Modeling Volcanic Shocktube Lightning
Abstract
Lightning discharges occur in ash plumes of volcanic eruptions. Identifying their radiofrequency emission may aid in forecasting volcanic ash along aviation routes and characterizing planetary volcanic activity. We propose a model for volcanic lightning in which 1) dust particles accelerate in shocks, 2) charge triboelectrically, 3) separate by their inertia, and 4) spark through streamer breakdown. In reality these distinct processes coincide partially or wholly. To validate the model, we implement a simulation in the block structured adaptive mesh refinement code AMRex of a volcanic shocktube experiment [1]. The experiment generates electrical discharges in a particle-laden gas jet formed during the decompression of a shocktube filled with argon and a well-controlled bimodal distribution of coarse and fine "dust". The fine dust advects with the gas and is implemented as a modified gas law considering the reduced volume and mass fraction. Coarse particles detach from the gas flow and are modeled as a separate fluid. A triboelectric charging model [2] tracks the transfer of high energy state electrons between particles during collisions. Breakdown events are identified by comparing integrals of the Townsend ionization coefficient along electric field lines to the Raether-Meek breakdown condition. Explosively-driven dust, such as is encountered in the volcano vent, presents a qualitatively different electrostatic environment than is present in steady flows. The high-gradient conditions inherent in the vicinity of the shock alter the processes that generate a spark. For example, charge magnitude and polarity are sensitive to the shocked drift velocity of small and large particle species. In addition, charge clustering in turbulent eddies creates rapid variations in the Townsend ionization coefficient leading to intermittent breakdown.
[1] Cimarelli, C. et al. (2014). Experimental generation of volcanic lightning. Geology, 42(1), 79-82 [2] Kok, J. et al. (2009). Electrification of granular systems of identical insulators. Physical Review E 79, 051304 LLNL-ABS-755319 is funded by the NNSA Office of Defense Nuclear Nonproliferation R&D and performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V43B..06V
- Keywords:
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- 3304 Atmospheric electricity;
- ATMOSPHERIC PROCESSESDE: 8419 Volcano monitoring;
- VOLCANOLOGYDE: 8428 Explosive volcanism;
- VOLCANOLOGYDE: 8488 Volcanic hazards and risks;
- VOLCANOLOGY