Atmospheric electrical effects during a strong explosive eruption of Bezymyanniy volcano (Kamchatka Peninsula, Russia) on December 20, 2017

A set of parameters of various geophysical methods is considered, which made it possible to quite clearly represent the mechanism of the explosive eruption of Bezymyanny volcano on January 20, 2017. The explosion began according to the «soft» scenario, which was reflected both in the seismic method and in the thunderstorm activity of the eruptive cloud (EC). The transition to Plinian activity was accompanied by the generation of an infrasonic signal and an increase in lightning discharges in the EC. The two-tiered EC was well traced in the potential gradient response in atmospheric electric field. The combination of various geophysical methods provides a detailed understanding of the mechanism of explosive eruptions and contributes to a better understanding of the physics of the explosive process on volcanoes.


Introduction
One of the most active volcanoes of the world is Bezymyanniy volcano (55.98° N, 160.59° E, height 2869 m 1 ). It is located in the central part of the Klyuchevskaya group of volcanoes (KGV) on Kamchatka peninsula (figure 1a). The closest neighbor of Bezymyanniy in the north is the extinct volcano Kamen (4670 m) next to which is the highest volcano of Europe and Asia, Klyuchevskoy volcano (4850 m). In the southern direction is the Ploskiy Tolbachik volcano, for which powerful lateral eruptions are typical (figure 1b). After the catastrophic eruption on March 30, 1956, the cone of Bezymyanniy, formed in the new crater (1.5×2.8 km, depth 700 m), continuously extruded [7] up to the end of 2012. The process had active periods with separate explosive eruptions and relatively calm periods. Manifestations of the volcanic activity are various: cone and block extrusions; explosions of different intensity including directed blasts; eruptions of pyroclastic flows (PF) and extrusion of small viscous lava flows. For more than almost a 65-year period of investigation of Bezymyanniy volcano, the events, enumerated above, often occurred there [5,10].
Since the end of 2012 to December 2016, the volcano was relatively calm. That was, evidently, associated with lateral eruption of Tolbachik volcano in 2012-2013 and four terminal eruptions of Klyuchevskoy volcano during that period [10]. In December 2016 it became active again. Two explosive eruptions occurred on March 9 and June 16, 2017. The third, the most powerful eruption IOP Conf. Series: Earth and Environmental Science 840 (2021) 012020 IOP Publishing doi:10.1088/1755-1315/840/1/012020 2 took place on December 20, 2017 at 03:39 2 . From 03:45 PF began to descent intensively. When they got into the deceleration area, they initiated secondary eruptive clouds. This process promoted multilayer eruptive clouds (EC).
EC height was 15 km and the main area of the territory, over which it was spread, was ~78 000 km 2 (figure 2а). Based on the satellite data, the PF deposits extension was 6 km and that of mud stream was up to 18 km. According to the model calculations of EC motion, the erupted tephra mass on the ground was estimated as ~ 3•10 7 t (~0.023 km 3 ) [6].

Instrumentation
There are sites for volcano observations and radiotelemetric seismic stations (RTSS) of Kamchatka Branch of Federal Research Center, Geophysical Survey RAS in KGV region. They allow us to monitor the seismisity of the regions near the active volcanoes (figure 1). Infrasound channels, the sensors of which are microbarographs (ISMB-03M), are installed at KZY and KLY sites. They record wave disturbances in the atmosphere during explosive eruptions. Moreover, an infrasound station IS44 of the international infrasound monitoring system is installed at Nachik site (figure 1a). The station includes an antenna array composed of four microbarographs M-2000 (France) with the aperture of ~1.8 km. It allows us to record infrasound signals in the frequency range of 0.003-5 Hz and to determine the azimuth of a source [9]. To record atmospheric electric potential gradient (PG) response in atmospheric electric field, occurring during EC passage over the observation sites, the electric field mills EF-4 was installed at KZY and KLY [3].  [4]. The World Wide Lightning Location Network [12] allows us to record EMPs from volcanic lightning [11]. Unfortunately, due to the widely spaced sites of the network in the region of the peninsula, WWLLN records only strong lightning strokes from the eruptions of Kamchatka volcanoes. Atmosphere stratification was determined from the data of height balloon sounding at Klyuchi observatory [13]. According to that data, at 00:00 on December 20, 2017, wind direction at the height of 6-16 km had the same value (~220°) whereas its velocity significantly changed from 10 to 30 km/s ( figure 3b, c). Based on the vertical profile, sound effective velocity stratification on the path BZM-IS44 was calculated by the formula

