INTRODUCTION

Volcanic activity has a strong effect on the Earth’s atmosphere (Firstov et al., 2019). Most prominent perturbations in the atmosphere are observed at the explosive stages of volcanic eruptions during the periods of formation of eruptive clouds. Besides a strong local perturbation that emerges in the atmosphere as a result of sharp ejection of a mixture of gases and pyroclastic material which leads, in particular, to the generation of air waves (Adushkin et al., 1984; Adushkin and Firstov, 2010), also atmospheric electric effects are observed (Firstov et al., 2019; James et al., 2000; Johnston, 1997; Lane and Gilbert, 1992; Mather and Harrison, 2006; Rulenko, 1979; 1985). In some cases, the electric field in the eruptive cloud is so strong that volcanic lightning emerge (Firstov et al., 2017). Air waves and electrization of the eruptive cloud determine the complex effect of the volcanic eruptions on the environment. An important point is that a volcanic eruption is not an instantaneous event: the process, as a rule, consists of an alternation of stages of explosive eruptions and less powerful prolonged stages of quasi-steady effusion and ejection of hot ash-and-gas mixture and pyroclastic material (Degterev and Chibisova, 2019).

Studies of the variations in physical fields caused by volcanic activity attracts particular interest among specialists, both as a means to expand the notions and promote understanding of mechanisms of the processes accompanying the volcanic eruptions and as an instrument for assessing and forecasting ecological implications of these processes.

In this work, based on the data of instrumental observations, we consider the acoustic effects and variations in the geomagnetic field and in electrical parameters of the near-surface atmosphere (electric field strength and atmospheric current) at the first and second explosive stages and during the period of intense “purging”—the most powerful stage of effusion of hot pyroclastic material during the eruption of the Stromboli volcano (Italy) that occurred in the summer of 2019.

The Stromboli volcano (38.786° N; 15.218° Е) is located on the island of the same name in the Tyrrhenian Sea. According to the Italian National Institute of Geophysics and Volcanology (INGV), the volcanic activity began to enhance on July 3, 2019 at ~13:45 UTFootnote 1 when intense degassing arose on a slope segment at a distance of ~100 m from the crater. One hour later, at ~14:45 UT, a high-power paroxysmal explosion with massive ejection of pyroclastic material from the crater and rapid formation of the eruptive cloud and then eruptive column took placeFootnote 2.

After the first explosive stage, there has been some quiet period that lasted up to ~20:00 UT of July 5, 2019. During that period, effusion of pyroclastic material sometimes accompanied with rare small explosions took place, and a lava flow was formed. The eruptive activity of the volcano in the form of “purging”—the effusion of ash-and-gas mixture into the atmosphere—resumed in the evening of July 5, 2019 and continued up to ~04:00 UT of July 6, 2019. The strongest outflow in the form of a vertical jet of an incandescent ash-gas mixture was noted at ~02:00 UT on July 6, 2019 (https://yandex.ru/video/preview? filmId=9168275947493259539&text=Strombolivolcanoeruptionvideo) with a brightness temperature of above 1000°С. The mixture was emitted to a height of a few km which led to the formation of a fairly large region in the atmosphere with a changed composition and significantly heightened temperature.

According to the National Civil Protection Service of Italy, the second stage of the explosive activity of the Stromboli volcano began on August 28, 2019 at ~10:00 UT and manifested itself by several weak explosions. The strongest explosions were observed on August 29, 2019 at ~20:45 UTFootnote 3. Moreover, after the main volcanic explosion at ~20:45 UT, yet two another volcanic blasts having lower energy occurred with the intervals of approximately 20 min, and these blasts were also accompanied with intense ash emission.

In this paper, we present some results of instrumental observations of the geophysical effects that accompanied the eruption of Stromboli volcano: acoustic signals, variations in the magnetic field, and variations in the electrical properties of the near-surface atmospheric layer.

DATA

This study mainly relies on the observations of geophysical fields carried out at the Geophysical observatory “Mikhvevo” (MHV) (54.94° N; 37.73° Е) and at the Center for Geophysical Monitoring in Moscow (CGM) (55.71° N; 37.57° Е) of the Sadovsky Institute of Geosphere Dynamics of the Russian Academy of Sciences. The distance between MHV and CGM is ~85 km (Fig. 1) (Adushkin et al., 2016; Spivak et al., 2016).

Fig. 1.
figure 1

Layout of magnetic observation points (triangles); asterisk marks position of Stromboli volcano.

