We develop theoretical basics of active experiments with two beams of
acoustic waves, radiated by a ground-based sound generator. These beams are
transformed into atmospheric acoustic gravity waves (AGWs), which have parameters
that enable them to penetrate to the altitudes of the ionospheric E and F
regions where they influence the electron concentration of the ionosphere.
Acoustic waves are generated by the ground-based parametric sound generator
(PSG) at the two close frequencies. The main idea of the experiment is to
design the output parameters of the PSG to build a cascade scheme of
nonlinear wave frequency downshift transformations to provide the necessary
conditions for their vertical propagation and to enable penetration to
ionospheric altitudes. The PSG generates sound waves (SWs) with frequencies
The generation of low-frequency sound waves in advance of earthquakes forms a
part of the modern description of the processes in the upper crust. A paper
by Xia et al. (2011) described 240 abnormal infrasound signals that have been
observed in Beijing and, in total, 92 cases worldwide for earthquakes with
During the last 25 years the presence of this phenomena occurring in both the ionosphere and atmosphere before earthquakes has been found. These phenomena are, in particular, variations in total electron content (TEC) (Liu et al., 2014), variations in the surface thermal anomalies (Genzano et al., 2007), increase and anomalous variability in outgoing long-wave radiation (Ouzounov et al., 2007), and the abnormal release of the heat energy (Kafatos et al., 2007). There are two main mechanisms of seismo-ionospheric phenomena, one of which is connected with electromagnetic and the other with atmosphere acoustic and internal gravity waves (Hayakawa, 2015; Pulinets and Boyarchuk, 2004). In addition, a mixed electromagnetic–atmospheric gravity wave mechanism may be important. In particular, atmosphere gravity waves can be excited due to the above-mentioned thermal anomalies. To realize the possibility of earthquake prediction, it is important to understand the mechanisms of seismo-ionospheric coupling, in particular, to distinguish the two possible mechanisms mentioned above. The present investigation proposes the mechanism of the penetration of atmospheric acoustic gravity waves from the lower atmosphere into the ionosphere. It is very useful to look, both theoretically and experimentally, how controllable sound from a ground-based generator would penetrate to the ionospheric altitudes and what effects they would cause in the ionosphere.
The acoustic lithosphere–ionosphere coupling is increasingly considered as one of the important factors of seismo-ionospheric phenomena (Sorokin and Hayakawa, 2013; Klimenko et al., 2011). The need for modelling of powerful disasters such as earthquakes (Gokhberg and Shalimov, 2000) has produced many studies of the acoustic effects in the ionosphere of powerful nuclear and technical explosions. In particular, the effect of the transformation of acoustic perturbations into electromagnetic waves was clearly demonstrated in the experiment MASSA (Gokhberg and Shalimov, 2000). However, the interpretation of the collection of natural and technical acoustic impacts on the ionosphere is quite diverse.
In particular, as shown in the paper Zettergren and Snively (2013), acoustic waves generated by
tropospheric sources may attain significant amplitudes in the ionosphere,
achieving temperature and vertical wind perturbations on the order of
approximately tens of kelvins and metres per second throughout the E and
F regions. The perturbations of the total electron content are predicted to
be detectable by ground-based radar and GPS receivers. Acoustic waves also
drive field-aligned currents that may be detectable by in situ magnetometers
(Iyemori et al., 2015). A recent paper (Zettergren and Snively, 2015) reports
that the recent measurements of GPS-derived TEC reveals acoustic wave periods
from
Moreover, in order to search the seismogenic effects in the ionosphere, it is necessary to unify the well-developed model of the electromagnetic channel to seismo-ionospheric coupling (Molchanov et al., 1995; Rapoport et al., 2004b; Grimalsky et al., 1999; Pulinets and Boyarchuk, 2004; Sorokin and Hayakawa, 2013) with the model of the acoustic channel (Gokhberg and Shalimov, 2000; Rapoport et al., 2004a, 2009; Koshevaya et al., 2002; Grimalsky et al., 2003; Zettergren and Snively, 2015; Iyemori et al., 2015). Traditionally, AGWs and electromagnetic waves have been considered as the basis of two main competitive mechanisms of seismo-ionospheric coupling (Sorokin and Hayakawa, 2013; Klimenko et al., 2011; Pulinets and Boyarchuk, 2004). The synergy approach has been put forward recently (Pulinets, 2011). We are also working in the field of the unified description of electromagnetic and acoustic channels of seismo-ionospheric coupling (Rapoport et al., 2014a, b).
