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Bistatic HF diagnostics of TIDs over the Antarctic Peninsula
ARTICLE IN PRESS
Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 403–410 www.elsevier.com/locate/jastp
Bistatic HF diagnostics of TIDs over the Antarctic Peninsula V.G. Galushkoa,, A.S. Kashcheyeva, S.B. Kashcheyeva, A.V. Koloskova, I.I. Pikulika, Y.M. Yampolskia, V.A. Litvinovb, G.P. Milinevskyb, S. Rakusa-Suszczewskic a
Institute of Radio Astronomy, National Academy of Sciences of Ukraine, 4, Chervonopraporna Street, Kharkov, 61002 Ukraine National Antarctic Scientific Center, Ministry for Education and Science of Ukraine, 16, Tarasa Shevchenka Boulevard, Kyiv, 01601 Ukraine c Polish Academy of Sciences, Department of Antarctic Biology, 10, Ustrzycka Street, 02-141 Warsaw, Poland
b
Received 20 January 2006; received in revised form 26 April 2006; accepted 10 May 2006 Available online 19 January 2007
Abstract Results of HF diagnostics of traveling ionospheric disturbances (TIDs) over the Antarctic Peninsula are presented that have been obtained in the frequency-and-angular sounding technique developed at the Institute of Radio Astronomy, National Academy of Sciences of Ukraine (Kharkov, Ukraine). A specially designed HF transmitter was deployed at the Polish Antarctic station Henryk Arctowski to radiate probe signals. Variations in trajectory parameters (angles of arrival and Doppler frequency shifts) of the probe signals used for recovering TID characteristics were recorded at the Ukrainian Antarctic station Akademik Vernadsky with a compact-size three-channel coherent receiver based on the Doppler interferometry technique. The propagation path length was approximately 440 km. Round-the-clock measurements were performed between 31 January and 16 March, 2004 at frequencies 5 to 7 MHz, depending on the ionospheric conditions. The total time of observations was about 1000 h. Results of preliminary processing of the measured data are reported. Characteristic parameters of the disturbances are presented and possible sources of their generation are discussed. r 2006 Elsevier Ltd. All rights reserved. Keywords: Traveling ionospheric disturbances; Frequency-and-angular sounding; Ionosphere; Sources of TID generation
1. Introduction Atmospheric gravity waves (AGW) are among the basic transport agents of energy exchange between the neutral and charged components of the nearEarth plasma. The AGWs represent density waves of the atmospheric gas which are able, owing to dispersion properties of the medium, to propagate over hundreds or thousands of kilometers away from their origin with a relatively small attenuation. For Corresponding author. Fax: +380-57-720-3462.
E-mail address: [email protected] (V.G. Galushko). 1364-6826/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2006.05.010
this reason, disturbances of the kind play an important role in the energy exchange and coupling between different regions of the upper atmosphere, transferring energy from the source of their excitation not only horizontally but also vertically from the Earth’s surface to ionospheric and higher altitudes and in the opposite direction. In addition, AGWs are kind of an indicator of various processes of their excitation both of natural and artificial (anthropogenic) origin. These can be, for example, earthquakes, particle precipitations in auroral regions, solar terminator, various meteorological perturbations or ionosopheric modification experiments, powerful
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explosions, chemical releases, etc. (Hunsucker, 1987). For this reason, investigations of AGWs are important for understanding the upper atmosphere dynamics and studying the space weather effects. When propagating through the ionosphere, gravity waves give rise to quasiperiodic variations in the electron density, known as traveling ionsopheric disturbances (TIDs). In their turn, these modulate parameters of radio signals reflected from or passing through the ionosphere. This modulation effect may deteriorate the radio waves propagating along ground-based or satellite links, but on the other hand allows using various remote radio sensing techniques to study ionospheric disturbances of the kind. These techniques are widely applied in investigating dynamic processes in the ionospheric plasma, with special attention given to the highlatitude ionosphere which is connected via the Earth’s magnetic field to the magnetosphere and near space, thus playing an important role in the geospace dynamics. Recently, a new figurative expression has been coined in the literature, comparing the polar ionosphere to a ‘‘TV screen’’ for surveying the solar-terrestrial interaction. Such investigations are quite intense in the Northern polar region which is saturated with a great number of remote-sensing instruments, while being comparatively rare in Antarctica because only a few such facilities are available there. This paper suggests results of a measurement campaign on TID diagnostics over the Antarctic Peninsula that was conducted between 31 January and 16 March, 2004, i.e. a summer season in the Southern hemisphere. This area is of special interest for two reasons. First, it is located rather close to the polar cusp, which is known as the range where TIDs can be generated in the ionosphere by particle precipitations during geomagnetic storms. Second, the area near the Antarctic Peninsula is characterized by a high cyclonic activity, thus being very suitable for investigating the imprint of meteorological perturbations at ionospheric heights and surveying vertical channels of energy exchange. Recently, a mechanism of such an energy transfer was suggested by Yampolski et al. (2004). The basic parameters of TIDs (amplitude, wavelength, dispersion law, velocity and direction of motion) have been recovered from observational data, using the model of a corrugated, perfectly reflecting surface located at ionospheric heights and the frequency-and-angular sounding technique (FAS) (Beley et al., 1995; Galushko et al., 2003)
developed at the Institute of Radio Astronomy, National Academy of Sciences of Ukraine (Kharkov, Ukraine). The technique proceeds from measuring trajectory parameters (angles of arrival and Doppler frequency shifts) of HF radio signals propagating along point-to-point paths. In the experiments, trajectory parameter variations to be used for TID diagnostics were measured for signals traveling between two Antarctic bases, namely the Polish station Henryk Arctowski (621 100 S, 581 280 W) and Akademik Vernadsky of Ukraine (651 150 S, 641 160 W). The surface length of the propagation path was nearly 440 km. The total observation time was about 1000 h. Statistical processing of all the data obtained has allowed determining the diurnal distribution of TID appearance and the most probable parameters of the perturbations. Based on these results, possible sources of TID generation are analyzed below. 2. TID restoration algorithm From the theoretical point of view, the TID diagnostics represents an inverse problem that was solved by Beley et al. (1995) for a model representation of the disturbance in the form of a perfectly reflecting surface z ¼ H(x, y, t) located at ionospheric heights (Lyon, 1979; Afraimovich, 1982). The input data for the diagnostic algorithm were time-varying angles of arrival (azimuth j(t), elevation angle e(t)) and Doppler frequency shift FD(t), of the probe signals. The geometry of the effective reflecting surface was specified as H ðx; y; tÞ ¼ H 0 ½1 þ hðx; y; tÞ. The horizontal-scale size of the inhomogeneities was assumed much longer than the size of the first Fresnel zone for the sounding radio wave, and the sphericity of the Earth was neglected. Provided that both the surface deviation from its average height H0 and the transverse (horizontal gradient plane) are sufficiently small, i.e. |h|, gx ¼ @H=@x, gy ¼ @H=@y51, explicit formulas for the varying trajectory parameters can be obtained within the ray optical approximation, D ¼ h sin 0 cos 0 þ gx , Dj ¼ gy tan 0 , 2H 0 _ h sin 0 . FD ¼ l
ð1Þ
Here and below, the dot denotes differentiation with respect to time, l is the (radio) wavelength, 0 ¼ hi ¼ tan1 ð2H 0 =DÞ, with the angular brackets _ gx and gy are denoting statistical averaging, and h, h,
