Observations of Volcanic Lightning During the 2009 Eruption of Redoubt Volcano

Observations of volcanic lightning during the eruption of Redoubt Volcano in March and April 2009 were made with the Lightning Mapping Array. During the eruption twenty-three distinct episodes of volcanic lightning were observed. Electrical activity occurred as either a volcanic lightning storm with up to thousands of lightning discharges or as a weak electrical event with only a handful of lightning discharges. During the volcanic lightning storms we observed two phases of electrical activity: the explosive phase and the plume phase. The explosive phase consisted of very small discharges (on the order of 10–100 m) occurring directly above the vent while an explosive eruption was ongoing, whereas the plume phase was comprised of discharges occurring throughout the plume subsequent to the explosive eruption. The area of discharges during the explosive phase ranged from less than 1 km2 to 50 km2 or more. The electrical activity at the beginning of the plume phase was dominated by small discharges. Over time the horizontal extent of the flashes increased, with the largest flashes occurring at the end of the plume phase. The distribution of the horizontal size of the discharges over the lifetime of the storm indicate that the charge structure of the plume evolved from a complex and ‘clumpy’ structure to a more simple horizontally stratified structure. Plume height was shown to be a key factor in the quantity of lightning in a storm. The volcanic lightning storms occurred in plumes with column heights greater than 10 km. The tall plumes may contribute to the efficiency of charge generation through ice collisions by providing strong updrafts from the large thermal energy input from the eruption