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Atmospheric pressure and other meteorological parameters characterizing the state of the atmosphere were recorded at MHV and CGM by Davis Vantage Pro 2 digital weather stations. Microbaric variations were recorded by MB-03 microbarograph which provides stable measurements of acoustic signals with amplitudes from 0.01 to 200 Pa in the frequency band from 0.0003 to 10 Hz. The acoustic signals generated by the volcanic eruption were identified based on the analysis of the original record in the frequency band from 0.005 to 1 Hz with the allowance for the distance to the source of the signal (~2450 km) and probable signal propagation velocity in the stratospheric waveguide (280–310 m/s) (Kulichkov et al., 2004). Due to the signal recording at two measurement points (MHV and CGM), it was not only possible to estimate the magnitude of the velocity of signal propagation but also to determine the direction to the source.

Magnetic induction components Bx, By and BzFootnote 4 were measured by LEMI-018 digital fluxgate magnetometer which records the magnetic field in the range of ±68000 nT with a resolution of 10 pT (sampling frequency 1 s–1). For analyzing the magnetic effect caused by the eruption of Stromboli volcano, we also used the records of three components of the magnetic field from a number of the midlatitude INTERMAGNET observatories (Fig. 1) located at different distances R from the volcano (Table 1).

Table 1.   Geomagnetic observation points
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Electrical perturbations in the surface atmospheric layer were described based on the results of continuous recording of the vertical electric field Е and vertical atmospheric current I at MHV and vertical component of the atmospheric electric field Е at CGM. The electric field and atmospheric current were measured by the INEP electrostatic fluxmeter (Adushkin et al., 2018) in the frequency band 0–20 Hz and compensating current meter (Baryshev et al., 2009) with sampling frequency 1 s–1. The analysis of the electric field Е was based on the time series of digital data formed with a discreteness of 5 s. Considering the high variability of the atmospheric current I, for identifying the variations induced by the volcanic activity we analyzed the root mean square deviations I0 averaged over 1-min intervals.

We considered the data obtained during the above eruption stages of Stromboli volcano on July 3 and 6 and on August 29, 2019. During the main eruption stages, magnetic activity was low (Table 2) and significant natural and manmade perturbations in the electric field at MHV were absent, which substantially simplified the analysis of the effects of volcanic activity in the physical fields during these periods.

Table 2.   K-index of magnetic activity during main stages of Stromboli volcano eruption according to MHV data
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ACOUSTIC SIGNALS GENERATED BY THE ERUPTION OF STROMBOLI VOLCANO

Acoustic Signal from Eruptive Explosion of July, 2019

Strong impacts exerted on the atmosphere by the volcanic explosions, particularly eruptive type ones, excite oscillations in the atmospheric pressure in a wide frequency range and in some cases cause shock waves (Firstov et al., 1978). Together with the aerodynamic noise (frequency band f ≈ 20–1000 Hz), pulse-type acoustic perturbations (f ≈ 1–10 Hz), and long-period oscillations (f < 0.003 Hz), volcanic eruptions generate infrasonic waves in the frequency band 0.003–1 Hz, which, due to the weak attenuation, propagate to fairly large distances from the source (Adushkin et al., 1984; Firstov, 1994; 1996; Murayama, 1969; Reed, 1980; Stewart, 1959).

For isolating the acoustic signal caused by the volcanic explosion, we band-pass filtered the primary digital record in the frequency band 0.05–1 Hz. The resulting waveforms are shown in Figs. 2a and 2b. From Fig. 2 it follows that signal arrival at MHV and CGM was recorded at ~17:09 and 17:11 UT, respectively. The duration of the signals was ~3–4 min. The frequency dependence of spectral density of the oscillations is shown in Fig. 2c. We note that in this case, the presence of several spectral maxima located close to each other is a characteristic feature of the infrasound signal produced by the volcanic explosion (Storcheus et al., 2006).

Fig. 2.
figure 2

Microbaric pulsations in frequency band 0.05–0.5 Hz caused by Stromboli volcano explosion on July 3, 2019 according to (a) MHV and (b) CGM data; (c) frequency dependence of spectral density of the induced acoustic signal at CGM.

