Exploring Properties of Volcanic Lightning using Electric Field Change Measurements and 3D Lightning Mapping Data

Abstract:

Volcanic lightning and its origins remain poorly understood as opposed to well-studied thundercloud lightning. Understanding the similarities and differences between these two types of lightning helps us understand volcanic eruptions and how plumes evolve. This study combines three-dimensional Lightning Mapping Array (LMA) data with electric field change measurements from a slow antenna recorded from several eruptions of Sakurajima volcano in Japan. These data were collected during a field campaign between May and June in 2015. Nine LMA stations surrounding the volcano measured very high frequency (VHF) radiation emitted by the electrical breakdown of air during the propagation of lightning. Three-dimensional maps of lightning discharges are generated using time-of-arrival methods. Combining the 3D LMA and electric field datasets allows calculation of total charge deposited to the ground in return strokes, discharge frequency, polarity, and starting location. Preliminary results find volcanic lightning is similar to thundercloud lightning, however, total discharge amounts, duration, and channel length are one to two orders of magnitude smaller. Most of the lightning occurs within the first tens of seconds of an eruption. In general, the discharges are primarily small cloud-to-ground (CG) flashes while a few appear to be intra-cloud (IC) flashes taking place in the plume above the vent. Combining LMA and slow antenna measurements gives more insight into the mechanisms of volcanic lightning than either method alone.