**4. Solar eclipse event – signatures in the ionospheric plasma**

It has been proposed by Chimonas and Hines (1970) that solar eclipses can act as sources for AGWs. The lunar shadow creates a cool spot in the atmosphere that sweeps at supersonic speed across the Earth. The sharp border between sunlit and eclipsed regions, characterized by strong gradients in temperature and ionization flux, moves throughout atmosphere and drives it into a non-equilibrium state. Earth atmosphere shows variable sensitivity to the changes of ionization flux.

## **4.1 Experiments**

Solar eclipse event represents phenomenon that can be precisely predicted, hence many observational campaigns are organised around the world. Effects of the solar eclipses on the ionospheric plasma are studied by mean of GPS techniques, radars, vertical ionospheric soundings etc. Study limitations lay mainly in the fact that there are no identical solar eclipse events. Moreover, solar eclipse induced effects are easily to be mixed with effects caused by geomagnetic field variations, diurnal changes of the ionosphere, seasonal variability of the atmosphere/ionosphere etc. In the upper atmosphere, AGWs can be observed either directly as neutral gas fluctuations or indirectly as induced ionospheric

activity including coordinated experiments or unwilling accidents. AGWs influence on the upper atmosphere is not yet understood enough. They produce a great amount of variability and contribute to the background conditions in a specific parcel of the atmosphere. Gravity waves propagating from lower laying atmosphere have been long regarded as a very important source of the energy and momentum transfer in the upper atmosphere (Hines, 1960). The breaking of the upward propagating waves affects wind system, generates

Waves that reach upper atmosphere produce travelling atmospheric disturbances (TAD) or travelling ionospheric disturbances (TID) and even form the ionospheric inhomogenities which grow and finally break into the plasma instabilities observed by radar techniques that might cause scintillation of the communication signals propagating through the ionosphere. From the observation it is evident that the thermosphere is continuously swept by the acoustic-gravity waves. Statistically, the waves show a moderate preference for southward travel, with this preference being reduced or shifted to southeastward travel during disturbed times (Oliver et al., 1997). Experimental studies show that AGW activity in the ionosphere slightly increases during dawn and dusk periods of the day (Galushko et al., 1998; Somsikov & Ganguly, 1995; Sauli et al., 2005 among others). Influence of infrasonic waves generated by ground experimental sources on the ionosphere was reported for

Solar eclipse represents well defined source of the AGW in the atmosphere and ionosphere systems. During solar eclipse event, solar ionization flux decreases producing well-defined cool spot in the atmosphere that moves through the Earth's atmosphere. Moving source in the atmosphere can emit both acoustic and gravity waves. Supersonic motion of the source forms wave field with bow wave. Both acoustic and gravity waves can be radiated in association with supersonic motion in the atmosphere. When the source is moving within atmosphere with subsonic velocity only gravity waves can be emitted

It has been proposed by Chimonas and Hines (1970) that solar eclipses can act as sources for AGWs. The lunar shadow creates a cool spot in the atmosphere that sweeps at supersonic speed across the Earth. The sharp border between sunlit and eclipsed regions, characterized by strong gradients in temperature and ionization flux, moves throughout atmosphere and drives it into a non-equilibrium state. Earth atmosphere shows variable sensitivity to the

Solar eclipse event represents phenomenon that can be precisely predicted, hence many observational campaigns are organised around the world. Effects of the solar eclipses on the ionospheric plasma are studied by mean of GPS techniques, radars, vertical ionospheric soundings etc. Study limitations lay mainly in the fact that there are no identical solar eclipse events. Moreover, solar eclipse induced effects are easily to be mixed with effects caused by geomagnetic field variations, diurnal changes of the ionosphere, seasonal variability of the atmosphere/ionosphere etc. In the upper atmosphere, AGWs can be observed either directly as neutral gas fluctuations or indirectly as induced ionospheric

**4. Solar eclipse event – signatures in the ionospheric plasma** 

turbulence and heats the atmospheric gas.

instance by Rapoport et al. (2004).

(Kato et al., 1977).

changes of ionization flux.

**4.1 Experiments** 

plasma variations. Despite intensive research many questions in the problem of the generation and propagation remain to be understood.