This research field is important in relation to natural hazards, being able to provide a new basis for the warning of the occurrence of devastating natural disasters such as volcanic eruptions and earthquakes. The subject of the current paper is the consideration of the acoustic channel in the interaction between the lower atmosphere and the ionosphere. The acoustic channel of seismo-ionospheric coupling can be investigated by means of active experiments on the penetration of modulated intense sound waves from man-made sources from the Earth's surface into the ionosphere. It is connected, in particular, with the role of the frequency transformation of sound excited by the parametric sound generator (PSG), including its experimental and theoretical investigation (Parrot et al., 2007; Koshovyy, 1999; Kotsarenko et al., 1999; Koshevaya et al., 2005).
In Koshevaya et al. (2005), the use of an acoustic generator is as the source
of the acoustic signal that is similar to natural hazards caused by
underground eruptions. This approach may help to investigate the acoustic
signal effects in the ionosphere that should be released by the PSG (Soroka
et al., 2006; Aramyan et al., 2008). In previous papers (Kotsarenko et al.,
1999; Grimalsky et al., 2003; Koshevaya et al., 2005), the theory of the
sound wave penetration produced by the two-frequency PSG has been proposed.
Kotsarenko et al. (1999) described the change in the ionosphere transparency
for radio waves from galactic radio sources that is connected with the
spatial resonance between the periodical variations in the plasma density
caused by AGWs and the radio
waves. The influence of nonlinearity, gravity, dissipation (in the form of
viscosity), non-isothermality, and diffraction on the infrasonic waves
generated by the PSG has been considered (Koshevaya et al., 2005). Important
theoretical results (Koshevaya et al., 2005) have demonstrated the nonlinear
frequency conversion in the course of the penetration of intermediate
frequency sound waves (ISWs) into the atmosphere with further transformation
to AGWs and a change in transparency for radio waves, propagating through the
non-uniform plasma due to
In the present paper, we continue the experimental and theoretical works
(Parrot et al., 2007; Koshovyy, 1999; Kotsarenko et al., 1999; Koshevaya et
al., 2005; Grimalsky et al., 2003) on the penetration of SWs excited by the
PSG into the atmosphere and ionosphere. We formulate and solve the new
problems not considered in previous papers (Koshevaya et al., 2005; Krasnov
and Kuleshov, 2014; Rapoport, et al., 2004). We propose a new hydrodynamic
analytical–numerical model which includes the two-stage (consequent)
transformation of SWs: (1) parametric transformations of two SW packets with
frequencies
We will present in this paper the analytical–numerical model of neutral atmospheric disturbances. Then, the results of the corresponding modelling will be used for the evaluations of some effects caused by AGWs, penetrating from the lower atmosphere into the ionospheric altitudes, on the ionospheric plasma and propagation of the electromagnetic waves in the ionosphere. The comparison with the experiment is connected with the analysis of the propagation time of acoustic gravity waves from the lower atmosphere to ionosphere altitudes as well as the dynamics of the neutral excitations, including their amplitudes and characteristic transverse sizes of the region of hydrodynamic excitations at ionospheric altitudes.
A possible impact of AGWs on the ionosphere has been estimated from the simulated amplitudes of the velocity of the neutral component of the ionosphere in AGWs at different heights and the evolution of transverse sizes of AGW beams. In addition, the experimental results on the increase in the ionosphere transmission/transparency for HF and VLF electromagnetic waves, obtained using the radio telescope URAN (Koshovy et al., 1997; Koshovyy and Soroka, 1998; Koshovyy, 1999; Kotsarenko et al., 1999) and satellite DEMETER (Parrot et al., 2007), are discussed. The increase in HF and VLF electromagnetic/radio wave intensity and ionospheric transparency for these waves are presented by Helliwell (1965) for ground-based measurements and in DEMETER (Parrot et al., 2007; Soroka et al., 2006) by satellite observation methods. We present simple evaluations for the effects in the ionosphere, and the detailed theory of the ionospheric response will be presented in subsequent papers.
Finally, the idea for a future active ground–satellite experiment involving electromagnetic and acoustics signals that are possibly connected with optical glows and effects in aerosol response (Aramyan et al., 2008) is formulated.
