Unsteadiness Timescales of Volcanic Explosions Through Electrical Monitoring of Ash Plumes.

Observations at active volcanoes indicate that gas-particle mass flux during explosive volcanic eruptions can dramatically change showing high unsteadiness even during short-lived explosions (e.g. Taddeucci et al, 2012; Scharff et al., 2015). How the timescales of these fluctuations imprint on the development of the resulting volcanic plume is far from being well understood. Model calculations suggest that eruption column height strongly depends on temporal mass flux output (Scharff, 2012), with crucial consequences on tephra distribution in the environment. Field observation and experimental work on plume electrification show that electrical discharge rate is dependent on mass eruption rate (Cimarelli et al. 2014; 2016; Gaudin et al., 2018) thus suggesting that volcanic lightning monitoring can provide complementary information on the dynamics of the explosions. Here we present observation of the explosive activity of Minamidake crater at Sakurajima volcano (southern Japan) using combined information derived from synchronised Thermal Infrared (TIR) imaging, high-speed Doppler radar and electrical measurements. Two antennas at different distances from the active vents within Minamidake nested crater. The mini-array of antennas reveals a effective tool to monitor the explosive activity at Sakurajima, able to provide continuous recording of ash plume activity in all weather conditions and allowing for the discrimination of the explosion sources. Comparison of TIR video recordings with electrical activity shows a positive correlation between magnitude of the explosions and height of the eruptive columns with the number and size of the associated electrical discharges. Our data show excellent match between the occurrence of electrical discharges and the peek velocity of particle ejection thus enabling the distinction of discrete consecutive pulses during larger explosive events.