Studies by Fritts and Luo (1993) suggest that perturbations generated by the eclipse induced ozone heating interruption may propagate upwards into the thermosphere–ionosphere system where they have an important influence. Temperature fluctuations and electron density changes propagate as a wave, away from the totality path, cf. Muller-Wodarg et al. (1998). By means of vertical ionospheric sounding, Liu et al. (1998) detected waves excited during solar eclipse event at F1 layer heights and their generation and/or enhancement attributed to changes of temperatures and variations of the height of the transition level for the loss coefficient and the height of the peak of electron production. Studies reported by Farges et al. (2001) suggest a longitudinal diversity of the disturbances with respect to prenoon and postnoon phases. Xinmiao et al. (2010) reported synchronous oscillations in the Es and F layer during the recovery phase of the solar eclipse. Ivanov et al. (1998) found that during solar eclipse with maximum obscuration of about 70% the F-region electron density decreased by 6-8% compared to a control day and detected travelling ionospheric disturbances. Additionally, they detected strong variations in the difference group delays with a period about 40 minutes associated with the start and end of the eclipse. Oscillations in the ionosphere, similar to gravity waves, were observed following some solar eclipse events (Chimonas and Hines, 1970; Cheng et al., 1992; Liu et al., 1998; Sauli et al., 2006). Investigation of the latitudinal dependence of NmF2 (the maximum electron density of the F2 layer) indicated that the strongest response was at middle latitudes (Le et al., 2009). The response of the sporadic-E (Es) layer also differed in each solar eclipse event. A remarkable decrease in Es layer ionization was observed during the eclipse of 20 July 1963 (Davis et al., 1964). Enhancement of Es layer ionization has also been reported and it has been suggested that it is related to internal gravity waves generated in the atmosphere during the solar eclipse (Datta, 1972).

### **4.2 Processes induced by solar eclipse**

During the solar eclipse, on the time scale shorter than day-night change, the ionosphere reconfigures itself into a state similar to that of night situation. Photochemical ionization falls heavily almost to a night-time level. With the decreasing solar flux, atmospheric temperature falls in the moon shadow creating a cool spot with well defined border. Then the increasing solar flux starts ionization processes and warms the atmosphere again to daytime level.

Such changes in the ionization cause variation in the reflection heights, decrease/increase in electron concentration at all ionospheric heights, decrease/increase in the total electron content, rising/falling of the layer height. Such effects are characteristic for the processes during sunrise/sunset in the ionosphere. However, supersonic movement of the eclipsed region represents a key difference from the regular solar terminator motion at sunrise and sunset times. These changes in the neutral atmosphere and ionosphere induced by solar eclipse force the evolution of the ionospheric plasma toward a new equilibrium state. The return to equilibrium is likely accompanied by the eclipse induced wave motions excited in the atmosphere. Any moving discontinuity of gas parameters such as temperature, pressure etc. will generate transit-like waves. In the upper ionosphere, waves can be generated by a strong horizontal electron pressure gradient. Possible mechanisms contributing to the wave generation in the region of solar terminator are in detail discussed by Somsikov & Ganguly (1995).

Acoustic–Gravity Waves in the Ionosphere During Solar Eclipse Events 313

Fig. 3. Sequence of raw ionograms measured by the ionosonde KEL Aerospace IPS 42 at the observatory Pruhonice. During the special campaign ionograms were recorded with oneminute resolution in order to study rapid ionospheric changes during the solar eclipse.

Solar eclipse induces changes in all atmospheric regions extend from the upper atmosphere down to ground level. Despite the low magnitude of the eclipse induced effects at ground level, Jones et al. (1992) reported wave-like oscillation related to eclipse on the microbarometer pressure records. The cooling effect of the Moon's shadow may induce the powerful meridional airflow in the atmosphere, which accelerates the ionized clouds in the Es layer and forms the wind shear to raise the observed Doppler frequency shift and foEs values, respectively (Chen et al., 2010).

## **5. Solar eclipse observed by vertical ionospheric sounding in midlatitudes**

Vertical sounding measurements provide local information on the electron density distribution of the bottomside ionosphere. Electron concentration in the plasma and its corresponding plasma frequency are related via following equation:

$$\text{f}\_p^2 = \frac{\text{Ne}^2}{4\pi\text{e}\_0\text{m}}\tag{25}$$

where fp denotes plasma frequency and N, e, ε0 and m stand for the electron concentration, the charge of electron, permittivity of free space, and the mass of the electron, respectively.

