When do Volcanic Eruptions make Lightning? Observations from Sakurajima, Japan

Previous radio frequency (RF) observations of volcanic lightning have revealed that electrical activity frequently occurs concurrent with the onset of an explosive volcanic event. Typically, a myriad of electrical impulses originating from directly above the vent are observed first and the ensemble has durations of several seconds. The impulses are distinct from those produced by typical types of thunderstorm lightning, and have earned the moniker “continuous RF” due to their high rate and long-lasting nature . Several seconds after the onset of these impulses, small (100s of meters to several kilometers) lightning discharges occur in the plume and near the vent, and have electrical signatures similar to typical thunderstorm lightning. In eruptions with plume heights reaching 8-10 km or more, large scale (10s of kilometers) lightning discharges are observed throughout the plume several minutes after the onset of an explosive event. In May 2015, a campaign began to study the various types of small-scale electrical activity, including continuous RF, during explosive eruptions of Sakurajima volcano in Kyushu, Japan. The volcano was instrumented with two seismometers, two infrasound arrays, a high sensitivity video camera, an infrared camera, two high speed video cameras, still cameras, a 10-station Lightning Mapping Array, slow and fast electric field change sensors, and a broadband very high frequency (VHF) antenna. With these instruments, a robust data set of both the volcanic activity and electrical activity was collected. The preliminary data have revealed brief (1-2 seconds) bursts of continuous RF simultaneous with the onset of the more energetic explosions. Occurrence of continuous RF may be linked to mass eruption rate, explosivity, or grain size. Due to its unique nature, detection of continuous RF is an unambiguous indicator of explosive volcanic activity and is therefore useful for real-time volcano monitoring