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taken at the middle point of the propagation path length D along the Earth’s surface. The reflecting surface form h(x, y, t) is represented as a Fourier expansion in polar coordinates, viz. Z 1 Z 2p Z 1 ~ w; yÞ hðx; y; tÞ ¼ dO eiOt dw dy hðO; 1
0
0
exp½iwðx cos y þ y sin yÞ, ~ w; yÞ is the complex spectrum of surface where hðO; qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi fluctuations, w ¼ w2x þ w2y and y ¼ tan1 ðwy =wx Þ. Now, assuming that the dispersive properties of the reflecting surface are such that each temporal harmonic corresponds to a single spatial wave moving in its own direction y0(o), viz. ~ w; yÞ ¼ hðOÞdðw ~ hðO; wðOÞÞdðy y0 ðOÞÞ, the complex-valued spectra of signal trajectory parameters can be written as ~ S ðOÞ ¼ hðOÞ½cos 0 sin 0 þ iH 0 wðOÞ cos y0 ðOÞ, ~ S j ðOÞ ¼ iH 0 wðOÞhðOÞ sin y0 ðOÞ tan 0 ,
(2)
2H 0 ~ OhðOÞ sin 0 , l which were used by Beley et al. (1995) as a set for determining the disturbance characteristics. As can be easily seen, the solution of the set Eq. (2) is S F ðOÞ ¼ i
~ Re hðOÞ ¼
l Im S F ðOÞ , 2H 0 O sin 0
l Re SF ðOÞ ~ , Im hðOÞ ¼ 2H 0 O sin 0 y0 ðOÞ ¼ 1 tan
1 Re Sj ðOÞRe SF ðOÞ þ Im Sj ðOÞIm S F ðOÞ , tan 0 Re S ðOÞRe SF ðOÞ þ Im S ðOÞIm S F ðOÞ
ð3Þ
wðOÞ ¼
405
the disturbances, and hence follow the TID dynamics and map their propagation, which is very suitable for space weather investigations. It is worth noting that the suggested diagnostic algorithm remains effective not only with dedicated transmitters of probe signals, but rather with any kind of radiation sources available, like, for example, broadcasting HF stations, and allows reconstructing dispersion laws and basic parameters of TIDs from variations in the sounding signal, even at a single radio path. However, it requires that the frequency and general spectral distribution of the transmitted signal were known and the probe signal trajectory parameters could be measured with high accuracy. As was found through computer simulations, the measurement accuracy of angles of arrival and Doppler frequency shifts should not be worse than 0.51 and 0.05 Hz, respectively, to provide for reliable TID reconstruction. In our early investigations (Beley et al., 1995) this accuracy was secured through the use of a large phased antenna array of the decameter wavelength radio telescope UTR-2 (Braude et al., 1978). Obviously enough, a unique instrument like that (there are only a few such antennas in the world) cannot be involved in longterm global observations of TIDs. Meanwhile, an alternative to large antenna arrays is offered by modern ionospheric sounders, like Digisonde Portable Sounder (DPS) of the University of Massachusetts, Lowell (Reinisch, 1996). Despite the use of a small-baseline antenna array the instrument is capable of high-accuracy measurements of angles of arrival and Doppler frequency shifts of the probe signals, based on the Doppler interferometry principle (Bibl and Reinisch, 1978). There is a sizable number (more than 60) of such instruments operating all over the world. Hence, incorporation of the FAS technique in the DPS system would permit using the radiation of the great amount of HF radios (according to the International Tele-
n 2 2O Re Sj ðOÞRe S F ðOÞ þ Im S j ðOÞIm S F ðOÞ cos2 0 2 ljS F ðOÞj 1=2 þ ½Re S ðOÞRe S F ðOÞ þ Im S ðOÞIm SF ðOÞ2 sin2 0 .
Thus, the technique allows recovering the basic parameters of the model TIDs, namely, the surface fluctuation spectrum, and propagation direction and wavenumber (or horizontal velocity, V ¼ O/w) of each spatial harmonic. As a result, it is possible to visualize
communication Union, there are about 6000 radios broadcasting over the globe), thus enabling effective TID diagnostics on the global scale. In view of this, in the spring of 2001 the FAS technique was implemented in a DPS of the University of
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Massachusetts and tested through simultaneous observations of TIDs with the Millstone Hill Incoherent Scatter Radar (Galushko et al., 2003). The radio signals used for frequency-and-angular sounding of TIDs were emissions from Radio CHU of the Canadian Time Service, which were received with a DPS system at the MIT Haystack Observatory (Westford, MA). The propagation path length was about 450 km. It is worth noting that the receiving site lay very close to the magnetically conjugate point with respect to the Ukrainian Antarctic base Akademik Vernadsky. This allowed us not only testing the TID diagnostic technique but also collecting data on the wavelike disturbances for similar geomagnetic, however over different weather conditions. During three days, specifically March 14 through 16, the DPS-based observations of TIDs were verified by parallel operation of the Incoherent Scatter Radar of the MIT Haystack observatory that followed a special program. The fixed, vertically oriented and the steerable antenna of the radar were used to measure the electron number density and the electron and ion temperatures of the ionospheric plasma through the height range 130 to 800 km. The fully steerable antenna was pointed in three consecutive directions in order that the expected reflection point of the broadcast sounding signal should occur within the corresponding angular sector formed by the radar beams. The linear separation of the beams was about 70 km at the height of 260 km. Such experimental setup allowed using the triangulation method to estimate the direction of motion and speed of the wavelike ionospheric disturbances. The data-taking cycle to interrogate all the antennas and orientations lasted for 5 min. The absolute calibration of the ISR measurements was made using the ionograms obtained every 5 min from another collocated DPS system that operated in the standard vertical sounding mode. The TID parameters recovered in the FAS technique and from the ISR data were compared for the strongest spectral components of the disturbances. The result was that the spatial characteristics of the TIDs obtained in the two techniques were in good agreement. For example, relative differences between the TID wavelengths and speeds were about 10% to 12%, while the motion directions differed by no more than 151. Since then, 3 versions of the data-taking/dataprocessing systems for the frequency-and-angular sounding have been constructed and used for TID diagnostics, mostly at midlatitudes. In 2002–2003
one of these was deployed in Antarctica and applied in 2004 for investigating the impact of weather activity on TID generation and propagation. 3. Experimental The Ukrainian station Akademik Vernadsky is saturated with a great number of various electromagnetic sensors. For this reason, the basic requirements for the radio sensing systems planned for deployment at the site are low power consumption, electromagnetic purity of the sounding signal, allocation of the transmitters out of the station territory, and finally performance reliability and robust operation. The frequency-and-angular sounding technique suggested for TID diagnostics is well suited to be implemented with allowance for all these requirements. The input data for the TID reconstruction algorithm are long records of timevarying angles of arrival (azimuth and elevation angle) and Doppler frequency shift of HF radio signals propagating along point-to-point paths. To measure such variations in Antarctica, a compactsize data-taking/data-processing complex was used. The complex has been constructed based on the Doppler interferometry technique (Bibl and Reinisch, 1978) consisting in signal reception at least in three spaced points, Doppler filtering to separate signal components in the spectral domain, and application of the phase difference direction finding to each individual component. The functional diagram of the complex is shown in Fig. 1. As can be seen, it includes an antenna-transmission line system, a three-channel coherent receiver and a data-processing system. Three spaced pairs of symmetric horizontal and asymmetric vertical dipoles are used as receiving antennas, which allows Data Processing System
Antenna Transmission Line System
Receiver control unit PreAmp PreAmp
Receiver
Data processing unit
Channel 0 Channel 1
Visualization unit
Channel 2 Fig. 1. Functional diagram of the data-taking/data-processing complex.
