Chapter 6 - Electrostatics and In Situ Sampling of Volcanic Plumes

Abstract

The spectacular lightning displays that can sometimes be seen during volcanic eruptions demonstrate that the emitted plumes readily become electrically charged, but the relationships between volcanic parameters, meteorological conditions, and lightning type and rate are not well understood. The variability and range of physical processes involved, and difficulties of in situ measurements—here defined as those made in and near the plume—are the major reasons for this uncertainty. Lightning in volcanic plumes has now been measured in some detail using established remote sensing systems that can characterize both lightning location and flash rate. It seems likely that the meteorological mechanisms generating lightning in thunderstorms are enhanced by the presence of ash, a mechanism that we refer to as the ash-rich icy electrification system (ARIES). The ash brings its own fractoemission and triboelectric (frictional) charging mechanisms, which are thought to dominate close to the vent where large numbers of weaker lightning strikes are seen. Charging in distal plumes has been detected by enhanced meteorological radiosondes, indicating that self-charging continues during dispersion. Laboratory measurement of ash samples under controlled conditions suggest that the self-charging efficiency of atmospheric plumes is related to the size distribution of the particles and therefore that self-charging can occur in all plumes. Measurements of laboratory electrical charging alone are inadequate to fully characterize plume electrostatics, which has motivated integrated radiosonde packages for in situ plume measurements, such as particle size and location, turbulence, and thermodynamic structure.