Location of Plume Charge Stratification as Indicated by Volcanic Ash Grain Size Distributions

Lightning is a common phenomenon during explosive volcanic eruptions, yet little is known about how ash grain properties may influence lightning activity in real-world settings. Ash falls from explosive eruptions will provide information on the intensity of the event through the grain size distribution (GSD) properties. These properties can also provide information on electrical activity within the dispersing ash plume, even if direct measurements or physical evidence of volcanic lightning is limited or absent. Peaks in volcanic lightning activity will occur at explosion onset, when triboelectrification of colliding tephra particles causes development of charge clusters within the rising ash column. As this column continues to buoyantly ascend to its maximum height in the atmosphere, tephra are progressively deposited. Once the ash plume begins to disperse radially away from the vent, coarser particles continue to sediment, and this includes the larger components of the ash-sized (< 2 mm) population. In this study, we hypothesize that characteristics of the ash fall GSD can provide information on potential electrical characteristics of the dispersing ash plume. We present analyses of 40 individual ash fall samples collected from 16 different volcanoes. The samples used for this study were sourced from eruptions that span a range of explosive intensities, global locations (which determines atmospheric temperature above the vent), and amounts of external water involved, offering an array of potential variables that may influence lightning generation. Using laser diffraction particle size analyses, we have examined single ash fall deposits from several different eruptions where lightning was directly observed, documented, or measured with instruments. In addition to these individual samples, we have also analyzed multiple ash fall deposits from eruptions where volcanic lightning was either directly measured with instruments or was likely to have occurred due to the type of eruption. These multiple samples (three or more) were collected at various distances from the vent. The span of the GSD indicates the variation in ash particle sizes at a single location, and the span value, in addition to the surface area of the sample, is controlled by the volume proportion of very fine particles (< 32 µm). Near the vent, the span of the GSD will be relatively higher amongst a set of ash samples and will decrease with distance until reaching a minimum value. At this distance, or range of distances where span is minimized, coarser ash particles have been deposited and finer grains remain aloft in the dispersing plume. The separation of charged finer particles from oppositely charged coarser particles may electrically stratify the ash plume and generate the potential necessary to induce volcanic lightning. Although lightning will occur throughout the eruption and will be highest immediately above the vent following the initial explosion(s), volcanic lightning may also occur at more distal locations when charge of the dispersing plume becomes stratified due to particle settling. At this distance from the volcanic vent, volcanic lightning (a.k.a., plume lightning) may be more similar to thunderstorm lightning in terms of channel length, potentially producing hazardous cloud-to-ground flashes tens to hundreds of kilometers from the volcano.