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switching between horizontal and vertical polarizations. Signals from the antennas are fed into the three-channel receiver (the operating frequency range is 5 to 25 MHz) where they are filtered and converted to a lower frequency range (10 to 20 Hz), thus facilitating further processing. The phasedifference variations between the channels are less than 31 or 51. The data-processing system provides estimation of arrival angles and Doppler frequency shifts of the received signals, reconstruction of TID parameters and visual representation of the results, as well as control of the receiver. A more detailed description of the data-taking/data-processing complex is given in (Pikulik et al., 2003). In the March of 2002 the complex was deployed at the Ukrainian Antarctic base Akademik Vernadsky and tested with the use of radiation from communication HF transmitters at Antarctic stations Bellingshausen (Russia), Henryk Arctowski (Poland) and Rothera (United Kingdom). Three pairs of the antennas were located at the vertices of an irregular triangle with sides 45.5, 57.7 and 65.7 m. For the given phase-difference instability of the receiver channels this geometry secured the measurement accuracy for angles of arrival no worse than 0.51 in case of signal-to-noise ratios exceeding 40 dB. During the wintering campaign of 2003–2004 the complex was involved in round-theclock monitoring of TIDs. The probe signals were radiations from HF broadcasting stations located in South America. Unfortunately, the propagation paths were rather long (no shorter than 2500 km), and hence, the recovered TID parameters corresponded to the ionospheric region lying too far from station Vernadsky whose vicinity is of special interest for us. The TIDs were monitored basically over Tierra del Fuego and the Northern part of the Drake Strait. The use of shorter radio paths was impossible for the lack of nearer broadcasting stations. For this reason it was found expedient to organize a shorter diagnostic path between Vernadsky and a nearby station. To that end a programmable research transmitter was developed specially. The transmitter is capable of radiating CW signals at 3 to 10 MHz with the relative frequency instability less than 109. The power consumption is about 500 W with the maximum radiated power 250 W. The transmitting antenna represents a symmetric broadband dipole 14 by 0.7 m in size fixed at a height of 7 m (see Fig. 2). Under an agreement between the Polish Academy of Sciences and the National Antarctic Scientific
407
Fig. 2. Radiating antenna of the transmitter for TID diagnostics.
Center of Ukraine the transmitter was deployed in late January 2004 at the Polish station Henryk Arctowski, separated from Vernadsky by about 440 km. This allowed recovering TID parameters directly above the Antarctic Peninsula, which actually was the aim of this study. 4. Results and discussion Time variations of the signal trajectory parameters at the Arctowski–Vernadsky propagation path were measured practically 24 h a day between 2 February and 16 March, 2004. The research transmitter radiated one of the following six frequencies 5575, 5581, 6025, 6061, 6292 and 6934 kHz. The specific value of the sounding frequency was selected, depending on ionospheric conditions which were monitored by a vertical sounder at Vernadsky every 15 min. Also, the measurements were accompanied by monitoring weather and geomagnetic variations with the use of several meteorological and magnetic stations. The total observation time was about 1000 h with nearly 15% of these showing the presence of TIDs that were determined as wave trains in the measured parameters. It should be noted that the 15% probability of TID occurrence is a rather underestimated figure, since registration of probe signal reflections from the F-region were impeded by the presence of sporadic E layers which were identified in the vertical sounder ionograms, especially in the daytime. As an example, Fig. 3 shows a time varying Doppler frequency shift, elevation angle and azimuth of the sounding signal at 5575 kHz recorded on 8 February, 2004 (top panel) and a fragment of the effective reflecting surface recon-
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14
Number of events
12 10 8 6 4 2 0
0
2
4
6
8
10 12 14 16 18 20 22 24 Local Time
Fig. 4. Diurnal distribution of the TID occurrence frequency. The light- and dark-gray bars correspond, respectively, to the sunrise and sunset times at the ionospheric heights.
18 16
Number of events
14 12 10 8 6 4 2 0
0
20
40
60 80 100 TID period,min
120
140
Fig. 5. Frequency diagram of TID periods.
Fig. 3. Time-varying Doppler frequency shift, elevation angle and azimuth of the probe signal at the frequency 5575 kHz recorded on 8 February, 2004 (a) and a fragment of the effective reflecting surface reconstructed after the technique suggested (b).
structed after the technique suggested (bottom panel). One can clearly see quasiperiodic variations both in the signal trajectory parameters and in the height of the reflecting surface, which is evidence for the presence of wavelike ionospheric disturbances. The total number of the TID events identified in the measurements is 73. Further analysis consisted in determining periods, amplitudes, wavelengths, and velocities and directions of motion for each spectral harmonic of TIDs. Statistical treatment of all the
data showed that TID events were observed for the most part near the sunrise and sunset terminators, namely, a few hours after passage of the respective terminator (see Fig. 4, light- and dark-gray bars, respectively). Thus it can be supposed that the main regular source of TID generation is the moving solar terminator, which is in full conformity with the results of our investigations at midlatitudes (Beley et al., 1995; Galushko et al., 1998, 2003). Shown in Fig. 5 is the probability density histogram of TID periods. As can be seen, the characteristic periods of the disturbances lie between 30 and 60 min, which is once again similar to the midlatitude processes. At the same time the histogram shows the presence of another maximum at about
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409
20
16
18
14
16 Number of events
Number of events
12 10 8 6
14 12 10 8 6
4
4
2
2
0
0 1
2
4 3 TID amplitude, %
5
6
Fig. 6. Probability density histogram of TID amplitudes.
