informing computer simulations
Rapid expansion experiments were performed with starting confining pressures of 6.9 and 8.9 MPa depending on the strength of the diaphragms used (Supplementary Table 1)
Summary
Volcanic jet flows in explosive eruptions emit radio frequency signatures, indicative of their fluid dynamic and electrostatic conditions. In particular the occurrence of electrical discharges at active volcanoes under unrest can be regarded as an indication of the onset of hazardous explosive activity and the production of ash plumes[4,5,6] Both observable discharges and RF emissions can reveal the mechanisms that initiate the discharges[7]. Lower frequencies are absent during most of the duration of the CRF they do occur sporadically, and coincide with prominent visual discharges These observations suggest that supersonic shock flows may alter the breakdown process hierarchy, so that frequent electrical discharges are occurring with only sporadic leader formation[13]. The hot, opaque plume makes it difficult to determine how the discharges are altered
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