Infrasound in ATMOSPHERE And ITS CONNECTION With SPACE And GEOSPHERic PROCESSES

Infrasound fluctuation in the Earth atmosphere resulted from the action of numerous geospheric processes and processes in near space. Action of the energy of a space origin on the Earth processes is usually connected to electromagnetic radiations and solar corpuscular flows. The interaction of electromagnetic radiation with heterogeneities of the atmosphere transparency can result in generation of acoustic fluctuations in a wide range of frequencies. It is important to note, that the transparency heterogeneities of the atmosphere are significantly determined by solar activity (SA). One can expect, that in the spectrum of the infrasound fluctuations of the atmosphere (IFA) the SA rhythmic should be manifested. The widely known connection of SA with biospheric processes can occur through the acoustic channel [1,2,3]. 

The IFA are caused by seismic activity. In this case it is possible to consider two situations. The IFA can be both the external influence on preparatory processes and their result.

It is known, that the intensity of seismic processes is caused by SA. This connection was found during the analysis of global seismicity on the Earth and the 11-year cycles of SA. It is considered, that this connection occurs through cyclonic activity in the atmosphere. However it is possible to assume, that SA influences the intensity of the infrasound waves in the atmosphere, which, in turn, influence the seismic processes. Thus, the Sun, interplanetary environment, atmosphere and lithosphere represent a uniform system, in which the infrasonic waves play an essential role during interaction of geospheres.

In LC ISR the researches are carried out in three directions:

	researches of the influence of SA on infrasonic  fluctuation in the Earth atmosphere;
	researches of connection of seismic activity with infrasound in the atmosphere;
	researches of the influence of artificial acoustic disturbance on the “atmosphere –      ionosphere system”.

The purpose of the first direction researches was the search of rythmic solar activity in the atmospheric infrasound spectrum. As a result of the analysis of the infrasound spectrum for the period of 1997-2000 the annual, seasonal, 27 daily and daily periods of fluctuations were found out. The hypothesis about the increase in infrasound energy with reduction of SA was confirmed. The diagram of the influence mechanism of SA on IFA was proposed.

Within the framework of the second direction, the researches of infrasound changes before large earthquakes and after them were carried out. If to assume, that the preparation of large earthquake is accompanied by small changes in seismic activity on the large territory, these changes will not be noticeable during seismic measurements at one point. In IFA the large changes due to their integrated character and due to the dependence on many factors, accompanying preparation of earthquakes on the large territories, can be observed. The characteristic changes of the spectrum and phase diagrams of infrasonic fluctuations in the atmosphere before large earthquakes are found out as a result of the carried out researches.

When performing researches in the third direction the numerous experiments on observation of the electromagnetic responses on acoustic disturbance in the atmosphere are carried out [4]. As a result of use of mobile acoustic radiator the effect of increase in acoustic disturbance in the atmosphere was found [5]. The connection of infrasound with geomagnetic variations is was experimentally proved.

Infrasonic waves in the ATMOSPHERE And SOLAR ACTIVITY

It is possible to name some sources of energy of a space origin inducing acoustic fluctuation in the Earth atmosphere. They are: the gravitational influence of the Moon and the Sun, the fall of meteors etc. The researches of connection of IFA with SA were carried out.

Proceeding from this fact, that the maximal values of infrasound amplitudes were observed at the moment of SA decrease, the hypothesis was put forward, that the infrasound level in the atmosphere depends on galactic cosmic rays (GCR). 

In Fig. 1 the changes of annual energy of the infrasound and SA for the period from 1997 to 2000 are shown. The maximal annual energy of the infrasound was observed in 1997, when SA was minimum. Similar was observed during short-term (5-10days) changes of SA. The infrasound measurements were carried out at the Western regional centre of the special control NSAU at a point with coordinates 48°41¢ N, 26°30¢ E.

The connection of SA with infrasound in the atmosphere can be presented as follows (Fig. 2). 

The SA changes result in modulation of GCR. The modulated flow of GCR during interaction with the bottom atmosphere changes its transparency by formation of aerosols and variations of small compounds of atmosphere (NO₂, H₂O, O₃ etc.). The changes of the optical transparency lead to the spatial variations of the solar energy absorption in the atmosphere. As a result in various zones of the atmosphere the temperature gradients and thermal instabilities, inducing acoustic fluctuation are formed. The formed infrasound can influence the fluctuation of the intensity of space beams interaction with atmospheric aerosols. In Fig. 2 the introduction of reverse connection is shown. The infrasonic fluctuations can strengthen modulation of the transparency and the effect of opto-acoustic transformation in the atmosphere.

