First field evidence of the electrical multipolar nature of volcanic aggregates.

The description of the formation of ash aggregates is crucial to our understanding of tephra dispersal and, therefore, to the forecasting of volcanic ash concentration in the atmosphere. Regardless of the importance of electrostatic processes, particle and aggregate charge is of complex determination both in the field and in the laboratory. We have measured the bulk charge carried by single aggregates at Sakurajima volcano (Japan) by inverting the sedimentation trajectory across two high-voltage copper plates filmed using a high-speed camera. Some aggregates were filmed during their breakage caused by the external electric field, which revealed the presence of inner highly charged multipoles with bulk charge up to one order of magnitude larger than the bulk aggregate. This electrical multipolar nature reveals the key role played by multipoles in aggregate formation and the shielding effect of aggregation on particle charge.