0
100
200 300 400 Horizontal velocity, m/s
500
600
Fig. 8. Probability density histogram of TID motion velocities.
16
340
0
20
14
320
40
8
Number of events
12
6 10
300
60 Normal to SRT
8
2
280
6
4 80
4
260
2 0
0
Normal to SST
200 400 600 800 1000 1200 1400 1600 1800 2000 TID wavelength, km
100
240
120
Fig. 7. Probability density distribution of TID wavelengths.
220
140 200
90 min. This suggests existence of additional sources of TID generation. For example, similar periods were observed in the simultaneous atmospheric pressure variations (Yampolski et al., 2004; Lytvynenko and Yampolski, 2005). Hence it can be assumed that these wavelike disturbances are generated owing to meteorological effects in the atmosphere. The most probable magnitudes of the other TID parameters, such as amplitudes of a few per cent (Fig. 6), wavelengths of hundreds of kilometers (Fig. 7) and motion velocities between 100 and 200 m/s (Fig. 8), correspond to those observed at midlatitudes. Analysis of the histogram of TID azimuths (see Fig. 9) shows two preferential directions of motion, one along the normal to the
180
160
Fig. 9. Histogram of TID azimuths. The light-gray and darkgray arrows are along the normals to the sunrise (SRT) and sunset (SST) terminators, respectively.
sunrise terminator and the other oriented northward. Most probably the latter can be associated with the wavelike disturbances generated by geomagnetic perturbations. However, identification of specific mechanisms of TID generation requires a detailed dynamic analysis of the measured data, i.e. comparing individual realizations with the respective weather and geomagnetic conditions. It should be noted that we failed to obtain explicit evidence for the projection of weather activity effects on
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ionospheric perturbations during this measuring campaign. The reason might be the lower cyclonic activity during the Antarctic summer. 5. Conclusions A data-taking complex for frequency-and-angular sounding of wavelike ionospheric disturbances has been constructed at the Ukrainian Antarctic station Akademik Vernadsky. More than 1000 h of observations of TID events were conducted in the bistatic mode, using long time records of trajectory parameter variations along the Arctowski–Vernadsky propagation path. Most probable parameters of the disturbances have been estimated for the Antarctic summer, and several possible mechanisms of TID generation are discussed. It has been shown in particular that the main regular source of TIDs under quiet geomagnetic conditions is the moving solar terminator. Distribution of TID azimuths shows the presence of a meridional component in the motion direction which can be associated with the impact of geomagnetic perturbations. Unfortunately, no explicit evidence of meteorological impact on TID generation was found during the campaign. As we suppose, this can be explained by the lower cyclonic activity in the summer and also by impedance on the part of the sporadic E layers whose probability of occurrence was very high during the season. For this reason we plan to conduct a measuring campaign on multifrequency diagnostics of TID in the autumn or winter. On the one hand, this would allow reducing the impeding effect of sporadic layers, while on the other, enable recovering the height distribution of wavelike disturbances and follow their dynamics in the vertical direction and the transfer of weather perturbations to ionospheric heights. The transmitting site will be equipped either at Henryk Arctowski or at the Rothera station which is even closer to Vernadsky. Acknowledgments The authors would like to thank the Polish Antarctic crew for providing an opportunity to
operate the HF transmitter from their base. The efforts of the Ukrainian team were partially supported by the Science and Technology Center in Ukraine (STCU) through Project Agreement 827c, and Projects Antarctida, Yamb and Resonances of the National Academy of Sciences and Ministry of Education and Science of Ukraine.
References Afraimovich, E.L., 1982. Interference Methods of the Ionospheric Sounding. Nauka, Moscow (in Russian). Beley, V.S., Galushko, V.G., Yampolski, Y.M., 1995. Traveling ionospheric disturbance diagnostics using HF signal trajectory parameter variations. Radio Science 30 (6), 1739–1752. Bibl, K., Reinisch, B.W., 1978. Universal digital ionosonde. Radio Science 13 (3), 519–530. Braude, S.Y., et al., 1978. Decametric survey of discrete sources in the northern sky. Astrophysics and Space Science 54 (3). Galushko, V.G., Paznukhov, V.V., Yampolski, Y.M., Foster, J.C., 1998. Incoherent scatter radar observations of AGW/ TID events generated by the moving solar terminator. Annales Geophysicae 16, 821–827. Galushko, V.G., Beley, V.S., Koloskov, A.V., Yampolski, Y.M., Paznukhov, V.V., Reinisch, B.W., Foster, J.C., Erickson, P., 2003. Frequency-and-angular HF sounding and ISR diagnostics of TIDs. Radio Science 38(6), 1102, doi:10.1029/ 2002RS002861. Hunsucker, R.D., 1987. The source of gravity waves. Nature 328, 204. Lyon, G.F., 1979. The corrugated model for one-hop oblique propagation. Journal of Atmosphere and Terrestrial Physics 41 (5). Lytvynenko, L.N., Yampolski, Y.M. (Eds.), 2005. Electromagnetic manifestations of geophysical effects in the Antarctic. Institute of Radio Astronomy, National Academy of Sciences of Ukraine, Kharkov, 331 p. Pikulik, I.I., Kascheev, S.B., Galushko, V.G., Yampolski, Y.M., 2003. A data taking/data processing complex for frequencyand-angular sounding of ionospheric disturbances. Ukrainian Antarctic Journal 1, 61–69 (in Russian). Reinisch, B.W., 1996. Modern ionosondes. In: Kohl, H., Ru¨ster, R., Schlegel, K. (Eds.), Modern Ionospheric Science. EGS, Katlenburg-Lindau, Germany, pp. 440–458. Yampolski, Y.M., Zalizovski, A.V., Lytvynenko, L.N., Lizunov, G.V., Groves, K.M., Moldwin, M.B., 2004. Geomagnetic field variations in Antarctica and in the magnetically conjugate region (New England) produced by cyclonic activity. Radio Physics and Radio Astronomy 9 (2), 130–151 (in Russian).
Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 403–410 www.elsevier.com/locate/jastp
Bistatic HF diagnostics of TIDs over the Antarctic Peninsula V.G. Galushkoa,, A.S. Kashcheyeva, S.B. Kashcheyeva, A.V. Koloskova, I.I. Pikulika, Y.M. Yampolskia, V.A. Litvinovb, G.P. Milinevskyb, S. Rakusa-Suszczewskic a
Institute of Radio Astronomy, National Academy of Sciences of Ukraine, 4, Chervonopraporna Street, Kharkov, 61002 Ukraine National Antarctic Scientific Center, Ministry for Education and Science of Ukraine, 16, Tarasa Shevchenka Boulevard, Kyiv, 01601 Ukraine c Polish Academy of Sciences, Department of Antarctic Biology, 10, Ustrzycka Street, 02-141 Warsaw, Poland
b
Received 20 January 2006; received in revised form 26 April 2006; accepted 10 May 2006 Available online 19 January 2007
Abstract Results of HF diagnostics of traveling ionospheric disturbances (TIDs) over the Antarctic Peninsula are presented that have been obtained in the frequency-and-angular sounding technique developed at the Institute of Radio Astronomy, National Academy of Sciences of Ukraine (Kharkov, Ukraine). A specially designed HF transmitter was deployed at the Polish Antarctic station Henryk Arctowski to radiate probe signals. Variations in trajectory parameters (angles of arrival and Doppler frequency shifts) of the probe signals used for recovering TID characteristics were recorded at the Ukrainian Antarctic station Akademik Vernadsky with a compact-size three-channel coherent receiver based on the Doppler interferometry technique. The propagation path length was approximately 440 km. Round-the-clock measurements were performed between 31 January and 16 March, 2004 at frequencies 5 to 7 MHz, depending on the ionospheric conditions. The total time of observations was about 1000 h. Results of preliminary processing of the measured data are reported. Characteristic parameters of the disturbances are presented and possible sources of their generation are discussed. r 2006 Elsevier Ltd. All rights reserved. Keywords: Traveling ionospheric disturbances; Frequency-and-angular sounding; Ionosphere; Sources of TID generation
1. Introduction Atmospheric gravity waves (AGW) are among the basic transport agents of energy exchange between the neutral and charged components of the nearEarth plasma. The AGWs represent density waves of the atmospheric gas which are able, owing to dispersion properties of the medium, to propagate over hundreds or thousands of kilometers away from their origin with a relatively small attenuation. For Corresponding author. Fax: +380-57-720-3462.
E-mail address: [email protected] (V.G. Galushko). 1364-6826/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2006.05.010
this reason, disturbances of the kind play an important role in the energy exchange and coupling between different regions of the upper atmosphere, transferring energy from the source of their excitation not only horizontally but also vertically from the Earth’s surface to ionospheric and higher altitudes and in the opposite direction. In addition, AGWs are kind of an indicator of various processes of their excitation both of natural and artificial (anthropogenic) origin. These can be, for example, earthquakes, particle precipitations in auroral regions, solar terminator, various meteorological perturbations or ionosopheric modification experiments, powerful
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explosions, chemical releases, etc. (Hunsucker, 1987). For this reason, investigations of AGWs are important for understanding the upper atmosphere dynamics and studying the space weather effects. When propagating through the ionosphere, gravity waves give rise to quasiperiodic variations in the electron density, known as traveling ionsopheric disturbances (TIDs). In their turn, these modulate parameters of radio signals reflected from or passing through the ionosphere. This modulation effect may deteriorate the radio waves propagating along ground-based or satellite links, but on the other hand allows using various remote radio sensing techniques to study ionospheric disturbances of the kind. These techniques are widely applied in investigating dynamic processes in the ionospheric plasma, with special attention given to the highlatitude ionosphere which is connected via the Earth’s magnetic field to the magnetosphere and near space, thus playing an important role in the geospace dynamics. Recently, a new figurative expression has been coined in the literature, comparing the polar ionosphere to a ‘‘TV screen’’ for surveying the solar-terrestrial interaction. Such investigations are quite intense in the Northern polar region which is saturated with a great number of remote-sensing instruments, while being comparatively rare in Antarctica because only a few such facilities are available there. This paper suggests results of a measurement campaign on TID diagnostics over the Antarctic Peninsula that was conducted between 31 January and 16 March, 2004, i.e. a summer season in the Southern hemisphere. This area is of special interest for two reasons. First, it is located rather close to the polar cusp, which is known as the range where TIDs can be generated in the ionosphere by particle precipitations during geomagnetic storms. Second, the area near the Antarctic Peninsula is characterized by a high cyclonic activity, thus being very suitable for investigating the imprint of meteorological perturbations at ionospheric heights and surveying vertical channels of energy exchange. Recently, a mechanism of such an energy transfer was suggested by Yampolski et al. (2004). The basic parameters of TIDs (amplitude, wavelength, dispersion law, velocity and direction of motion) have been recovered from observational data, using the model of a corrugated, perfectly reflecting surface located at ionospheric heights and the frequency-and-angular sounding technique (FAS) (Beley et al., 1995; Galushko et al., 2003)
developed at the Institute of Radio Astronomy, National Academy of Sciences of Ukraine (Kharkov, Ukraine). The technique proceeds from measuring trajectory parameters (angles of arrival and Doppler frequency shifts) of HF radio signals propagating along point-to-point paths. In the experiments, trajectory parameter variations to be used for TID diagnostics were measured for signals traveling between two Antarctic bases, namely the Polish station Henryk Arctowski (621 100 S, 581 280 W) and Akademik Vernadsky of Ukraine (651 150 S, 641 160 W). The surface length of the propagation path was nearly 440 km. The total observation time was about 1000 h. Statistical processing of all the data obtained has allowed determining the diurnal distribution of TID appearance and the most probable parameters of the perturbations. Based on these results, possible sources of TID generation are analyzed below. 2. TID restoration algorithm From the theoretical point of view, the TID diagnostics represents an inverse problem that was solved by Beley et al. (1995) for a model representation of the disturbance in the form of a perfectly reflecting surface z ¼ H(x, y, t) located at ionospheric heights (Lyon, 1979; Afraimovich, 1982). The input data for the diagnostic algorithm were time-varying angles of arrival (azimuth j(t), elevation angle e(t)) and Doppler frequency shift FD(t), of the probe signals. The geometry of the effective reflecting surface was specified as H ðx; y; tÞ ¼ H 0 ½1 þ hðx; y; tÞ. The horizontal-scale size of the inhomogeneities was assumed much longer than the size of the first Fresnel zone for the sounding radio wave, and the sphericity of the Earth was neglected. Provided that both the surface deviation from its average height H0 and the transverse (horizontal gradient plane) are sufficiently small, i.e. |h|, gx ¼ @H=@x, gy ¼ @H=@y51, explicit formulas for the varying trajectory parameters can be obtained within the ray optical approximation, D ¼ h sin 0 cos 0 þ gx , Dj ¼ gy tan 0 , 2H 0 _ h sin 0 . FD ¼ l
ð1Þ
Here and below, the dot denotes differentiation with respect to time, l is the (radio) wavelength, 0 ¼ hi ¼ tan1 ð2H 0 =DÞ, with the angular brackets _ gx and gy are denoting statistical averaging, and h, h,