The model includes the ground-based sound generator, which produces the
strong acoustic emission with spectrum at the two SW frequencies
The nonlinear propagation of the acoustic pulses in the atmosphere is
described by the following set of hydrodynamic equations (Rudenko, 1995):
To consider the propagation of acoustic waves in the atmosphere and the
ionosphere, it is necessary to take into account the dependencies of kinetic
temperature
The dependencies
Due to the wave acoustic dispersion being small for the circular frequencies
This transform is as follows: the initial (carrier) SW frequencies are
As a result of the nonlinear interaction of the SWs frequency components
The second frequency transform is described by the set of equations with the same form as Eqs. (7) and (8), but the acoustic harmonics are of the intermediate frequency generated under the first frequency transform and the lower frequency component is AGWs. The difference between the first frequency transform and the second one is in the spatial scales and the values of air velocities within the acoustic wave.
Equation (5) is augmented by the boundary conditions to simulate this second
frequency transform:
The double frequency transform
600 Hz
During sound wave propagation, an intermediate wave frequency ISW at the
intermediate frequency
As a result of the first frequency transform, the ISW at the intermediate
frequency
Spatial distribution of the air velocity
Selected results of simulations of the double frequency transform, or the
double frequency down-conversion, are presented in Figs. 2 and 3. One can see
that the air velocities within ULF AGWs can reach the values of
As our simulations have demonstrated, the amplitudes of the air velocity of
ULF AGWs at
The results of simulations of the one-stage frequency transform
25 Hz
Our results have been compared with earlier simulations by Krasnov and
Kuleshov (2014), where the input values of the air velocities were
In this section, we present the experimental results of the PSG-injected sound wave influence on the effective ionospheric radio wave transparency for the different frequency ranges measured by ground-based and satellite instruments.
Panels
Experimental investigations with the use of a radio telescope can be realized
both with ground-based sources and with the radio-astronomical method. The
first one is the method of the scattering of HF radio waves by small-scale
inhomogeneities of the electron concentration within the ionosphere. This
method uses remote radar as HF (MHz) transmitter and the radio telescope
URAN-3 as a receiver. The set of experiments were conducted in October 1997
(Koshovy and Ivantyshyn, 1998, 1999; Koshovyy,
1999). The signal frequency was
Experimental investigations of artificial acousto-ionospheric disturbances were also conducted using the broadband registration system of the radio telescope URAN-3, which records the dynamic spectra of HF range, or the decametre wave range of the radio emission from galactic radio sources.
The acoustic wave injected by the controlled ground-based acoustic generator, which functioned in a single mode, consists of a sequence of intervals of the acoustic soundings with duration of 60 s, each with a 60 s pause between them.
The results of observations of the galactic background radio emission in the absence of infrasound excitation of the ionosphere (see Fig. 5a) and during the excitation (see Fig. 5b) are presented below. Measurements of the galactic background radio emission were carried out in a selected area of the celestial sphere, which was chosen to account for the locations of the radio telescope URAN-3 and the acoustic generator, as well as the estimated height that would be achieved by the waves.
Figure 5a shows the experimental record of the dynamic radio spectrum of galactic background in the band 20–30 MHz (the upper part in Fig. 5a) and its time profile at a frequency of 24 MHz (the lower part in Fig. 5a). These measurements were carried out on 24 November 2013 in the absence of infrasound excitation. Figure 5b shows the similar data record from 28 November 2013 in the period from 07:00:00 to 08:00:00 UT with infrasound excitation. The start time of the infrasound excitation (07:07:00 UT) is represented by the yellow arrow. The acoustic generator was operated in the mode of the three 60 s periods of acoustic injection with a 60 s pause in between.
The recording of the galactic background radio emission in the absence of infrasound excitation (see Fig. 5a) was characterized by a fairly uniform level of background radio emission across the band of 20–30 MHz.
After
After
The results of the observations of the artificial acoustic modification of the ionosphere obtained in the experiment 28 November 2013 are in good agreement with investigations provided by the Lviv Centre of the Institute of Space Research and the Physical–Mechanical Institute, Ukraine, of artificial acousto-ionospheric disturbances during November and December 1996 and April and October 1997 using the radio telescope URAN-3 at 25 MHz and the controlled ground-based acoustic generator (Kotsarenko et al., 1999; Koshovyy and Soroka, 1998; Koshovyy et al., 2005).
