VHF Radiation Observed During Eruptions of Sakurajima Volcano, Japan, Part II: Continuous RF Discharge Mechanisms

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We investigate possible mechanisms for the production of Continuous RF (CRF) emissions during explosive volcanic eruptions of Sakurajima volcano in Japan. CRF is a continual production of VHF emissions during the ash production phase of an eruption, lasting anywhere from a fraction of a second up to a minute. CRF has been observed during eruptions of various volcanoes over the last 10 years; in the case of Sakurajima volcano, the CRF phase generally lasts only a few seconds.

Multi-parametric observations of CRF were made during a field campaign at Sakurajima volcano, Japan in May and June of 2015. The instrumentation included a 10-station Lightning Mapping Array (LMA), a log-RF waveform derived from one of the LMA stations, a broadband VHF antenna, slow and fast delta-E antennas, high-speed video, and still photography. Data were recorded for several hundred explosive events.

We test the hypothesis that CRF is produced by numerous small (~100 m) leader discharges at the vent of the volcano. We also investigate other possible discharge mechanisms that may be responsible for CRF, by comparing CRF characteristics to VHF emissions from known sources, such as corona discharge and spark discharges. Preliminary results show that CRF is distinct from lightning, as no charge transfer is observed on delta-E instruments, unlike the charge transfer from leader stepping in regular lightning flashes that occur later on during an eruption.

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