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taken at the middle point of the propagation path length D along the Earth’s surface. The reflecting surface form h(x, y, t) is represented as a Fourier expansion in polar coordinates, viz. Z 1 Z 2p Z 1 ~ w; yÞ hðx; y; tÞ ¼ dO eiOt dw dy hðO; 1
0
0
exp½iwðx cos y þ y sin yÞ, ~ w; yÞ is the complex spectrum of surface where hðO; qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi fluctuations, w ¼ w2x þ w2y and y ¼ tan1 ðwy =wx Þ. Now, assuming that the dispersive properties of the reflecting surface are such that each temporal harmonic corresponds to a single spatial wave moving in its own direction y0(o), viz. ~ w; yÞ ¼ hðOÞdðw ~ hðO; wðOÞÞdðy y0 ðOÞÞ, the complex-valued spectra of signal trajectory parameters can be written as ~ S ðOÞ ¼ hðOÞ½cos 0 sin 0 þ iH 0 wðOÞ cos y0 ðOÞ, ~ S j ðOÞ ¼ iH 0 wðOÞhðOÞ sin y0 ðOÞ tan 0 ,
(2)
2H 0 ~ OhðOÞ sin 0 , l which were used by Beley et al. (1995) as a set for determining the disturbance characteristics. As can be easily seen, the solution of the set Eq. (2) is S F ðOÞ ¼ i
~ Re hðOÞ ¼
l Im S F ðOÞ , 2H 0 O sin 0
l Re SF ðOÞ ~ , Im hðOÞ ¼ 2H 0 O sin 0 y0 ðOÞ ¼ 1 tan
1 Re Sj ðOÞRe SF ðOÞ þ Im Sj ðOÞIm S F ðOÞ , tan 0 Re S ðOÞRe SF ðOÞ þ Im S ðOÞIm S F ðOÞ
ð3Þ
wðOÞ ¼
405
the disturbances, and hence follow the TID dynamics and map their propagation, which is very suitable for space weather investigations. It is worth noting that the suggested diagnostic algorithm remains effective not only with dedicated transmitters of probe signals, but rather with any kind of radiation sources available, like, for example, broadcasting HF stations, and allows reconstructing dispersion laws and basic parameters of TIDs from variations in the sounding signal, even at a single radio path. However, it requires that the frequency and general spectral distribution of the transmitted signal were known and the probe signal trajectory parameters could be measured with high accuracy. As was found through computer simulations, the measurement accuracy of angles of arrival and Doppler frequency shifts should not be worse than 0.51 and 0.05 Hz, respectively, to provide for reliable TID reconstruction. In our early investigations (Beley et al., 1995) this accuracy was secured through the use of a large phased antenna array of the decameter wavelength radio telescope UTR-2 (Braude et al., 1978). Obviously enough, a unique instrument like that (there are only a few such antennas in the world) cannot be involved in longterm global observations of TIDs. Meanwhile, an alternative to large antenna arrays is offered by modern ionospheric sounders, like Digisonde Portable Sounder (DPS) of the University of Massachusetts, Lowell (Reinisch, 1996). Despite the use of a small-baseline antenna array the instrument is capable of high-accuracy measurements of angles of arrival and Doppler frequency shifts of the probe signals, based on the Doppler interferometry principle (Bibl and Reinisch, 1978). There is a sizable number (more than 60) of such instruments operating all over the world. Hence, incorporation of the FAS technique in the DPS system would permit using the radiation of the great amount of HF radios (according to the International Tele-
n 2 2O Re Sj ðOÞRe S F ðOÞ þ Im S j ðOÞIm S F ðOÞ cos2 0 2 ljS F ðOÞj 1=2 þ ½Re S ðOÞRe S F ðOÞ þ Im S ðOÞIm SF ðOÞ2 sin2 0 .
Thus, the technique allows recovering the basic parameters of the model TIDs, namely, the surface fluctuation spectrum, and propagation direction and wavenumber (or horizontal velocity, V ¼ O/w) of each spatial harmonic. As a result, it is possible to visualize
communication Union, there are about 6000 radios broadcasting over the globe), thus enabling effective TID diagnostics on the global scale. In view of this, in the spring of 2001 the FAS technique was implemented in a DPS of the University of
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Massachusetts and tested through simultaneous observations of TIDs with the Millstone Hill Incoherent Scatter Radar (Galushko et al., 2003). The radio signals used for frequency-and-angular sounding of TIDs were emissions from Radio CHU of the Canadian Time Service, which were received with a DPS system at the MIT Haystack Observatory (Westford, MA). The propagation path length was about 450 km. It is worth noting that the receiving site lay very close to the magnetically conjugate point with respect to the Ukrainian Antarctic base Akademik Vernadsky. This allowed us not only testing the TID diagnostic technique but also collecting data on the wavelike disturbances for similar geomagnetic, however over different weather conditions. During three days, specifically March 14 through 16, the DPS-based observations of TIDs were verified by parallel operation of the Incoherent Scatter Radar of the MIT Haystack observatory that followed a special program. The fixed, vertically oriented and the steerable antenna of the radar were used to measure the electron number density and the electron and ion temperatures of the ionospheric plasma through the height range 130 to 800 km. The fully steerable antenna was pointed in three consecutive directions in order that the expected reflection point of the broadcast sounding signal should occur within the corresponding angular sector formed by the radar beams. The linear separation of the beams was about 70 km at the height of 260 km. Such experimental setup allowed using the triangulation method to estimate the direction of motion and speed of the wavelike ionospheric disturbances. The data-taking cycle to interrogate all the antennas and orientations lasted for 5 min. The absolute calibration of the ISR measurements was made using the ionograms obtained every 5 min from another collocated DPS system that operated in the standard vertical sounding mode. The TID parameters recovered in the FAS technique and from the ISR data were compared for the strongest spectral components of the disturbances. The result was that the spatial characteristics of the TIDs obtained in the two techniques were in good agreement. For example, relative differences between the TID wavelengths and speeds were about 10% to 12%, while the motion directions differed by no more than 151. Since then, 3 versions of the data-taking/dataprocessing systems for the frequency-and-angular sounding have been constructed and used for TID diagnostics, mostly at midlatitudes. In 2002–2003
one of these was deployed in Antarctica and applied in 2004 for investigating the impact of weather activity on TID generation and propagation. 3. Experimental The Ukrainian station Akademik Vernadsky is saturated with a great number of various electromagnetic sensors. For this reason, the basic requirements for the radio sensing systems planned for deployment at the site are low power consumption, electromagnetic purity of the sounding signal, allocation of the transmitters out of the station territory, and finally performance reliability and robust operation. The frequency-and-angular sounding technique suggested for TID diagnostics is well suited to be implemented with allowance for all these requirements. The input data for the TID reconstruction algorithm are long records of timevarying angles of arrival (azimuth and elevation angle) and Doppler frequency shift of HF radio signals propagating along point-to-point paths. To measure such variations in Antarctica, a compactsize data-taking/data-processing complex was used. The complex has been constructed based on the Doppler interferometry technique (Bibl and Reinisch, 1978) consisting in signal reception at least in three spaced points, Doppler filtering to separate signal components in the spectral domain, and application of the phase difference direction finding to each individual component. The functional diagram of the complex is shown in Fig. 1. As can be seen, it includes an antenna-transmission line system, a three-channel coherent receiver and a data-processing system. Three spaced pairs of symmetric horizontal and asymmetric vertical dipoles are used as receiving antennas, which allows Data Processing System
Antenna Transmission Line System
Receiver control unit PreAmp PreAmp
Receiver
Data processing unit
Channel 0 Channel 1
Visualization unit
Channel 2 Fig. 1. Functional diagram of the data-taking/data-processing complex.