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Acoustic Signal Caused by Volcanic Explosions of August 29, 2019

The acoustic signal generated by the repeated explosive eruption of Stromboli is shown in Fig. 3a. The signal arrival is detected at ~23:03 UT. Clear responses to the main paroxysm and two subsequent volcanic explosions which occurred with an interval of ~25 min can be seen in the record. Figure 3b shows a more detailed illustration of a signal waveform from the first, most powerful explosion. The spectrum of the signal recorded at CGM is shown in Fig. 3c. We note that the acoustic signals from the explosions of July 3 and August 29, 2019 have similar spectral shapes while the main contribution to spectral energy in these cases is observed in the substantially different frequency bands.

Fig. 3.
figure 3

Microbaric pulsations in frequency band 0.01–0.04 Hz caused by Stromboli volcano explosion on August 29, 2019 according to (a) CGM data and (b) its fragment; (c) frequency dependence of spectral density of the induced acoustic signal.

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Air Wave at the Stage of “Purging”—Highly Intense Quasi Steady Effusion of Stromboli Volcano

The specificity of this eruption stage is that explosions are absent. The volcanic activity had the form of a sudden ejection of a vertical jet of highly heated matter forming a luminous column which extended to a height of a few hundred meters. As a result, a fairly large region with the excess temperatures was formed above the crater within the first tens of minutes. It is known that the presence of a dynamically formed high-temperature region is a cause of generation of infrasound and internal gravity waves (Gossard and Hook, 1975)Footnote 5. This phenomenon is particularly pronounced in large-scale explosions and catastrophic fires (Gostintsev et al., 1983; 1985; Gostintsev and Shatskikh, 1987). The results of recording the air waves excited by intense “purging” (effusion) (Storcheus et al., 2006) of the Stromboli volcano are shown in Figs. 4a and 4b. The main microbaric variations associated with signal arrival are recorded from ~04:20 to 04:30 UT. Figures 4a and 4b shows that, in contrast to the explosive eruption stages, intense “purging” mainly generates internal air waves (IGW) in the frequency range ~0.0005–0.003 Hz, which is soundly supported by spectral estimates (Fig. 4c). In this case, the perturbations in the IGW range are caused by the formation of a tall eruptive column which, according to some authors, reaches the tropopause (Firstov, 1997).

Fig. 4.
figure 4

Microbaric pulsations in frequency band 0.0005–0.05 Hz during intense effusion (“purging”) of Stromboli volcano on July 6, 2019 according to (a) MHV and (b) CGM data; (c) frequency dependence of spectral density of the induced acoustic signal at CGM; filling shows period of anomalous microbaric variations.

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The estimates conducted with the allowance for the arrival times of the acoustic signal at a recording station suggest that signal propagation velocity in the atmospheric waveguide is in all cases 290–310 m/s.

GEOMAGNETIC EFFECT OF THE VOLCANIC ERUPTION

Instrumental observations show that during the periods of volcanic activity, variations in the Earth’s magnetic field intensify (Johnston, 1997). The results of our study indicate that anomalous geomagnetic variations are not only observed during the period of explosions but also during the arrival of the acoustic signal at the recording point. The geomagnetic variations during the arrival of the acoustic signal from the explosion of July 3, 2019 at different observatories are displayed in Fig. 5 (the signal propagation velocity in the thermospheric waveguide (or in the ionosphere (Kunitsyn and Shalimov, 2011)) estimated from the magnetic records with the allowance for the distance to the source is ~295–305 m/s). The amplitudes of the induced geomagnetic variations recorded in this case are presented in Table 3.

Fig. 5.
figure 5

Geomagnetic variations during the arrival periods of acoustic signal caused by explosion of July 3, 2019 according to data from different observatories (hereinafter in the figures, vertical arrows indicate the arrivals of acoustic signal).

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The response of the magnetic field to the arrival of the acoustic signal from the volcanic explosion of August 29, 2019 is illustrated in Fig. 6. This figure shows that anomalous geomagnetic variations are also present during the arrival of the acoustic signal whose propagation velocity in the thermospheric waveguide estimated from the magnetic records with the allowance for the distance to the source is in this case ~290–310 m/s. At the remote recording stations NVS, IRT and KHB, the effect was not observed.

Fig. 6.
figure 6

Geomagnetic variations during arrival periods of acoustic signal caused by Stromboli explosion of August 29, 2019 according to data from different observatories.

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In addition we note that as in the cases of the explosions of July 3 and August 29, 2019, the acoustic signal generated by the intense “purging” of the volcano caused variations in BH (Fig. 7) with amplitudes ~1–5.5 nT (Table 3) which are fairly distinctly recognized in the magnetic records.

