Multiparametric observation of volcanic lightning: Sakurajima Volcano, Japan
C. Cimarelli, M. A. Alatorre-Ibargüengoitia, K. Aizawa, A. Yokoo, A. Díaz-Marina, M. Iguchi and D. B. Dingwell, 2016
Location of monitoring array at Sakurajima Volcano. Insert shows position of HS and 30 fps cameras (CAM) and microphones (IS1-4) at Kurokami Observatory (KUR). SRH and MTK indicate position of magnetotelluric stations. JMA shows the location of JapanMeteorological Agency branch office in Kagoshima fromwhere weather balloons are launched at 12 h intervals.
We recorded volcanic lightning generated by Vulcanian explosions at Sakurajima Volcano using a synchronized multiparametric array. Physical properties of lightning are related to plume dynamics, and associated electromagnetic field variations are revealed by video observations (high speed and normal speed) together with infrasound and high sampling rate magnetotelluric signals. Data show that volcanic lightning at Sakurajima mainly occurs in the plume gas thrust region at a few hundred meters above the crater rim, where the overpressure of the turbulent volcanic jets determines the electrification of particles generating a complex charge structure in the growing plume. Organization of charges may be achieved at later stages when the plume transitions from the jet phase to the convective phase. Comparison with atmospheric sounding and maximum plume height data show that the effect of hydrometeors on flash generation at Sakurajima is negligible and can be more prudently considered as an additional factor contributing to the electrification of volcanic plumes.
(a) Spatial distribution of recorded flashes above Showa crater as detected by HS videos. Color-coded images showing propagation of (b) IC and (c) CG flashes as captured by consecutive frames in HS videos. Flashes in Figures 2b and 2c are 31 and 6 frames long, respectively (frame interval is 333.34 μs in both images). See Text S1 in the supporting information for analysis procedure.
Volcanic lightning occurs in eruptive plumes as the result of the electrification of ash. Evidence is mounting that electrification is a common process in explosive eruptions in the form of a growing number of volcanic light- ning reports from recent eruptions (Eyjafjallajökull 2010, Grimsvötn 2011, Puyehue 2011, Kirishima 2011, Etna 2013, Sinabung 2014, Villarica 2015, Calbuco 2015, Colima 2015, and Etna 2015). Those reports complement numerous historical reports of similar phenomena [McNutt and Williams, 2010].
To date, volcanic lightning has been mainly detected via electric field changes [Anderson et al., 1965; James et al., 1998; Miura et al., 2002] and via radio frequency radiation generated by the plume electrical activity [Bennett et al., 2010; Thomas et al., 2010; Behnke et al., 2013]. Thus, volcanic lightning may provide a valuable monitoring tool for active volcanoes, allowing detection of ash emissions from safe distance and in inclement weather conditions [Behnke and McNutt, 2014]. However, the use of volcanic lightning to probe the properties of volcanic plumes (ash concentration, mass eruption rate, turbulence, etc) has been hampered so far largely by (i) the lack of systematic instrumental observation of electric activity in volcanic plumes and (ii) the limited number of experimental investigations on the electrification processes of volcanic materials [James et al., 2000; Houghton et al., 2013; Méndez-Harper et al., 2015] and the mechanism of plume electrification [Cimarelli et al., 2014].
We present here the first results of the high-speed (HS) video recording of volcanic lightning, paired to synchro- nized magnetotelluric (MT) and infrasound recordings of volcanic explosions associated with electrical activity at Sakurajima Volcano. We report quantitative measurements on the physical properties of lightning discharges and their occurrence with respect to the observed plume dynamics. We compare our observations with maximum plume height measurements and atmospheric soundings and derive conclusions concerning the atmospheric conditions that are favorable for the occurrence of volcanic lightning.