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switching between horizontal and vertical polarizations. Signals from the antennas are fed into the three-channel receiver (the operating frequency range is 5 to 25 MHz) where they are filtered and converted to a lower frequency range (10 to 20 Hz), thus facilitating further processing. The phasedifference variations between the channels are less than 31 or 51. The data-processing system provides estimation of arrival angles and Doppler frequency shifts of the received signals, reconstruction of TID parameters and visual representation of the results, as well as control of the receiver. A more detailed description of the data-taking/data-processing complex is given in (Pikulik et al., 2003). In the March of 2002 the complex was deployed at the Ukrainian Antarctic base Akademik Vernadsky and tested with the use of radiation from communication HF transmitters at Antarctic stations Bellingshausen (Russia), Henryk Arctowski (Poland) and Rothera (United Kingdom). Three pairs of the antennas were located at the vertices of an irregular triangle with sides 45.5, 57.7 and 65.7 m. For the given phase-difference instability of the receiver channels this geometry secured the measurement accuracy for angles of arrival no worse than 0.51 in case of signal-to-noise ratios exceeding 40 dB. During the wintering campaign of 2003–2004 the complex was involved in round-theclock monitoring of TIDs. The probe signals were radiations from HF broadcasting stations located in South America. Unfortunately, the propagation paths were rather long (no shorter than 2500 km), and hence, the recovered TID parameters corresponded to the ionospheric region lying too far from station Vernadsky whose vicinity is of special interest for us. The TIDs were monitored basically over Tierra del Fuego and the Northern part of the Drake Strait. The use of shorter radio paths was impossible for the lack of nearer broadcasting stations. For this reason it was found expedient to organize a shorter diagnostic path between Vernadsky and a nearby station. To that end a programmable research transmitter was developed specially. The transmitter is capable of radiating CW signals at 3 to 10 MHz with the relative frequency instability less than 109. The power consumption is about 500 W with the maximum radiated power 250 W. The transmitting antenna represents a symmetric broadband dipole 14 by 0.7 m in size fixed at a height of 7 m (see Fig. 2). Under an agreement between the Polish Academy of Sciences and the National Antarctic Scientific
407
Fig. 2. Radiating antenna of the transmitter for TID diagnostics.
Center of Ukraine the transmitter was deployed in late January 2004 at the Polish station Henryk Arctowski, separated from Vernadsky by about 440 km. This allowed recovering TID parameters directly above the Antarctic Peninsula, which actually was the aim of this study. 4. Results and discussion Time variations of the signal trajectory parameters at the Arctowski–Vernadsky propagation path were measured practically 24 h a day between 2 February and 16 March, 2004. The research transmitter radiated one of the following six frequencies 5575, 5581, 6025, 6061, 6292 and 6934 kHz. The specific value of the sounding frequency was selected, depending on ionospheric conditions which were monitored by a vertical sounder at Vernadsky every 15 min. Also, the measurements were accompanied by monitoring weather and geomagnetic variations with the use of several meteorological and magnetic stations. The total observation time was about 1000 h with nearly 15% of these showing the presence of TIDs that were determined as wave trains in the measured parameters. It should be noted that the 15% probability of TID occurrence is a rather underestimated figure, since registration of probe signal reflections from the F-region were impeded by the presence of sporadic E layers which were identified in the vertical sounder ionograms, especially in the daytime. As an example, Fig. 3 shows a time varying Doppler frequency shift, elevation angle and azimuth of the sounding signal at 5575 kHz recorded on 8 February, 2004 (top panel) and a fragment of the effective reflecting surface recon-
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14
Number of events
12 10 8 6 4 2 0
0
2
4
6
8
10 12 14 16 18 20 22 24 Local Time
Fig. 4. Diurnal distribution of the TID occurrence frequency. The light- and dark-gray bars correspond, respectively, to the sunrise and sunset times at the ionospheric heights.
18 16
Number of events
14 12 10 8 6 4 2 0
0
20
40
60 80 100 TID period,min
120
140
Fig. 5. Frequency diagram of TID periods.
Fig. 3. Time-varying Doppler frequency shift, elevation angle and azimuth of the probe signal at the frequency 5575 kHz recorded on 8 February, 2004 (a) and a fragment of the effective reflecting surface reconstructed after the technique suggested (b).
structed after the technique suggested (bottom panel). One can clearly see quasiperiodic variations both in the signal trajectory parameters and in the height of the reflecting surface, which is evidence for the presence of wavelike ionospheric disturbances. The total number of the TID events identified in the measurements is 73. Further analysis consisted in determining periods, amplitudes, wavelengths, and velocities and directions of motion for each spectral harmonic of TIDs. Statistical treatment of all the
data showed that TID events were observed for the most part near the sunrise and sunset terminators, namely, a few hours after passage of the respective terminator (see Fig. 4, light- and dark-gray bars, respectively). Thus it can be supposed that the main regular source of TID generation is the moving solar terminator, which is in full conformity with the results of our investigations at midlatitudes (Beley et al., 1995; Galushko et al., 1998, 2003). Shown in Fig. 5 is the probability density histogram of TID periods. As can be seen, the characteristic periods of the disturbances lie between 30 and 60 min, which is once again similar to the midlatitude processes. At the same time the histogram shows the presence of another maximum at about
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409
20
16
18
14
16 Number of events
Number of events
12 10 8 6
14 12 10 8 6
4
4
2
2
0
0 1
2
4 3 TID amplitude, %
5
6
Fig. 6. Probability density histogram of TID amplitudes.