Fig. 7.
figure 7

Geomagnetic variations during arrival periods of acoustic signal caused by intense effusion (“purging”) of Stromboli volcano on July 6, 2019 according to data from different observatories.

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Table 3.   Maximum amplitude of variations in ВН at different stages of Stromboli volcano eruption
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VARIATIONS IN THE ELECTRICAL CHARACTERISTICS OF THE ATMOSPHERE DURING THE ERUPTION OF STROMBOLI VOLCANO

Atmospheric Electric Field

The volcanic explosion of July 3, 2019 has almost immediately caused a change in the dependence of the electric field strength Е on time t at MHV—the observatory which is located quite far from Stromboli. Figure 8a shows the record of the vertical electric field at MHV during the explosive stage of the eruption of July 3, 2019. It can be seen that from 13:40 to 13:45 UT, well-pronounced anomalous variations appear in Е(t) in the form of the periodic pulsations with a period of ~45 min and average amplitude of ~35 V/m.

Fig. 8.
figure 8

Variations in vertical electric field (a) during Stromboli volcano explosion on July 3, 2019 and (b) during the arrival of acoustic signal caused by the explosion according to MHV data.

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During the volcanic explosion of August 29, 2019, the response of the electric field also had the form of the enhanced periodic variations in Е(t) at MHV and more distinctly pronounced Е(t) variations at CGM (Figs. 9a and 9b).

Fig. 9.
figure 9

Variations in the vertical electric field at (a) MHV and (a) CGM during Stromboli volcano explosion on August 29, 2019 and during the arrival of acoustic signal caused by the explosion of August 30, 2019 according to (c) MHV and (d) CGM data.

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The anomalous variations in the electric field were also detected during the arrival of the acoustic signal produced by the volcanic explosions. Figure 8b shows the variations in E caused by the arrival of the acoustic signal from the explosion of July 3, 2019 at the MHV. Figures 9c and 9d show the variations in E observed on August 30, 2019 during the arrival of the acoustic signal from the explosion on August 29, 2019, which were recorded at MHV and CGM, respectively. It can be seen that in both cases, the acoustic perturbation is concomitant with the enhanced alternating-sign variations in E.

Variations in the electric field recorded at MHV and CGM during the intense effusion (“purging”) of Stromboli volcano on July 6, 2019 have a characteristic form shown in Figs. 10a and 10b, respectively. Intense heating of the atmosphere caused a bay-like negative variation in E with an amplitude of ~55 V/m. It is worth noting that variations in the electric field E in this case are morphologically similar to the morphology of the air wave observed during the eruption of the Bezymyannyi volcano, Kamchatka, on March 30, 1956 (Pasechnik, 1958).

Fig. 10.
figure 10

Variations in vertical electric field during period of intense effusion (“purging”) of Stromboli volcano on July 6, 2019 according to (a) MHV and (b) CGM data and during the arrival of acoustic signal according to (c) MHV and (b) CGM data.

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The arrival of the IGWs that developed during the intense “purging” of the volcano caused a bay-like decrease in the electric field at MHV and CGM which is complicated by the alternating-sign variations in E (Figs. 9c and 9d, respectively). Overall, the amplitude of the change in E in this case was ~25 V/m.

Vertical Atmospheric Current

The measurement results demonstrate a fairly strong response of the atmospheric current to the volcanic eruption. Figure 11 shows the variations in the vertical component of the atmospheric current in terms of rms deviation I0 during the first explosive stage of the Stromboli eruption on July 3, 2019 (Fig. 11a) and during the arrival of the acoustic signal from the explosion recorded at MHV (Fig. 11b). The figures clearly show a significant increase in I0 in both cases. Similar variations in the atmospheric current were recorded during the intense “purging” of the volcano on July 6, 2019 and during the arrival of IGWs caused by the intense effusion (“purging”) of the volcano (Fig. 12).

Fig. 11.
figure 11

Root mean square deviation of vertical component of atmospheric current (a) during Stromboli volcano explosion on July 3, 2019 and (b) during the arrival of induced acoustic signal according to MHV data.

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Fig. 12.
figure 12

Root mean square deviation of vertical component of atmospheric current (a) during intense effusion of Stromboli volcano on July 6, 2019 and (b) during the arrival of induced acoustic signal according to MHV data.

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The response of the atmospheric current to the second explosive stage of the Stromboli eruption of August 29, 2019 is more complex (Fig. 13). In this case, both during the explosions and during the arrival of the acoustic signal generated by these explosions, alternating-sign variations with ~2–2.5 times higher amplitude than background variations are observed in I0.