This section summarizes experimental results from the midlatitude ionospheric observatory Pruhonice (50N, 15E). At the observatory, the vertical sounding measurements were performed with ionosonde IPS 42 KEL Aerospace till the end of year 2003. Then this older equipment was replaced by digisonde DPS 4. Special campaigns of rapid sequence soundings were organized in order to study in detail ionospheric behavior during partial solar eclipses of 11 August 1999, 4 January 2011 and annular solar eclipse 3 October 2005. All three analyzed events were characterized by low geomagnetic activity; hence they represent a good occasion to observe mostly solar eclipse induced effects in the ionosphere. However, inconclusive results of the solar eclipse observations rise from the fact that different solar eclipses produce different plasma motions. Indeed, the travel cone geometry and its angular effects on the magnetized plasmas are different for each eclipse.

Solar eclipse of 11 August 1999 (as a total seen in place as close as 200 km from the measurement point) represents so far the event of the highest solar disc coverage observed in the Observatory Pruhonice. Figure 3 depicts sequence of raw ionograms measured during this event by IPS 42 KEL Aerospace equipment. The ionograms were recorded with the cadence of 1 minute. On the ionograms there is clearly seen that the eclipse event affects whole electron density profile. Critical frequencies in the E and F layer decrease before maximum disc occultation and then increase again. The electron density decrease in the E layer is much stronger than in the F layer due to different dominant type of the recombination. Electron density fall and increase occur simultaneously with occultation and de-occultation of the solar disc in the E and F1 layer while the F2 layer electron density reacts with slight delay. There are special structures of the spread F type developed on the profile after beginning of the solar disc occultation (clearly seen on ionograms at 9.14 UT and 9.16 UT). Shape of the F layer is affected as well. Unfortunately, effects in the F1 region cannot be discussed here in details because F1 layer is blanketed by strong sporadic E layer during part of the solar eclipse.

Solar eclipse induces changes in all atmospheric regions extend from the upper atmosphere down to ground level. Despite the low magnitude of the eclipse induced effects at ground level, Jones et al. (1992) reported wave-like oscillation related to eclipse on the microbarometer pressure records. The cooling effect of the Moon's shadow may induce the powerful meridional airflow in the atmosphere, which accelerates the ionized clouds in the Es layer and forms the wind shear to raise the observed Doppler frequency shift and foEs

**5. Solar eclipse observed by vertical ionospheric sounding in midlatitudes**  Vertical sounding measurements provide local information on the electron density distribution of the bottomside ionosphere. Electron concentration in the plasma and its

> 2 p

Ne <sup>f</sup> 4 m <sup>=</sup> πε

where fp denotes plasma frequency and N, e, ε0 and m stand for the electron concentration, the charge of electron, permittivity of free space, and the mass of the

This section summarizes experimental results from the midlatitude ionospheric observatory Pruhonice (50N, 15E). At the observatory, the vertical sounding measurements were performed with ionosonde IPS 42 KEL Aerospace till the end of year 2003. Then this older equipment was replaced by digisonde DPS 4. Special campaigns of rapid sequence soundings were organized in order to study in detail ionospheric behavior during partial solar eclipses of 11 August 1999, 4 January 2011 and annular solar eclipse 3 October 2005. All three analyzed events were characterized by low geomagnetic activity; hence they represent a good occasion to observe mostly solar eclipse induced effects in the ionosphere. However, inconclusive results of the solar eclipse observations rise from the fact that different solar eclipses produce different plasma motions. Indeed, the travel cone geometry and its angular

Solar eclipse of 11 August 1999 (as a total seen in place as close as 200 km from the measurement point) represents so far the event of the highest solar disc coverage observed in the Observatory Pruhonice. Figure 3 depicts sequence of raw ionograms measured during this event by IPS 42 KEL Aerospace equipment. The ionograms were recorded with the cadence of 1 minute. On the ionograms there is clearly seen that the eclipse event affects whole electron density profile. Critical frequencies in the E and F layer decrease before maximum disc occultation and then increase again. The electron density decrease in the E layer is much stronger than in the F layer due to different dominant type of the recombination. Electron density fall and increase occur simultaneously with occultation and de-occultation of the solar disc in the E and F1 layer while the F2 layer electron density reacts with slight delay. There are special structures of the spread F type developed on the profile after beginning of the solar disc occultation (clearly seen on ionograms at 9.14 UT and 9.16 UT). Shape of the F layer is affected as well. Unfortunately, effects in the F1 region cannot be discussed here in details because F1 layer is blanketed

2

(25)

0

corresponding plasma frequency are related via following equation:

effects on the magnetized plasmas are different for each eclipse.

by strong sporadic E layer during part of the solar eclipse.

values, respectively (Chen et al., 2010).

electron, respectively.