Experimental record of the galactic radio emission has been observed
with the correlation radiometer of the HF radio telescope URAN-3 at
25 MHz (1997). The figure shows a module of the mutual correlation function
Measurements of the galactic radio emission have been obtained by the correlation radiometer of the HF radio telescope URAN-3 at 25 MHz as illustrated in Fig. 6. The pulsed bursts of radio signals were revealed by processing the data registered during 24 sessions of the measurements. The bursts have been identified as a reaction of the ionosphere to the infrasound excitation (see Fig. 6). The time delay of the ionospheric response relative to the start of the acoustic sounding is the main evidence in this method.
According to the statistical processing of these results, a series of the
time delay variations in the detected atmospheric effects have been obtained:
5.8
Time delay variations in the detected atmospheric effects:
confidence intervals for the probability distribution
The results of the statistical processing in MATLAB are presented in Table 1.
Here
A histogram of the time delay of the ionospheric response relative
to the start of the acoustic generation. According statistical processing
results, a series of time delay variations in the detected atmospheric
effects have been obtained:
The time delays of the revealed ionospheric disturbances steadily correlate
with the propagation time of the sound packet up to the ionosphere. We have
interpreted this as evidence that the ground-based acoustic generator creates
the artificial acousto-ionospheric disturbances. To confirm this, the
following parameters were taken into account:
the differences in
recordings of galactic radio source signals in the presence of the acoustic
excitation compared with the same measurements without it; the first
reaction of the ionosphere taking place with a time delay that corresponds to
the time of vertical propagation of this wave to the altitudes of the order
of the repetition of the ionosphere reactions across a
relatively stable series of time delays: 5, 20, 30, 40, and 60 min; the
reproducibility of the time pattern of the ionospheric reaction for various
galactic radio sources.
The delay time of the ionosphere response to acoustic signals from the ground
generator occurs in one or more of the ranges, as we measured. However, the time lag between switching on the generator and the appearance
of a response from the signal from a third party would be random. Moreover,
in view of the independence (lack of physical links) of such events, we can
assume an equal probability for the occurrence of small delays (e.g. equal
to zero) as well as large delays (e.g. equal to 24 h). A histogram of
the time delay variations was constructed relative to the time sequence of
switching on the radiator. Although this sequence is selected randomly (on
different days, at different times of the day), the delays nonetheless turned
out to be grouped in each case. Grouping is present in these cases and
reveals the presence of a non-random component in the delays. If the radiator
is absent (or is off), we must select an event with respect to which we will
determine the delay. We note that, in almost all experiments carried out, a
calibration session was carried out before in which the same measurement was made with the generator switched off. Therefore, a direct comparison of
the measurement results of the transmitted electromagnetic field in cases of
the presence and absence of influence of the parametric acoustic generator on
the ionosphere can be made using Fig. 5a and b.
Time distribution of the whistler density before (00:00–01:00) and
after (01:00–02:00) the acoustic disturbance (time is measured relative to
the beginning of epoch). AD is the moment of acoustic disturbance.
Distribution (
The time distributions of the intensities of VLF range signals registered on board the DEMETER satellite, normalized to the corresponding maximum values:
It is known that the lower boundary of the ionosphere acts as a shield for the VLF range of 3–30 kHz electromagnetic wave propagation. Whistlers are natural representatives of VLF electromagnetic waves. They are created when electromagnetic pulses emitted by lightnings enter the magnetic flux tube of the Earth's magnetosphere (ducting). Electromagnetic pulses passing through magnetic flux tubes are subject to the dispersive transformation or dissolution into the frequency set, where lower frequencies propagate slower than higher frequency waves (Helliwell, 1965). In the Lviv Centre of the Institute of Space Research, a study of acoustic disturbances in the atmosphere interacting with VLF electromagnetic waves passing through the ionosphere from above was conducted. Signals were recorded by a ground-based VLF receiver and analysed by comparing the whistler density occurring throughout 1 h before the acoustic generation was switched on and 1 h after. Figure 8a, which shows the time distribution of the whistler density before and after the acoustic sounding was switched on, was created with the superposed epoch analysis method (Lühr et al., 1998) for 1 s accounts for data from 32 experiments, which were carried out in 2004 from 15 February to 20 May during the second half of day. The result of processing has made it possible to distinguish the non-random stable character of the reaction of the ionosphere to the artificial acoustic influence. The time interval from the 5th to the 50th minute after the start of the acoustic sounding is characterized by the whistler density increasing. This suggests an increase in the ionospheric transparency for VLF electromagnetic waves. The 5 min delay of the response (Fig. 8a) is due to the time span required for the acoustic wave propagation from the Earth's surface to ionospheric altitudes. This further suggests that the acoustic disturbance influences the ionospheric transparency and provides an opportunity for VLF waves to propagate from the magnetosphere to the Earth's surface. To exclude an influence of possible single abnormally large events on the results of the measurements, median filtration (Rabiner, 1978; Arce, 2005) has been applied to the 1 min counts of the intensity of whistlers; see Fig. 8b. The median values of the intensity of whistlers are depicted at the vertical axis from the set of 32 experiments. In Fig. 8a and b, an increase is seen in the intensity of whistlers and the density of their occurrence after the acoustic sounding during approximately 40 min, which can be explained by an increase in the transparency of the ionosphere after the artificial acoustic perturbation.