0
100
200 300 400 Horizontal velocity, m/s
500
600
Fig. 8. Probability density histogram of TID motion velocities.
16
340
0
20
14
320
40
8
Number of events
12
6 10
300
60 Normal to SRT
8
2
280
6
4 80
4
260
2 0
0
Normal to SST
200 400 600 800 1000 1200 1400 1600 1800 2000 TID wavelength, km
100
240
120
Fig. 7. Probability density distribution of TID wavelengths.
220
140 200
90 min. This suggests existence of additional sources of TID generation. For example, similar periods were observed in the simultaneous atmospheric pressure variations (Yampolski et al., 2004; Lytvynenko and Yampolski, 2005). Hence it can be assumed that these wavelike disturbances are generated owing to meteorological effects in the atmosphere. The most probable magnitudes of the other TID parameters, such as amplitudes of a few per cent (Fig. 6), wavelengths of hundreds of kilometers (Fig. 7) and motion velocities between 100 and 200 m/s (Fig. 8), correspond to those observed at midlatitudes. Analysis of the histogram of TID azimuths (see Fig. 9) shows two preferential directions of motion, one along the normal to the
180
160
Fig. 9. Histogram of TID azimuths. The light-gray and darkgray arrows are along the normals to the sunrise (SRT) and sunset (SST) terminators, respectively.
sunrise terminator and the other oriented northward. Most probably the latter can be associated with the wavelike disturbances generated by geomagnetic perturbations. However, identification of specific mechanisms of TID generation requires a detailed dynamic analysis of the measured data, i.e. comparing individual realizations with the respective weather and geomagnetic conditions. It should be noted that we failed to obtain explicit evidence for the projection of weather activity effects on
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ionospheric perturbations during this measuring campaign. The reason might be the lower cyclonic activity during the Antarctic summer. 5. Conclusions A data-taking complex for frequency-and-angular sounding of wavelike ionospheric disturbances has been constructed at the Ukrainian Antarctic station Akademik Vernadsky. More than 1000 h of observations of TID events were conducted in the bistatic mode, using long time records of trajectory parameter variations along the Arctowski–Vernadsky propagation path. Most probable parameters of the disturbances have been estimated for the Antarctic summer, and several possible mechanisms of TID generation are discussed. It has been shown in particular that the main regular source of TIDs under quiet geomagnetic conditions is the moving solar terminator. Distribution of TID azimuths shows the presence of a meridional component in the motion direction which can be associated with the impact of geomagnetic perturbations. Unfortunately, no explicit evidence of meteorological impact on TID generation was found during the campaign. As we suppose, this can be explained by the lower cyclonic activity in the summer and also by impedance on the part of the sporadic E layers whose probability of occurrence was very high during the season. For this reason we plan to conduct a measuring campaign on multifrequency diagnostics of TID in the autumn or winter. On the one hand, this would allow reducing the impeding effect of sporadic layers, while on the other, enable recovering the height distribution of wavelike disturbances and follow their dynamics in the vertical direction and the transfer of weather perturbations to ionospheric heights. The transmitting site will be equipped either at Henryk Arctowski or at the Rothera station which is even closer to Vernadsky. Acknowledgments The authors would like to thank the Polish Antarctic crew for providing an opportunity to
operate the HF transmitter from their base. The efforts of the Ukrainian team were partially supported by the Science and Technology Center in Ukraine (STCU) through Project Agreement 827c, and Projects Antarctida, Yamb and Resonances of the National Academy of Sciences and Ministry of Education and Science of Ukraine.
References Afraimovich, E.L., 1982. Interference Methods of the Ionospheric Sounding. Nauka, Moscow (in Russian). Beley, V.S., Galushko, V.G., Yampolski, Y.M., 1995. Traveling ionospheric disturbance diagnostics using HF signal trajectory parameter variations. Radio Science 30 (6), 1739–1752. Bibl, K., Reinisch, B.W., 1978. Universal digital ionosonde. Radio Science 13 (3), 519–530. Braude, S.Y., et al., 1978. Decametric survey of discrete sources in the northern sky. Astrophysics and Space Science 54 (3). Galushko, V.G., Paznukhov, V.V., Yampolski, Y.M., Foster, J.C., 1998. Incoherent scatter radar observations of AGW/ TID events generated by the moving solar terminator. Annales Geophysicae 16, 821–827. Galushko, V.G., Beley, V.S., Koloskov, A.V., Yampolski, Y.M., Paznukhov, V.V., Reinisch, B.W., Foster, J.C., Erickson, P., 2003. Frequency-and-angular HF sounding and ISR diagnostics of TIDs. Radio Science 38(6), 1102, doi:10.1029/ 2002RS002861. Hunsucker, R.D., 1987. The source of gravity waves. Nature 328, 204. Lyon, G.F., 1979. The corrugated model for one-hop oblique propagation. Journal of Atmosphere and Terrestrial Physics 41 (5). Lytvynenko, L.N., Yampolski, Y.M. (Eds.), 2005. Electromagnetic manifestations of geophysical effects in the Antarctic. Institute of Radio Astronomy, National Academy of Sciences of Ukraine, Kharkov, 331 p. Pikulik, I.I., Kascheev, S.B., Galushko, V.G., Yampolski, Y.M., 2003. A data taking/data processing complex for frequencyand-angular sounding of ionospheric disturbances. Ukrainian Antarctic Journal 1, 61–69 (in Russian). Reinisch, B.W., 1996. Modern ionosondes. In: Kohl, H., Ru¨ster, R., Schlegel, K. (Eds.), Modern Ionospheric Science. EGS, Katlenburg-Lindau, Germany, pp. 440–458. Yampolski, Y.M., Zalizovski, A.V., Lytvynenko, L.N., Lizunov, G.V., Groves, K.M., Moldwin, M.B., 2004. Geomagnetic field variations in Antarctica and in the magnetically conjugate region (New England) produced by cyclonic activity. Radio Physics and Radio Astronomy 9 (2), 130–151 (in Russian).