Fig. 3. Sequence of raw ionograms measured by the ionosonde KEL Aerospace IPS 42 at the observatory Pruhonice. During the special campaign ionograms were recorded with oneminute resolution in order to study rapid ionospheric changes during the solar eclipse.

Acoustic–Gravity Waves in the Ionosphere During Solar Eclipse Events 315

Fig. 5. Profilogram (height-time-plasma frequency development) during solar eclipse 3 October 2005 as measured by DPS 4. Ionograms were measured every 2 minutes. All ionograms were manually scaled and inverted into true-height profiles using True Height

Fig. 6. Parameters of acoustic-gravity wave structure detected within ionospheric plasma during solar eclipse event 3 October 2005 (Sauli et al. 2007). Panels: wave vector (a), phase velocity (b), packet velocity (c), wave number (d), energy (e) and phase (f) angles. For the vectors of first row, the '□' correspond to the measured (black) and computed (empty) zcomponents, the '○' correspond to the horizontal components while the '∇ ' are related to the

Profile Inversion Tool NHPC.

modulus.

Detail analysis of electron concentration by mean of spectral analysis reveals that within oscillation of electron concentration there occur several clear wave-like oscillations. It has been shown by Sauli et al. (2007) that wavelet spectral analysis is very convenient approach for such wave detection. The advantage of the wavelet based analysis is identification of the structure occurrence time which helps to associate particular wave-like structure to the agent. Figure 4 shows estimated wave parameters for selected structure that is coherent through all studied heights. Parametrization of the wave-like structure is based on AGW approximation described in Section 2. From Figure 4 it is evident that wave originates at height of about 200 km and propagates upward and downward from the source region.

Fig. 4. Parameters of acoustic-gravity wave structure detected within ionospheric plasma during solar eclipse event 11 August 1999 (Sauli et al., 2007). Panels: wave vector (a), phase velocity (b), packet velocity (c), wave number (d), energy (e) and phase (f) angles. For the vectors of first row, the '□' correspond to the measured (full squares) and computed (empty symbols) z-components, the '○' correspond to the horizontal components while the '∇ ' are related to the modulus.

Another representation of the rapid changes in the ionospheric plasma is shown on the profilogram (Figure 5) measured during solar eclipse 3 October 2005 by DPS 4. Decrease in the plasma frequency at all heights is well developed. Within plasma frequency oscillation, several wave coherent structures were found that can be attributed to the eclipse event. These structures occur in the plasma at the maximum of the eclipse and after the event. In all cases we detected a component of upward energy progression. Due to the occurrence time and low geomagnetic activity the detected wave-like oscillations in the ionospheric plasma are likely signatures of bow shock and possibly waves excited by cooling of ozone in the lower laying atmosphere. Estimated velocities for one particular structure are shown in Figure 6.

Detail analysis of electron concentration by mean of spectral analysis reveals that within oscillation of electron concentration there occur several clear wave-like oscillations. It has been shown by Sauli et al. (2007) that wavelet spectral analysis is very convenient approach for such wave detection. The advantage of the wavelet based analysis is identification of the structure occurrence time which helps to associate particular wave-like structure to the agent. Figure 4 shows estimated wave parameters for selected structure that is coherent through all studied heights. Parametrization of the wave-like structure is based on AGW approximation described in Section 2. From Figure 4 it is evident that wave originates at height of about 200 km and propagates upward and downward from the source region.

Fig. 4. Parameters of acoustic-gravity wave structure detected within ionospheric plasma during solar eclipse event 11 August 1999 (Sauli et al., 2007). Panels: wave vector (a), phase velocity (b), packet velocity (c), wave number (d), energy (e) and phase (f) angles. For the vectors of first row, the '□' correspond to the measured (full squares) and computed (empty symbols) z-components, the '○' correspond to the horizontal components while the '∇ ' are

Another representation of the rapid changes in the ionospheric plasma is shown on the profilogram (Figure 5) measured during solar eclipse 3 October 2005 by DPS 4. Decrease in the plasma frequency at all heights is well developed. Within plasma frequency oscillation, several wave coherent structures were found that can be attributed to the eclipse event. These structures occur in the plasma at the maximum of the eclipse and after the event. In all cases we detected a component of upward energy progression. Due to the occurrence time and low geomagnetic activity the detected wave-like oscillations in the ionospheric plasma are likely signatures of bow shock and possibly waves excited by cooling of ozone in the lower laying atmosphere. Estimated velocities for one particular structure are shown in

related to the modulus.