Data and analysis
It follows from RTSS (BZW, BZP) records of the explosive earthquake, occurred during the eruption, that the eruption began at 3:39 on May 20, 2017. During the first ~5 minutes (I), seismic signal intensity gradually increased. Then the signal level increased rapidly and during the following 4 minutes it exceeded the instrumentation dynamic range (II). Evidently, during this period a Plinian eruption with powerful ejection of ash-gas mixture into the atmosphere began. After that, signal amplitude decreased and remained quasi-constant for almost 5 minutes followed by a decrease up to the background in 10 minutes (III) (figure 4b).
Turbulent column and EC were the sources of air infrasonic waves recorded at the infrasonic site IS44 and at KZY (figure 4a). A weak inversion (с ef =275 m/s) is observed on the vertical profile of sound effective velocity (с ef ) on the path BZM-IS44 at the height of 11 km. It may be a reflecting boundary. Making an assumption that infrasonic waves propagate mainly along this boundary, figure  4a shows the times for separate wave group onsets from the moments of their occurrences. Infrasonic signal occurrence (figure 4a) coincides with the end of the region of ground vibration maximum velocity at BZW. It is likely to be associated with EC formation in the floating zone and PF descent. PF deposit area could be the source of heat-mass release. This source formed the second layer of EC at lower height. EC ash fall axis was located ~20 km from KZY. Fair weather conditions allowed us to record the response from its passage, lasting for about two hours, by the electrostatic fluxmeter. The response consisted of two bay-like signals of negative polarity with the amplitude ' 1 V = 60 V and ' 2 V = 50 V/m and duration of 20 and 90 minutes. EC propagation at different heights had different velocity under wind effect that was recorded in PG. Based on the atmosphere stratification, the first cloud layer propagated at the height of ~ 13 km, and the second one propagated at the height of 8 km. The volumetric charge for the both parts of EC was calculated from its trajectory and the recorded response in PG by the formula , where ε 0 is the dielectric constant, R min =25 km is the minimum distance from the recorder to the horizontal projection of EC trajectory, z=13 km is the EC propagation height. EC charge was -9 and -7.5 C, respectively.
We consider EC lightning activity dynamics according to the data of the VLF direction finder when increased amount of EMPs (inset in figure 5a) was recorded for ~ 40 minutes from the direction to Shiveluch volcano (23.6°). A weak increase of EMP number was recorded during the first 6 minutes (I) in minute intervals (N) with the total number of 9 pulses. This fact indicated the eruption beginning according to the «smooth» scenario that agreed with earthquake record (figure 4b). During that period EMPs had negative polarity of the first half period (figure 6a). All these pulses have a negative phase of the first half-wave, which indicates a predominantly vertical orientation of the emitting dipole with a positive pole at the bottom. The next 11 minutes (II+III) are characterized by N increase up to 54 pul/min followed by a decay during 15 minutes (IV) to N =12 pul/min. For the both periods, the average maximum amplitude ( max A ) was within 0.4 -1.0 mV/m.  Some increase of N and significant increase of max A and E occurred in the section V (marked by pink in figure 5). During that period, EC interacted with meteorological clouds stretching along the Kamchatka coast. From 4:00, EMP had mainly positive onsets of the first quasiperiod (figure 6b) that indicated the prevalence of in-cloud strokes. The same was observed for other volcanoes [11]. Almost