According to proposed diagram of the infrasound generation in its spectrum the solar action should be shown. 

In Fig. 3 the spectral density of solar activity and infrasound is shown. The data for the period of 1997-2000 are used. The realizations of infrasound recordings were centred also smoothed. The index F10.7 was used for SA evaluation. The spectral density and SA are well-coordinated with the range of periods of 24-35 days. At higher frequencies the seismic activity more likely influences IFA. A daily and a half-day periods in infrasonic fluctuations are well observed. The diagrams of spectral density of the hour sums of infrasound modules for the period of about~2 months are shown in Fig.4a,b. In the "quiet" periods two harmonics only (Fig.4a) are shown. In the interseasonal period at the presence of disturbances in the atmosphere there appear the additional harmonics (Fig. 4b).

Thus, spectral analysis of the infrasound has shown the presence of its connection with SA. It is possible to assume, that the influence of SA on biosphere occurs through the acoustic channel. Experimental researches [2] have shown the presence of the infrasound influence on electroconductivity and viscosity of solutions. The infrasound influence on capillary processes is also revealed.

CONNECTION of SEISMIC ACTIVITY With infrasound IN THE ATMOSPHERE

The second factor that significantly influences IFA, is the seismic activity. The influence of seismic activity on IFA is very a complex process and does not consists only in piston radiation of oscillating lithosphere plates. It is necessary here to take into account the various physicochemical processes both in lithosphere, and in atmosphere. The IFA can be caused by gas release from lithosphere cracks during increase of seismic activity, oscillations of lithospheric plates, aerosol heterogeneities in the atmosphere.

The IFA can create alternate stresses on the Earth surface and penetrate at significant depths in lithosphere. Penetrating into lithosphere the infrasonic fluctuation influence the speed of the movement of fluids, telluric electrical fields and also on local seismic fluctuations. Such processes occur on the large territories and can render the essential influence on seismic activity. Thus, the infrasound in the atmosphere can be both the result of seismic fluctuations and also actively influence them.  The character of the interchange oscillatory energy between lithosphere and atmosphere can indicate the processes of preparation of large earthquakes.

To investigate the acoustic channel of lithosphere -atmospheric connections two indexes of seismic activity were introduced. First one is proportional to the square of the maximal magnitude in the given day in the given region, the second – to the square of the sum magnitudes of all seismic events with magnitude ³3 for day in the given region. Two regions were considered. One with the sizes longitude 10° -45°E and latitude 35° -60° N, and the second: longitude 10° -55° E and latitude 20° -60° N. The first and the second regions included the basic zones of the increased seismicity of Central and  East Europe , and also  Turkey . The infrasound was measured at the point with coordinates 48°-41° N, 26°-30° E.

The connection of seismicity with the infrasound for the period 1997-2000 was analysed. The spectral characteristics infrasound and seismic activity agree well.  

In Fig.5 the spectral density of the daily energy of the infrasound and seismic activity for the period of 1997-2000 are shown. 

The investigations have shown, that the infrasonic fluctuations "are sensitive" to the seismic activity changes in radius up to 2000km. The optimum size of the radius of this area of seismic activity account is in the range of 1000-1500km. 

The greatest interest represents the analysis of IFA before catastrophic earthquakes in the region close to the point of infrasound measurement. 

In Fig.6 the dynamics of the change of spectral density of the infrasound envelope before catastrophic earthquake in  Turkey  on  August 17, 1999 is shown. Approaching the moment of the earthquake the infrasound spectral density changes, and many spectral components not typical of the “quiet” period appear. In “quiet” time in the spectrum density two components with the periods 24 and 12 hours are observed which, most likely, are caused by the change of solar radiation (day, night). When approaching the moment of earthquake the daily characteristics of infrasound also change.

To analyze the seismic activity the phase portraits widely used in the theory of dynamic systems were used. In Fig.7 the phase portraits of seismic activity for the period of 1997-2000 are shown. On the abscissa axis the values of the index of seismic activity (in the given case the normalized value of the maximum magnitude square in the given region) are put. On the ordinate axis the normalized derivative of the function constructed on the values of the seismic activity index is given. 