Figure 6.

Fig. 5. Profilogram (height-time-plasma frequency development) during solar eclipse 3 October 2005 as measured by DPS 4. Ionograms were measured every 2 minutes. All ionograms were manually scaled and inverted into true-height profiles using True Height Profile Inversion Tool NHPC.

Fig. 6. Parameters of acoustic-gravity wave structure detected within ionospheric plasma during solar eclipse event 3 October 2005 (Sauli et al. 2007). Panels: wave vector (a), phase velocity (b), packet velocity (c), wave number (d), energy (e) and phase (f) angles. For the vectors of first row, the '□' correspond to the measured (black) and computed (empty) zcomponents, the '○' correspond to the horizontal components while the '∇ ' are related to the modulus.

Acoustic–Gravity Waves in the Ionosphere During Solar Eclipse Events 317

partial solar eclipse. During annular solar eclipse, significant acoustic-gravity wave type

Fig. 8. Virtual reflection heights of plasma frequency range 3.4 – 3.5 MHz derived from raw ionograms. From up to bottom: day before, day of eclipse, day after eclipse. Vertical lines in middle panel depict beginning and end of the eclipse. Time resolution is different for day of

Acoustic-Gravity waves play important role in the dynamic of the upper atmosphere. Vertical ionospheric sounding represents powerful tool that allows us to monitor acousticgravity wave activity in the ionosphere. Ionospheric observation of such a strong event as solar eclipse gives us an opportunity to better understand processes of creation and dissipation of the AGW in the area of the ionosphere. Although the acoustic-gravity waves are always present in the area of our interest, sharp temporally well-defined changes of

It is rather uneasy to unambiguously assess causality between the solar eclipse events and the detected wave structures in the ionospheric plasma. Difficulties result from the fact that there are no two exactly identical solar eclipse events and from limitations of sounding techniques. Despite the fact that various AGW sources have been identified, many others remain to be found. Amongst irregular AGW bursts, regular increase in AGW activity were found to occur around sunrise and sunset hours, excited by Solar Terminator movement. Most of other sources (meteorological systems, geomagnetic and solar disturbances, etc.) and corresponding wave-like oscillations contribute to the irregular patterns of AGW

solar flux during the solar eclipse give us a possibility to define sources of AGW.

bursts develop around and after maximum phase of the eclipse.

solar eclipse (5 min) and days before/after (15 min).

activity observed in the ionospheric plasma.

**6. Conclusion** 

Result of the annular eclipse is significantly different from the case of the total eclipse event of 1999 where the dominant AGW activity took place at the beginning of eclipse. The atmospheric cooling and decrease in radiation flux during an annular solar eclipse is not as strong as during a total eclipse and the ionospheric response occurs with time delay.

Fig. 7. Virtual reflection heights of plasma frequency in the range 4.2 - 4.3 MHz derived from raw ionograms. From up to bottom: day before, day of eclipse, day after eclipse. Vertical lines in middle panel depict beginning and end of the eclipse. Time resolution is different for day of solar eclipse (2 min) and days before/after (15 min).

In Figure 7 and Figure 8, there are plots of virtual reflection height variations at single frequency during three consecutive days, day of solar eclipse event and one day before and after the event. Variation of the reflection height during eclipse event of 3 October 2005 does not differ much from the corresponding variation during reference time span day before and day after. Wave-like oscillations excited by solar eclipse are of comparable magnitude as those induced by other sources preceding and consecutive day. On the contrary, clear difference in reflection height oscillation during reference days and solar eclipse event is perfectly seen in Figure 8. Records of virtual heights at fixed frequency from January 4, 2011 present strong ionospheric response which is exhibited as periodic changes in reflection height. Sharp changes in the reflection height develop immediately after the beginning of the solar disc occultation and last till the end of eclipse event. Higher wave-like activity remains remarkable whole day. In this partial solar eclipse event, wave-like oscillations can be very probably attributed to the solar eclipse.

Strong decrease in electron concentration in practically whole electron profile as well as the wave-like changes were observed during and after August 11, 1999 and January 4, 2011. Wave-like activity develops immediately after the start of the solar disc obscuration during partial solar eclipse. During annular solar eclipse, significant acoustic-gravity wave type bursts develop around and after maximum phase of the eclipse.

