  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.