An increase, after the acoustic impact on the atmosphere, in the number of the whistlers recorded per unit time (for 1 min) is evaluated statistically. A statistical significance
test of differences between mean values is applied (see in Waerden, 1971). It
is concluded that the increase in the number of whistlers after the acoustic
impact is of a non-random nature with an estimated probability
The intensity of VLF waves travelling in the opposite direction (penetrating into the ionosphere from the ground level) should also increase. To investigate this hypothesis, a test observational series with satellite DEMETER has been carried out (Parrot at al., 2007; Soroka at al., 2006). The VLF wave signal from navigational stations RSDN-20 (“Alpha”) with a frequency of 12 648.809 Hz has been used as a benchmark. In Fig. 9 the examples of the intensity distribution of VLF signals registered on board DEMETER are shown. The start of the acoustic sounding was chosen to be 10 min before the bypass of the satellite over PSG. This time is enough for AGWs to reaching the ionosphere. The time moment of the bypass of the satellite is marked by the vertical arrow in Fig. 9a–c. The intensity of VLF waves after the start of the acoustic generation (Fig. 9a, b) is significantly larger than in the absence of the acoustic generation (Fig. 9c). We compare the signals received during the DEMETER satellite flight over the Vrancea earthquake zone in Romania (130–210 s in Fig. 9b) with satellite signals registered later over the artificial acoustically disturbed zone (320–380 s in Fig. 9b). The comparison shows that natural and artificial acoustical disturbances are similar in shape, which allows us to assume the similarity of the influence of both disturbance sources on the ionosphere. This result can provide evidence that supports an idea and a new instrument for the study of seismic phenomena using satellite data.
As a result of Sect. 3.2, we conclude that acoustic disturbances in the atmosphere may cause a change in the ionosphere transparency.
We discuss the influence of the acoustic waves excited by a ground-based generator on the ionosphere. Acoustic waves cause an increase in transparency of the electromagnetic waves in HF (MHz) and VLF (kHz) frequency bands. This conclusion is revealed on the basis of experiments with the effect detected from ground-based and satellite instruments.
The sound waves' propagation time to the altitudes of E region and of the lower edge of the F region is equal to 5–7 min. It was shown experimentally that the corresponding group delay for electromagnetic waves in the HF range has the smallest delay dispersion. That corresponds to the expected direct influence of the sound waves on the ionosphere without any hypothetical accumulation effects. These direct effects could be, for example, a deformation or altitude shift of corresponding ionospheric layers, which can cause a change in characteristics of the wave propagation.
In addition, the Doppler shift corresponding to the acoustic oscillations was recorded in the ionosphere simultaneously with variations in the electromagnetic HF waves reflected from the ionospheric inhomogeneities. An increase in reflection can be caused by the deformation of existing inhomogeneities and by the shift due to acoustic gravity waves penetrating to the ionospheric altitudes, after transformation of the sound waves generated by the PSG.
The atmospheric acoustic events are accompanied by many changes in external
conditions which occur in the real atmosphere during the packet (60 s)
propagation. Then, the total energy released during our experiment (
The successful experiment outcomes in our study, connected with the
transparency increase for electromagnetic waves, depend on a number of
factors:
the acoustic power of the generator; the difference
frequency, which is formed in the area of the parametric antenna; a time diagram of the signal during its generation; the selected measurement method; the presence and degree of dynamic processes development in
the neutral atmosphere and ionosphere.