Fig. 8. Virtual reflection heights of plasma frequency range 3.4 – 3.5 MHz derived from raw ionograms. From up to bottom: day before, day of eclipse, day after eclipse. Vertical lines in middle panel depict beginning and end of the eclipse. Time resolution is different for day of solar eclipse (5 min) and days before/after (15 min).

### **6. Conclusion**

316 Acoustic Waves – From Microdevices to Helioseismology

Result of the annular eclipse is significantly different from the case of the total eclipse event of 1999 where the dominant AGW activity took place at the beginning of eclipse. The atmospheric cooling and decrease in radiation flux during an annular solar eclipse is not as

Fig. 7. Virtual reflection heights of plasma frequency in the range 4.2 - 4.3 MHz derived from raw ionograms. From up to bottom: day before, day of eclipse, day after eclipse. Vertical lines in middle panel depict beginning and end of the eclipse. Time resolution is different

In Figure 7 and Figure 8, there are plots of virtual reflection height variations at single frequency during three consecutive days, day of solar eclipse event and one day before and after the event. Variation of the reflection height during eclipse event of 3 October 2005 does not differ much from the corresponding variation during reference time span day before and day after. Wave-like oscillations excited by solar eclipse are of comparable magnitude as those induced by other sources preceding and consecutive day. On the contrary, clear difference in reflection height oscillation during reference days and solar eclipse event is perfectly seen in Figure 8. Records of virtual heights at fixed frequency from January 4, 2011 present strong ionospheric response which is exhibited as periodic changes in reflection height. Sharp changes in the reflection height develop immediately after the beginning of the solar disc occultation and last till the end of eclipse event. Higher wave-like activity remains remarkable whole day. In this partial solar eclipse event, wave-like oscillations can

Strong decrease in electron concentration in practically whole electron profile as well as the wave-like changes were observed during and after August 11, 1999 and January 4, 2011. Wave-like activity develops immediately after the start of the solar disc obscuration during

for day of solar eclipse (2 min) and days before/after (15 min).

be very probably attributed to the solar eclipse.

strong as during a total eclipse and the ionospheric response occurs with time delay.

Acoustic-Gravity waves play important role in the dynamic of the upper atmosphere. Vertical ionospheric sounding represents powerful tool that allows us to monitor acousticgravity wave activity in the ionosphere. Ionospheric observation of such a strong event as solar eclipse gives us an opportunity to better understand processes of creation and dissipation of the AGW in the area of the ionosphere. Although the acoustic-gravity waves are always present in the area of our interest, sharp temporally well-defined changes of solar flux during the solar eclipse give us a possibility to define sources of AGW.

It is rather uneasy to unambiguously assess causality between the solar eclipse events and the detected wave structures in the ionospheric plasma. Difficulties result from the fact that there are no two exactly identical solar eclipse events and from limitations of sounding techniques. Despite the fact that various AGW sources have been identified, many others remain to be found. Amongst irregular AGW bursts, regular increase in AGW activity were found to occur around sunrise and sunset hours, excited by Solar Terminator movement. Most of other sources (meteorological systems, geomagnetic and solar disturbances, etc.) and corresponding wave-like oscillations contribute to the irregular patterns of AGW activity observed in the ionospheric plasma.

Acoustic–Gravity Waves in the Ionosphere During Solar Eclipse Events 319

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As the solar eclipses, analyzed in the Section 5, occur sufficiently long time after the sunrise hours, one can assume that none of the reported waves are induced by solar terminator. During the analyzed sounding campaigns, no wave coming from auroral zone was expected, due to the quiet geomagnetic and solar activity. Additionally, meteorological analysis shows that meteorological systems very probably did not influence the ionosphere during studied events by means of AGW. The acoustic-gravity wave activity increases after a notably larger delay for the annular solar eclipse compared to the total solar eclipses: waves are found during the maximum phase of the eclipse only for the former while they occur during the initial phase for the latter. This discrepancy in gravity waves generation/occurrence can likely be explained by differences in the terrestrial atmosphere cooling: the border between sunlit and eclipsed region is much sharper in the case of total eclipse. Analyzing wave propagations, we observe predominantly upward propagating structures. The wave structure, that propagate upward and downward from the source region located around 200 km height, was created during an exceptional case related to the Solar eclipse of 11 August 1999.

### **7. References**


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**7. References** 


**Part 3** 

**Acoustic Waves as Manipulative Tools**

