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Volcanic jet flows in explosive eruptions emit radio frequency signatures,indicative of their fluid dynamic and electrostatic conditions. The emissionsoriginate from sparks supported by an electric field built up by the ejectedcharged volcanic particles. When shock-defined, low-pressure regions confinethe sparks, the signatures may be limited to high-frequency contentcorresponding to the early components of the avalanche-streamer-leaderhierarchy. Here, we image sparks and a standing shock together in a transientsupersonic jet of micro-diamonds entrained in argon. Fluid dynamic and kineticsimulations of the experiment demonstrate that the observed sparks originateupstream of the standing shock. The sparks are initiated in the rarefactionregion, and cut off at the shock, which would limit their radio frequencyemissions to a tell-tale high-frequency regime. We show that sparks transmit animpression of the explosive flow, and open the way for novel instrumentation todiagnose currently inaccessible explosive phenomena.

Standing shock prevents propagation of sparks in supersonic explosive flows