There were three series of experiments with approximately the same conditions in each series, including 32 experiments with VLF (kHz) electromagnetic waves/whistlers of natural origin (Fig. 8), 20 experiments with VLF (kHz) observations by the DEMETER satellite, and 24 experiments with galactic radio sources (in MHz range). The total number of
active sessions of measurements is 76 cases, when the appearance of
disturbances in the ionosphere and its delay determination has been
registered. Each active session was preceded by a session of calibration
measurements in which the local state of the ionosphere was characterized by
the sound generator being turned off.
Some outcomes supporting the general conclusions on influence by AGWs on the ionosphere from the PSG are also drawn from the case studies of (1) registration on the satellite DEMETER of kHz radiation generated by the ground-based navigation station (Fig. 9) and (2) the changes in reflection of HF electromagnetic waves, injected by a ground transmitter. This effect was accompanied by the recording of the Doppler shift of the reflected electromagnetic wave, caused by the sound wave from the PSG in the ionosphere (Fig. 4).
Note that in the recent ground–satellite measurements by the CHIBIS satellite (Cheremnykh et al., 2014) we were able to analyse the main factors of influence on the local ionosphere. It has been shown that parameters of the transmitted, reflected, and scattered radio signal are well correlated, in particular, in terms of the delay time, with parameters of the acoustic injection, as well as that the experiments recorded weak ionospheric disturbances caused by this radiation.
We can consider the possibility that, instead of artificial excitation, we registered the response from natural sources, in particular from lightning. The arguments which support the artificial reason of the ionosphere impact from the PSG are as follows. The PSG creates an acoustic signal of the artificial waveform. It differs from most natural signals in their time characteristics. First, natural sources like thunderstorms have an exposure time less than 1 s, but one packet in PSG signal has a duration of 60 s. Secondly, the PSG creates a series of signals with three packets having the 60 s interval between submissions. However, the appearance of three lightning strikes with the exact intervals between them of 60 s is very improbable.
As seen from Fig. 6, time delays in the ionosphere response less than 4 min are practically absent. Based on our even relatively small set of observations we can conclude that the ionospheric disturbance appeared exactly with 5–6 min delay after the PSG start operation. Therefore, due to the special artificial signal waveform, which makes it different from natural and other technical signals, and spatio-temporal characteristics of the ionospheric response, it is possible to assume the existence of an acousto-ionospheric effect created by the PSG.
The ionospheric response to a change in electromagnetic transmittance due to the influence of acoustic waves from the parametric generator is specific and cannot be explained by an influence of the lightning strike. In the VLF waveband (Fig. 8), the time distribution of the number of registered whistlers of a natural origin demonstrates a clear change after activating the acoustic parametric generator with a delay of 6 min. This time corresponds to the delay required for the acoustic gravity waves to reach the E region.
The possibility that the registered effect of the electromagnetic wave transmission changes could be caused by a powerful natural source, such as an earthquake, is considered as well. This case is possible, in principle, if the launch time of the sound generator coincides with an earthquake both in time and space. This is seen from the comparison between the signals shown in Fig. 9c, which concerns electromagnetic VLF signals from the navigation radio station registered on board the DEMETER satellite. Peaks in Fig. 9, presumably caused by infrasound waves from a seismoactive region and the activation of a parametric generator, are shown in Fig. 9b at relative times 170 and 330 s. Those peaks are of the same order of amplitude and similar in shape (bell-like).
Analysis of the experimental results has been carried out as well with regard to the state of solar activity in order to avoid the effect of solar flares on the ionosphere. Note that the time distribution of ionospheric disturbances caused by natural factors is random, while the responses caused by the acoustic injector have stable time delays relative to the time of the acoustic wave generation.
The simulations and experiments have demonstrated a high efficiency in the
generation of AGWs due to a cascade scheme of the double frequency transform
600 Hz
The present model is quite different from the previous models of the
penetration of sound waves from the lower atmosphere to ionospheric
altitudes. In particular, in contrast to Krasnov and Kuleshov (2014),
(1) effects of gravity are taken into account and (2) two-stage frequency
transforms in two ranges of altitudes (600–20 and 20–0.01 Hz at
0–100 m and from 100 m to
Experimental investigations of artificial acousto-ionospheric disturbances were also conducted using the broadband registration system of the radio telescope URAN-3, which records the dynamic spectra of HF range, or decametre wave range of the radio emission from galactic radio sources.
