Extremely Low Frequency Electrical Monitoring of Persistent Explosive Activity of Minamidake Crater (Sakurajima Volcano, Japan)

Abstract:

Very low frequency and wide-band thunderstorm networks have proven to be able to detect volcanic plumes of large magnitude sensing their intense electrical and lightning activity. However, the time delay and the high number of non-detected explosive episodes show the limits of these systems in the detection of smaller and more frequent ash-rich explosive events. Here, we use a data-effective thunderstorm detector to observe electrical discharges generated by the persistent Vulcanian activity of Minamidake crater at Sakurajima volcano in Japan from July 2018 to January 2020.

Two detectors recorded the electrical activity produced by explosions at Minamidake crater from a distance of 3 and 4 kilometres from the active vents. The detector measured the induced current due to the change in electric field with time within the extremely low frequency range (1-45 Hz). Using a volcanic lightning detection algorithm and the catalogue of volcanic explosions compiled by the Japan Meteorological Agency (JMA), the number of electrical discharges, the discharge rate and the amount of neutralised charge were determined for each individual explosive event. In addition, the start of the electrical discharges was compared to the approximated eruption onset, which was reported by the JMA to the minute precise.

The sensors detected electrical discharges in 71% of the 724 recorded eruptions. Our detection algorithm successfully recognised the presence/absence of electrical discharges with an accuracy of 70%. We find a non-linear positive correlation between the number of discharges and the plume height. More in detail, we find that the discharge rate and the maximum amount of neutralised charge by a single discharge also increases with plume height. Few electrified plumes exceeded the -10°C isotherm, indicating that ice nucleation did not play a major role in the electrification process. Finally, for 16% of the electrically-active eruptions, discharges were detected by the sensors before the approximated eruption onset. Our results show the ability of our detectors in pinpointing the inception of electrified explosive episodes in real time and in providing indications on the magnitude of the explosions, thus making them a cost- and data-effective instrumentation for the monitoring of explosive ash emissions at active volcanoes.