As it is visible from Fig.7 the phase trajectories are attracted to the compact area. Only during catastrophic earthquakes the phase trajectory leaves the attraction area. And, the phase trajectory going out from the attraction area and its penetration into the dangerous area (dotted lines in the picture) do not proceed instantly. It occurs for some days. In Fig.8 the phase portraits of seismic activity and infrasound before the catastrophic earthquake in   Turkey on  August 17, 1999 are shown. The phase portrait before the earthquake looks like a developed spiral. After the earthquake the phase trajectory comes back into the area of attraction. It is interesting to note that the phase portrait of changes of the infrasound daily energy behaves similarly (Fig.8b).

ACTIVE ACOUSTIC EXPERIMENTS IN THE ATMOSPHERE

The important research direction of the connection IFA with processes in geospheres is the artificial acoustic disturbance of the bottom atmosphere, and subsequent observation of the change of various geophysical fields. For acoustic disturbance modeling the large ground explosions were used. In such a way the researches on the influence of ground acoustic disturbance on ionosphere were carried out. The convincing facts confirming the influence of ground explosions on ionospheric plasma were received. However, recently many researchers put under doubt the possibility of the acoustic wave passage at ionospheric heights. They doubt not the fact of the acoustic waves influence in troposphere on ionospheric plasma, but only the direct passage of the acoustic wave into ionosphere. These doubts are based on the large attenuation of acoustic waves in the atmosphere during their vertical distribution owing to exponential fall of pressure with height. The pressure decrease causes the increase in the wave amplitude, transformation of its shape in a triangular one and, accordingly, the occurrence of nonlinear mechanisms of energy dissipation.

The purpose of the given research direction was the search of possible mechanisms explaining the occurrence of the electromagnetic responses to ground acoustic disturbance and creation of tools for realization of experimental tests.

For researches of the influences ground acoustic disturbances on system the atmosphere– ionosphere two acoustic stands are created: a stationary (Fig.9a) and a mobile (Fig.9b).  

The created equipment has allowed us to make the experiments considerably cheaper (in thousand times) in comparison with the explosive methods of the acoustic disturbance of the ionosphere. The stationary acoustic stand was used for the extensive experimental researches and the basic laws of the electromagnetic responses to acoustic disturbance of the atmosphere were established [4].

In Fig.10 the typical changes of geomagnetic variations in the range of 1-40Hz are shown. On the diagrams the energy of a magnetometer signal in the range of 1-40Hz before and after acoustic disturbance is presented. The character of signals changed for a durable time. 

In some cases the signal energy grew (Fig.10b), in others decreased (Fig.10а). These signals are not compatible with the Galperin-Hayakava well-known model. It is impossible to explain the signal decrease after acoustic disturbance by the presence in the atmosphere of only the mechanism of amplification.

To explain such strange behaviour of the signals - responses the hypothesis was put forward, that in the atmosphere there exist own generators of the infrasound. In some cases, under external acoustic influence, they strengthen the infrasound generation, and in others - the external acoustic influence destroys the atmospheric generators. In the atmosphere there is an original acoustic “trigger” mechanism. For this purpose it is necessary to assume, that in the atmosphere there is a hidden energy. In fact, the atmosphere cannot be considered as a passive environment consisting of the gases mixture. In the atmosphere there are chemical reactions, phase transitions of water, ionization under action of solar radiation and weathering of the radioactive dust from the Earth surface. All these processes result in spatial variations of release and absorption of energy in the atmosphere and formation of thermal instabilities. Spatial thermal heterogeneity in certain conditions can result in generation of the infrasound in the atmosphere. Local changes of atmospheric environment during the influence of the external acoustic wave can cause the additional release of thermal energy and, accordingly, amplification of acoustic waves generation. In this situation energy from environment passes in the acoustic wave. This process can be considered as the original acoustic “laser”. It is necessary to note, that the frequency of atmospheric acoustic fluctuations caused by external acoustic influence will not coincide with frequency of the influence. The external acoustic influence is a “trigger hook” for start of the large-scale atmospheric processes. In this case the energy from internal reservoirs of the atmosphere passes into acoustic fluctuations. In certain situations the external acoustic influence can destroy the atmospheric generators. In this case a level infrasound should decrease and, accordingly, the electromagnetic signals - responses also decrease. Three possible mechanisms of amplification of the infrasound in an atmosphere are offered [5].