In the experiments described in Sect. 3 of the present paper, the observed
minimum time delay of 4 to 7 min corresponds to the time taken for signal
propagation to 80–130 km altitudes (see Fig. 1). Our assumption is that an
influence of AGWs on the ionosphere E region with the minimum time delay may
be caused by some relatively low-inertial direct mechanism such as shifting
or deformation of some ionospheric inhomogeneity, without more inertial
hypothetical accumulation effects. This assumption is supported by the fact
that the corresponding delay in the response to HF waves in the ionosphere is
characterized by the minimum dispersion (see Fig. 7 and Table 1). The
modulation of the intensity of the radio waves (20–30 MHz) and the
variations in the intensity of whistlers (3–30 kHz), both coming from
above, can be explained by a change in the electron concentration within the
E layer produced by the acoustic signal. The width of the infrasound beams in
the E layer will rise to
Concerning the ionospheric responses with a time delay of about 10–30 min, these may presumably be connected with the processes in the F region of the ionosphere such as the photochemistry processes and the release of energy accumulated in the atmosphere–ionosphere system or dissipation (Emelyanov et al., 2015; Pulinets et al., 2011; Sorokin and Hayakawa, 2014). Another effect concerning a possible delay in the ionospheric response could be re-reflections from the boundaries of the “magnetosphere–ionosphere” or “ionosphere–atmosphere” during bounce oscillations.
For the considered model of the acoustic generator the amplitudes of the air
displacements within the resulting AGW are
On the basis of this and previous works, we would like to propose an
Acoustic active experiment proposed for verification of the model
and mechanisms of the influence of the acoustic soundings on the ionosphere.
Acoustic generator (
The experiment would include the impact from a ground-based acoustic generator towards the ionosphere, which includes the measurements of associated electromagnetic and acoustic fields, effects in aerosols on the sound wave attenuation/amplification, optical measurements of associated atmospheric glows, and radio physical measurements of the attenuation/amplification of emissions from galactic radio sources with a set of ground-based instrumentation. During the experiment (see Fig. 10) the acoustic generator will operate at two close frequencies. The signal at the differential frequency is within the infrasound acoustic wave band and, due to the nonlinear interaction in the atmosphere, propagates to the ionosphere. The acoustic receiver (acoustic spectral analyser) located at the same distance from the source detects the scatter by the atmospheric acoustic waves. The interaction area is the ionospheric local region disturbed by acoustic waves. The HF ionospheric vertical sounder (ionosonde) will be used to obtain ionospheric electron concentration measurements up to the F layer. The atmospheric light radar (lidar) will be used to measuring tropospheric parameters including aerosol height distribution and acoustic wave disturbances in the atmosphere. The radio telescope URAN-3 will provide measurements of radio emissions from sky radio sources (astronomical sources of radio emission including quasars and pulsars) in the HF band and ionospheric disturbances, including an influence of acoustic waves on radio wave propagation. The radio transmitter/receiver (radar in Fig. 10) allows detection of possible disturbances in the upper atmosphere/ionosphere. The aerosol sun photometer provides registration of the total aerosol content in the atmosphere when using the Sun's radiation, and the optical scanning photometer (airglow photometer) and low-light-level TV camera will record optical emissions in the lower ionosphere. Satellite instruments will detect (in situ and remotely) the atmospheric and ionospheric disturbances produced by acoustic waves.
The goal of the parametric two-frequency acoustic generator experiment is to check in practice the theoretical assumption on propagation and dissipation of acoustic waves in the atmosphere. The first step in the experiment development is the detailed study of the properties of the acoustic source. We have to estimate the frequency spectrum of the generator and its acoustic power to map velocity and pressure fields and to provide a detailed study of nonlinear transformation of frequencies in the atmosphere. When the frequency combination is generated, it is important to determine the power of radiation at the differential ISW frequency. The next step is the detailed study of the possible influence of the acoustic emission on the atmospheric and ionospheric constitutions. In previous experiments the change in the atmospheric aerosols state was measured under the influence of the acoustic source radiation. The aerosol sun photometer is suitable to study the influence of acoustic waves on atmospheric aerosols, if we expect aerosol optical thickness variations (Milinevsky et al., 2012; Bovchaliuk et al., 2013; Danylevsky et al., 2011). The ionosonde will be helpful for the study of ionospheric modification, as well as the diagnostics of the ionosphere by the URAN-3 HF radio telescope using astronomical radio sources. The study of the ionospheric emissions in visual and near-infrared spectral bands by photometers and low-light-level TV cameras will allow us to detect optical phenomena in the acoustic wave injection. The low-frequency radar and GPS satellite signals will provide additional information on ionospheric disturbances and the ionospheric electron concentration. In situ measurements from satellites at ionospheric altitudes will be important for diagnostics of the near-Earth environment. In general, the proposed future experiment would be able to verify a theoretical model and mechanisms of the influence of the acoustic soundings on the ionosphere.
