Volcanic lightning observations from the Redoubt and Eyjafjallajokul eruptions

At the 2009 AMS Annual Meeting in Phoenix we reported on lightning observations during the 2006 eruption of Mount St. Augustine, located in Alaska's Cook Inlet. The observations were obtained using VHF lightning mapping stations that readily sense discharges inside the optically dense volcanic cloud. We have since obtained similar observations of the 2009 eruption of Redoubt Volcano, also on the west side of Cook Inlet, and of the recent 2010 eruption of Eyjafjallajokull in Iceland.

Redoubt produced more than 30 distinct explosive eruptions over a two week period between 23 March and 5 April 2009. The eruptive activity was similar in nature to the Augustine eruption but substantially more energetic. Plume heights ranged in altitude from 4 to 20 km. Sixteen of the largest eruptions produced intense and spectacular lightning both during the explosion itself and in the downwind plumes as they propagated away from the volcano. The lightning activity during these explosions, numbering in the thousands of discharges (versus several hundred for Augustine), was essentially continuous for tens of minutes at a time, and was as strong or stronger than that of large severe thunderstorms.

By contrast, the Eyjafjallajokull eruption, which began explosively on 14 April 2010 and continued through mid-June, was less energetic than Redoubt or Augustine but was temporally more continuous and long-lasting, producing substantial downwind ash and aerosol clouds. The maximum plume height was 11 km, but on average was under 3 km for the majority of the eruptions. The lightning activity consisted of discrete discharges with flashing rates up to 100 discharges per hour. Visual observations indicate that the lightning discharges propagated upward from the volcano. In all eruptions observed so far, the volcanic eruption cloud appears to transport net positive charge away from the volcano.

VHF or electric field change measurements of the lightning provide an additional means of confirming or detecting the occurrence of an eruption, that is otherwise difficult or not possible during inclement weather conditions. As is the case for convective thunderstorms, the quantity and intensity of lightning in a volcanic eruption is an indicator of the intensity of the eruption and can provide a valuable measure of the volcano's ash and aerosol emissions.