The large-scale experiments related with observation of the atmospheric infrasound before and after powerful acoustic influence of duration of 1-3min were carried out. The mobile acoustic emitter with electric drive (560kVt) was established at a distance of ~300m from the infrasound measuring system. The characteristic changes in the infrasound amplitude and geomagnetic variations before acoustic disturbance (Fig.11) were found.

 The short-term acoustic influence resulted in the increase in the infrasound amplitude (Fig.11b), or in its decrease (Fig.11a). The synchronous measurements of geomagnetic variations have shown, that they change in a similar way (Fig.11). With the increase of the infrasound intensity the geomagnetic variations also increase (Fig.11b) and vice versa (Fig.11а). The spectral analysis has proved, that in the range of the periods from tens of minutes up to tens of seconds the spectra of the infrasound and geomagnetic variations have the identical character. The low-frequency part of the infrasound spectrum and of the geomagnetic variations is shown in Fig.12.

Thus, it was experimentally proved, that the short-term acoustic influence of high intensity changes the character of infrasonic fluctuations in the atmosphere for a long time. Reaching the ionospheric heights, the infrasonic fluctuations influence the ionospheric electrical currents and cause the changes of the geomagnetic field.

CONCLUSIONS

1.   The analysis of the infrasound spectrum for the period of 1997-2000 has shown the presence of frequencies with the periods typical of solar activity of 27 days, 24 hours, 12 hours. The energy of infrasound grows with decrease in the solar activity. The possible mechanism of the influence of solar activity on the infrasound in the atmosphere is proposed.

2.   The changes in the infrasound spectrum caused by lithospheric processes were found. During 5-10 days prior to large earthquakes the spectrum of infrasonic fluctuations in the atmosphere changes essentially, that can become a basis for creation of a new method of the earthquakes prediction. The phase diagram of the infrasound before a large earthquake looks like an untwisted spiral. 

3.   The effect of amplification a sound in the atmosphere was found for the first time. Short-term (~60sec) intensive acoustic influence on the atmosphere results in increase of the infrasonic fluctuations, which fade during a long time. Three possible mechanisms of this effect are proposed: condensation, chemical and relaxation of nonequilibrium warmed-up gas. In general, it can be formulated as the presence of nonequilibrium states in the atmosphere, which under the acoustic waves influence generate infrasound in the wide frequency range. 

4.   The range of frequencies is experimentally determined, in which the infrasound influences the ionospheric currents. The spectral densities of geomagnetic variations and infrasound coincide in the range of periods from 1 to 20 min.

5.   It is worth conducting the further active acoustic researches of ionosphere by synchronous ground and satellite measurements of the electromagnetic responses to ground acoustic disturbance. Such researches will allow us to prove the possibility of creation of the system of satellite monitoring of the Earth processes, which are accompanied by intensive acoustic radiation.

6.   It is worth expending the researches of the influence infrasound on biospheric processes and biological objects at the different levels of organization. It is possible, that by means of infrasound the of solar activity on the Earth influences the biosphere.

REFERENCES

1.   Negoda A.A., Soroka  S.A. The acoustic channel of space influence on biosphere of the Earth. Space science and tecnology, 2001, v.7, #5/6, p 85-93.

2.   Soroka S.А Solar activity and infrasonic fluctuation in the atmosphere of the Earth. The theses of the reports of the third All-Russia scientific conference “ Physical problems of ecology (ecological physics) ”.  Moscow, 2001, p. 48-49.

3.   Znak Z.О., Negoda А.А., Soroka S.А., Acoustic fluctuations in an atmosphere as the possible channel of space influence on biosphere. The theses of the reports of the IInd Ukrainian conference on perspective space researches. Kathsively, 2002, p. 152.

4.   Kalita B., Mezentcev V., Soroka S. Electromagnetic responses during acoustic disturbance in atmosphere. III International Workshop on Magnetic and Electromagnetic Methods in Seismology and Volcanology (MEEMSV-2002).  Moscow, 2002, p.205.

5.   Soroka S.A. To Galperin-Hayakava Model of Influence of Ground Acoustic disturbance on Ionosphere. International Symposium in memory of Professor Yuri Galperin. Aureal Phenomena and Solar-Terrestrial Relations.  Moscow , 2003, p.101.