VHF Radiation Observed During Eruptions of Sakurajima Volcano, Japan, Part I: Overview and Characteristics of Continuous RF

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Volcanic plumes resulting from explosive eruptions are commonly electrified and lightning is frequently observed during eruptions. Over the past 10 years VHF lightning mapping arrays (LMAs) have been used to study the electrical discharges in volcanic plumes from volcanoes in Alaska (USA), Iceland, Chile, and Japan. In addition to VHF emissions typical of lightning, during explosive eruptions LMAs have also recorded a source of VHF radiation distinct from lightning. This distinct source of radiation is referred to as continuous RF (CRF) and is characterized by relatively high rates of impulsive radiation events over long time scales (several to tens of seconds). Continuous RF is observed simultaneous with the onset of explosive eruptions and originates at low altitude -- at or near the vent of the volcano. Continuous RF has only been observed while the volcano is actively ejecting ash. The source of the CRF is unknown, but has been hypothesized to be due to numerous, small (~100 m), leader-forming discharges at the vent of the volcano.

A new study is underway to gain further insight into the source of CRF emissions. In May and June of 2015 multi-parametric observations of volcanic lightning were conducted at Sakurajima volcano in Japan. Sakurajima produces frequent small-scale eruptions (plume heights 1-3 km) and during the main observation period several hundred explosive events occurred. The electrical activity was measured with a 10 station LMA, log-RF waveform, a broadband VHF antenna, slow and fast antennas, high-speed video, and still photography. Seismometers, infrasound sensors, infrared video, and low light video were used to measure the eruptive activity. We present an overview of the CRF observations obtained during the field deployment in Japan, including the electrical characteristics of CRF and the characteristics of the eruptive activity producing the CRF.

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Presented at the AGU Fall Meeting on December 13, 2016 in San Francisco, CA