Fig. 13.
figure 13

Root mean square deviation of vertical component of atmospheric current (a) during Stromboli volcano explosion on August 29, 2019 and (b) during the arrival of induced acoustic signal on August 30, 2019 according to MHV data.

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DISCUSSION

The above results indicate that a clearly pronounced geophysical effect is present in the medium at all stages of the volcanic eruption. It is important that the effect is observed in the physical fields of different origin and at the significant distances from the source of perturbations. This expands the applications of the instrumental data from developing the models that only reproduce the activity of specific volcanoes to describing the behavior of the geophysical medium under strong external impacts.

The air wave generated by the explosive stage of a volcanic eruption contains information about the level of energy release in the source. Indeed, the existing data (Kulichkov and Bush, 2001; Adushkin et al., 2019) show that as the main parameter of the acoustic signal for estimating the total energy of the source W, one should consider the characteristic frequency f0 of the signal because the amplitude of the signal strongly depends on the distance and, which is most important, on the propagation path. In this work, we estimate W at the explosive stages and at the “purging” stage of Stromboli eruption using the dependence proposed in (Adushkin et al., 2019):

$$W \approx \frac{{9.24 \times {{{10}}^{{10}}}}}{{f_{0}^{{2.2}}}}\,\,{\text{J}}{\text{,}}$$
(1)

where f0 is measured in Hz.

It should be noted here that the energy estimates of the source of the air waves generated by a volcano at the stage of intense “purging” relate to the energy released during the interval of the most intense effusion of volcanic products. At this stage of volcanic activity, we can consider the IGW generation as most probably following the mechanism of a successive interaction of the buoyant volumes of heated air having a certain characteristic size r0 (Zaslavskii and Yuriev, 1999; Gossard and Hook, 1975).

The results of estimating the source energy W with the allowance for the spectral characteristics of the recorded acoustic signals (Figs. 2–4) are presented in Table 4.

Table 4.   Energy released at different stages of Stromboli volcano eruption
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Let us consider the stage of intense “purging” of the volcano and assess the dynamics of the heated region of the atmosphere in the scope of the known problems of thermo- and hydrodynamics (Gostintsev and Shatskikh, 1987; Zetser et al., 2012; Storcheus et al., 2006; Firstov et al., 1987). Since the heat increment of a unit mass of gas is:

$$\Delta Q = {{T}_{0}}\Delta S,$$

where ΔS is the change in specific entropy, T0 is the initial gas temperature at the height z of perturbation, using the results of (Orlov and Uralov, 1984), we obtain the following formula for the change in the gas temperature ΔТ:

$$\frac{{\Delta T}}{{{{T}_{0}}}} \cong \frac{{\left( {{{{\gamma }}^{2}} - 1} \right)}}{{12}}{{\left( {\frac{u}{C}} \right)}^{3}},$$

where \({\gamma } = \frac{{{{c}_{P}}}}{{{{c}_{V}}}};\) С is the speed of sound at height z; u is the velocity of the cocurrent gas flow which is about 500 m (Firstov et al., 1987).

The probable displacement of the heated region in the vertical direction (along the z-axis) is estimated (Orlov and Uralov, 1984) at

$${{z}_{0}} \approx \frac{g}{{{{{({\omega *})}}^{2}}}}\frac{1}{{{\gamma } - 1}}\frac{{{\Delta }T}}{{{{T}_{0}}}} \approx \frac{1}{5}\frac{g}{{{\omega }_{*}^{2}}}{{\left( {\frac{u}{C}} \right)}^{3}},$$
(2)

where \({\omega ^*}\) is Brunt–Väisälä frequency.

With the uncertainty of T0 taken into account, the estimates based on the dependence (2) give z0 ≈ 12–15 km.

In the stably stratified atmosphere, the heated domain having reached the maximum height begins oscillating about the level of thermal equilibrium with the decaying amplitude. Due to the turbulent structure of the ascending region, this motion entrains the surrounding air masses. The entrainment intensity is proportional to the product of the vertical velocity of the ascending domain and its current surface area with the empirical trapping coefficient α.

