
Image sequence of sparks and condensation cloud forming a sharp upper boundary, rising, and dropping captured by camera 1. Camera 1, exposing 20 μs frames at a rate of 13, 500 Hz, is triggered by a pressure increase at the nozzle (t = 0), and is viewing the nozzle opening and the region above the nozzle including portions of the inductive antennas (10.3 × 8.3 cm shown here, full frame shown in Supplementary Movie 1). Colored squares mark sequential phases of the decompression experiment: rise of condensation and formation of sharp upper boundary (blue), rapid drop of condensation boundary (orange) and electrical sparks (red), slower drop of condensation boundary (black). Sparks are visible in the 1.19 and 1.41 ms frames. Reflective ruptured diaphragm pieces are visible at 1.26, 1.33, and 1.41, and possibly 1.11 ms.
In nature, electrical discharges are frequently observed in widely diverse environments that, beside the common occurrence in thunderclouds1, include also volcanic plumes2 and other turbulent particle-laden flows such as dust devils 3, on Earth and other planets. The underlying processes are regulated by the mechanism of induction and separation of electrical charges. Upon electrical discharge, radio frequency (RF) emissions can be recorded, thus providing a means to track the progressive evolution in space and time of the discharge source. Analogous to the detection of thunderclouds and storms, RF detection is now also being used to detect, and inform on the hazards associated with ash-laden volcanic plumes and ash-clouds. 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 4–6. In addi- tion, both observable discharges and RF emissions can reveal the mechanisms that initiate the discharges 7. The broad RF spectrum associated with lightning discharges results from cascading processes on a hierarchy of time and spatial scales 8,9. Electric fields accelerate electrons, creating ionization avalanches10. A single, or several merging avalanches can collect enough space charge to form a streamer, and such streamers may merge to form a hot self-sustaining plasma channel: a leader. Avalanches and strea- mers emit very high frequencies (VHF) and leaders emit bright flashes of light together with lower frequencies 1,11,12.
Nature points us to examples where supersonic flows and shocks from explosive events may suppress parts of the hierarchy of the discharge phenomena, such as leaders 13. In particular, explosive volcanic eruptions produce supersonic flows through the sudden release of overpressured gases contained in the erupting magma, resulting in shock waves. Observation of erupting volcanoes in Alaska 14,15, Iceland 11, and Japan 13 have revealed that in the first few seconds following the onset of an explosive eruption, RF signatures distinct from those produced by leader-forming lightning are recorded in the vicinity (within 10’s to 100’s of meters) of volcano vents. This early quasi-continuous RF emission is called continual radio frequency (CRF). CRF consists of discrete VHF RF spikes, occurring at rates of tens of thousands per second. Lower frequencies are absent during most of the duration of the CRF although they do occur sporadically, and coincide with prominent visual discharges. These observa- tions suggest that supersonic shock flows may alter the break- down 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.
Rapid decompression shock tube experiments allow us to explore explosive flows in the laboratory16–18. In such experiments, a shock tube ejects a flow of gas and particles into an expansion chamber. Images of non-illuminated decompression reveal bright sparks that are mostly vertical immediately above the nozzle of the shock tube, but bend horizontally at a certain height19–21. Reference 22 suggests that the barrel shock structure of a high pressure outflow localizes sparks. Here, we report simultaneous imaging of the Mach disk and coinciding spark discharges, and we provide results of fluid dynamic and kinetic simulations describing the shock flows and breakdown processes. The spatial and temporal scales of the sparks convey an impression of the shock tube flow and kinetic simulations indicate that conditions for discharge are most favorable just upstream of the Mach disk.