Volcanic lightning reproduced in laboratory experiments

Ars Technica : The lightning produced by volcanic ash clouds is difficult to study at lower points in the plume. Now a team of researchers led by Corrado Cimarelli of Ludwig Maximilian University in Germany has created a device that re-creates the event in a lab. The apparatus used pressurized argon gas to send mixtures of volcanic ash and glass beads into a tank of air kept at surface temperature and pressure. They measured the electric potential and pressure at the point where the ash entered the tank and recorded the process with a high-speed camera. In the experiment, the researchers varied the size, mix, and chemical properties of the particulates they used. They found that lightning is triggered by the presence of particles, regardless of chemical type, that are small enough to separate from the main column of ash and create turbulent areas. The complex motion within the turbulent area causes the buildup of charge through particle collisions and also creates clusters of particles. If the size of the ash particles in a given plume can be determined, that information could be used to better predict the disruptive effects of eruptions.