The motion of the heated region is described by the following system of equations (Gostintsev et al., 1985):

$$\begin{gathered} \frac{{dz}}{{dt}} = u;\,\,\,\,\frac{{du}}{{dt}} = \frac{{{{{\rho }}_{0}} - {{{\rho }}_{z}}}}{{{{{\rho }}_{z}}}}g - \frac{{3{{{\rho }}_{0}}K}}{{8{{{\rho }}_{{\text{z}}}}{{r}_{0}}}}\frac{{dz}}{{dt}}, \\ \frac{{dM}}{{dt}} = 4\pi \alpha {{\rho }_{0}}r_{0}^{2}\frac{{dz}}{{dt}};\,\,\,\,\frac{{dF}}{{dt}} = - \omega _{*}^{2}r_{0}^{3}\frac{{dz}}{{dt}}, \\ \end{gathered} $$
(3)

where r0 is the effective size of the heated domain; \(M = \frac{4}{3}\pi {{\rho }_{0}}r_{0}^{3}\) is the mass of gas enclosed in this domain; ρ0 and ρz are the initial gas density and gas density at height z, respectively; \(F = \frac{4}{3}\pi gr_{0}^{3}\frac{{{{T}_{0}} - {{T}_{z}}}}{{{{T}_{z}}}}\) is buoyancy; K = KF + 8α is the effective coefficient of resistance; КF is the form resistance coefficient (according to data of (Gostintsev and Shatskikh, 1987), α ≈ 0.1; КF ≈ 0.5).

The system of equations (3) is supplemented by the gas state condition which in the convective approximation has the following form:

$$\frac{{{{\rho }_{0}} - {{\rho }_{z}}}}{{{{\rho }_{z}}}} = \frac{{{{T}_{0}} - {{T}_{z}}}}{{{{T}_{z}}}}.$$
(4)

The estimating calculations by formulas (3) and (4) show that the motion of the heated domain has the form of oscillations which in our case have a period of 12–17 min and amplitude 2200–2800 m. These oscillations result in the generation of IGWs. The obtained estimates are closely consistent with the results of observations presented above.

The mechanism of the influence of volcanic eruptions on the Earth’s magnetic and electric field is currently not quite clear. Unlike the local effects that can be attributed to the intense release of the strongly heated matter into the atmosphere, the long-range and close in time influence of the volcanic eruption which has been established in this work needs a further and more detailed study. It is not excluded that at the stages of intense perturbations (explosions, strong “purging”), rapid formation of an eruptive column or a large, vertically fairly extended jet of hot highly ionized ash-and-gas mixture leads to the emergence in the near-surface zone of the Earth of a source of strong acoustic and electrical influence on the ionosphere. As a result, the magnetohydrodynamic perturbation that emerged in the ionosphere at the epicenter of this source propagates through the ionosphere at high velocity (~22 km/s (Sorokin and Fedorovich, 1982)) causing variations in the electric field which arise close in time (with allowance for the accuracy of establishing the onset time of the induced variations) at different distances from the source. One of the alternative mechanisms could be, for example, the conversion of the acoustic perturbation into the magnetoacoustic wave (Kovaleva et al., 2013).

Considering the observed response of the geomagnetic and electric fields to the arrival of the acoustic signal, we can hypothesize that the air waves generated by the volcanic activity are at the same time the cause of the perturbations in the magnetic and electric fields, e.g., as a result of the formation of air-electricity structures in the surface atmospheric layer, or, more likely, by the scenarios with mediation of the ionosphere.

The questions concerning the probable mechanisms and the models of the effect of a volcanic eruption on geophysical fields that describe all the aspects of the process need special attention and additional research.

CONCLUSIONS

The results of our study demonstrate the complexity of geophysical processes accompanying active volcanism. Our data indicate that along with the well-known local effects concomitant with the volcanic activity (emission of hot matter, formation of an air wave and lava flow, perturbations of the local electric field in the near-surface atmosphere, etc.), the perturbations of the physical fields are detected at significant distances from the volcano. It is important to note that variations, e.g., in the electrical characteristics of the atmosphere are not limited to the very period of the volcanic eruption with its different stages but are also observed during the periods when the acoustic signal generated by volcanic activity and arrived at the measurement point disturbs the atmosphere.

As of now, it is barely possible to suggest a clear physical interpretation of the discussed effects. This can be achieved through the accumulation and detailed analysis of instrumental data and extensive analytical and numerical calculations, which is planned to be done in the future.

In our opinion, even in their current form, the results of our research supplement the corresponding database and can be useful for improving the known and developing the new models of the influence of volcanic activity on the geophysical environment as well as for verification of these models.