The influence on ionospheric transparency for the electromagnetic VLF and HF wave caused by the parametric sound-generator-excited acoustic waves has been observed in a number of experiments. This effect was observed for electromagnetic waves artificially excited from the ground and detected at ground-based observatories and satellites in different seasons, days, and times of day. Due to a special artificial signal waveform, which helps to separate it from natural signals, we can consider measured spatio-temporal characteristics of the ionospheric response as the influence of artificial injection. Our analysis shows that the other possible reasons for the observed increase in the ionospheric transparency for VLF and HF waves are practically excluded.
The new model of the penetration of acoustic waves excited by a ground-based PSG up to the ionospheric altitudes has been developed. It is shown that the finite size of the initial aperture, diffraction, dissipation, dispersion, and nonlinearity of the beam determines the characteristics of the AGW, which is finally able to reach the altitudes of the E and F regions.
The time delay of the ionospheric response to the excitation of sound waves by the PSG, as well as the value of the Doppler shift of the HF electromagnetic waves reflected from the ionosphere, has been determined on the basis of the developed nonlinear model. The above-mentioned characteristics correspond well to those obtained experimentally. The modulation of the intensity of the radio waves at frequencies between 20 and 30 MHz and the variations in the intensity of whistlers (3–30 kHz), both coming from above, can be explained by a change in the electron concentration within the E layer produced by the acoustic signal. At least a dimension of the region with changed electron concentration is suitable for the creation of a lens or a deflector for whistlers, as well as for the galactic radio waves. The AGW influence on the ionosphere has been estimated for the cases of the modulation of the ionospheric plasma concentration. The latter effect is shown as an alternative possible reason of the increasing of the ionosphere transparency for the propagation of HF waves.
Some mechanisms of direct and relatively fast impact of the AGW on the ionosphere (such as shift or deformation of the ionospheric inhomogeneities) and corresponding indirect and more inertial impact (such as accumulation) have been mentioned as hypothetical. Due to the complexity of the proposed nonlinear model of the PSG-injected acoustic wave transformation into AGWs and their penetration to ionospheric altitudes, we limited our consideration by simulation of the neutral perturbations only.
The active experiment for investigation of the artificial sound waves impact on the atmosphere–ionosphere system is proposed (in Sect. 4.4). Comprehensive observations during the active experiment with acoustic soundings and our theoretical model would be useful for improving the effectiveness of the controllable acoustic influence on the ionosphere and understanding the mechanisms of seismo-ionospheric coupling. The proposed future experiment based on the developed theoretical model would be able to verify the model results and the mechanisms of the influence of the acoustic waves on the ionosphere. In order to explain some ionospheric phenomena due to a controlled acoustic impact, the proposed theory needs further development and experimental verification, and this is the subject of further research.
In conclusion, based on previous experiments and computer simulation, we can see evidence that the increase in ionosphere transparency for the HF and VLF electromagnetic waves could be caused by the PSG-injected acoustic wave impact on the ionosphere as a result of cascade transformations.
Original experimental data are available by request to the following participants of the experimental teams conducting corresponding experimental studies, via e-mail: in particular, for data on HF measurements contact Oleh L. Ivantishin (o.ivantyshyn@gmail.com), and for data on VLF measurements contact Roman T. Nogach (r.t.nogach@gmail.com) or Valentyn P. Mezentsev (m5z5ncev@gmail.com).
This publication is based on work supported in part by STCU Project 6060. The work was also partly supported by project 16BF051-02 of the Taras Shevchenko National University of Kyiv and by a grant of the State Fund for Fundamental Research, project F73/115-2016. Viktor N. Fedun would like to acknowledge the Royal Society for support received. The topical editor, Christoph Jacobi, thanks O. A. Pokhotelov and the three anonymous referees for their help in evaluating this paper.