Volcanic Lightning: in nature and in the lab.

Abstract:

Ash-rich volcanic plumes are often associated with intense electrical activity and the generation of volcanic lightning. Monitoring electrical discharges associated with explosive eruptions can provide crucial information on the dynamics and structure of the plume as well as on the mass eruption rate and cargo of erupted fine ash. Nevertheless, our understanding of volcanic lightning is still limited due to lacking of i) systematic instrumental observations and ii) the limited number of experimental investigations on the electrical properties of volcanic materials and the opportunity of replicating volcanic plume conditions in the lab.
We recently contributed to the understanding of both these aspects by performing multi-parametric observation of volcanic lightning at Sakurajima volcano in Japan and by achieving volcanic lightning in particle-laden jets generated in the lab.
At Sakurajima volcano we combined high-speed imaging with magnetotelluric and acoustic measurements of ash-rich plumes and compared our observation with maximum plume height measurement and atmospheric soundings. Our observations at Sakurajima allow the measurement of flash properties with respect to the plume evolution as well as magnetic and electric field variation and associated transferred current. In addition, weather-balloon soundings rule out the contribution of hydrometeors in the electrification of the plume.
We complement the field observation by performing rapid decompression experiments of well-constrained (composition and granulometry) ash samples and analogue materials. The experiments show many similarities with the vulcanian explosions at Sakurajima and, most importantly, they highlight how lightning is controlled by the dynamics of the rapidly expanding particle-laden jet. Two main conditions are required to generate lightning: 1) triboelectrification of the particles and 2) clustering of the particles driven by the jet fluid dynamics. As observed in nature, the size of the flashes scales with the dimensions of the evolving jet and the presence of fine ash in the jet is key to generate electrical discharges.
We further invite discussions on cross-correlation of relevant monitoring and lab techniques and possible future developments of multi-parametric arrays relevant for volcanic lightning.