Radio-acoustic sounding of the ionosphere

We present the results of first experiments on radio-acoustic sounding of ionosphere at the altitudes from 70 to 85 km. The sounding was performed in autumn 2006, using a horn acoustic emitter and a radar on the basis of the “Sura” facility. The emitter had an acoustic power of about 1 kW and operated in the chirp-modulation regime with frequency variation from 15.9 to 18.4 Hz. The radar transmitter operated in the pulse regime at a frequency of 9 MHz and had an average power of 30 kW. The power of the radio signal scattered from a sound wave in the ionosphere did not exceed 10⁻¹⁶ W, and the measured values of the temperature in the scattering region ranged from 190 to 225 K. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 2, pp. 128–133, February 2009.     

On small-scale plasma inhomogeneities of the traveling ionospheric disturbances

We report on the results of the first special experiment on radio sounding of the midlatitude ionosphere by signals from in-orbit satellites at a frequency of 150 MHz under quiet geophysical conditions. Along with the conventional correlation processing, fractal processing of the received signals was also performed. Using the fractal approach, we obtained first data on the sources and generation mechanisms of small-scale plasma inhomogeneities of the traveling ionospheric disturbances (TIDs) in the upper ionosphere. It is noted that the phenomenon of nonlinear “ breaking” of the acoustic-gravity waves entering the ionosphere from the underlying atmosphere plays the crucial role in the formation of plasma inhomogeneities of the TIDs. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 7, pp. 561–569, July 2006.     

Parametric generation of low-frequency waves by plasma electrons accelerated under electron cyclotron resonance conditions

It has been shown experimentally that the diamagnetic effect appearing when electrons of a magnetized plasma in the antenna near field are accelerated under electron cyclotron resonance conditions can be used to generate low-frequency waves. The amplitude modulation of a signal supplied to the antenna is accompanied by the modulation of the diamagnetic effect and leads to the emission of waves at the modulation frequency to the surrounding plasma. In this process, the extended plasma region containing accelerated electrons serves as a parametric bodiless antenna. The results of the model laboratory experiments make it possible to propose a method for the parametric generation of low-frequency whistler waves in the Earth’s ionosphere by a powerful amplitude-modulated signal supplied to the satellite-borne antenna.     

On the excitation of the ionosphere by high-power radio waves in the geomagnetic equator region

We consider the features of the modification of the equatorial ionosphere by high-power radiowaves. A possibility for creating strong (compared to mid-latitude) quasi-periodic ionospheric F-layer irregularities, formed by the field of the high-power standing wave, producing artificial bubbles, and influencing the natural equatorial “bubbles” is discussed. The peculiarities of exciting the striction and thermal parametric instabilities in the equatorial ionosphere are considered. The problem of the modulation of the equatorial current jet is briefly discussed. Radiophysical Research Institute, Nizhny Novgorod, Russia; Swedish Institute of Space Physics, Uppsala Division, Uppsala, Sweden. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, Nos. 1–2, pp. 250–262, January–February, 1997.     

On a mechanism forming a broadband maximum in the spectrum of background noise at frequencies 2–6 Hz

We study the properties of a broadband spectral maximum (BSM) of the background noise at frequencies 2–6 Hz using the data of mid-latitude observations. We find that the parameters of the maximum depend on the local ionosphere properties and that small scales (several tens of km) are absent in the field distribution at the BSM frequencies. We propose a mechanism forming the BSM, which is related to the large gradients of the refractive indices of normal ionospheric-plasma waves at altitudes from 70 to 300 km. The performed simulation of BSM spectra for a plane-stratified model of anisotropic inhomogeneous ionosphere allowed us to explain the main experimentally observed BSM properties. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 7, pp. 607–623, July 2007.     

Modification of the earth’s ionosphere by high-power HF radio emission: Artificial periodic inhomogeneities and the sporadic <Emphasis Type="Italic">E</Emphasis> layer

We present the results of new studies of the sporadic E layer in the case of heating of the ionosphere by high-power HF radio emission. The measurements were performed at the “Sura” heating facility. Ionosphere was modified by high-power radio emission from the “Sura” facility and was sounded by the probing radio waves of the same frequency and mode. The heating of the ionosphere resulted in the formation of artificial periodic inhomogeneities, and an increase in the intensity of all signals scattered by the D, E, and F regions and the sporadic E layer by 5–20 dB was observed. The increase was observed during heating of the ionosphere by each magnetoionic component, but was smaller for heating by an ordinary-mode wave. This effect was resonant and disappeared as a result of the frequency detuning down to 85 kHz. During the ionospheric modification, the signal-intensity increased due to modulation of the natural profile of the electron number density by the artificial periodic structure. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 8, pp. 695–708, August 2007.     

Artificial focusing system in the ionosphere

We propose a method for the creation of an artificial focusing radio wave system in the ionospheric E layer on the basis of the recombination effect by production of a bagel-shaped disturbance region in the ionosphere in the two-dimensional case and a two-band disturbance region in the one-dimensional case. Two-band disturbance of the ionosphere was achieved by appropriate reconstruction of the antenna system of the SURA facility. First experiments on the diagnostics of a focusing ionospheric system have been performed by observations from the NASA WIND space vehicle. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, Nos. 1-2, pp. 101–110, January–February, 1997.     

The polar-ionosphere phenomena induced by high-power radio waves from the spear heating facility

We present the results of experimental studies of specific features in the behavior of small-scale artificial field-aligned irregularities (AFAIs) and the DM component in the spectra of stimulated electromagnetic emission (SEE). Analysis of experimental data shows that AFAIs in the polar ionosphere are generated under different background geophysical conditions (season, local time, the presence of sporadic layers in the E region, etc.). It is shown that AFAIs can be excited not only in the F region, but also in “thick” sporadic E _s layers of the polar ionosphere. The AFAIs were observed in some cycles of heating when the HF heater frequency exceeded the critical frequency by 0.3–0.5 MHz. Propagation paths of diagnostic HF radio waves scattered by AFAIs were modelled for geophysical conditions prevailing during the SPEAR heating experiments. Two components, namely, a narrow-banded one with a Doppler-spectrum width of up to 2 Hz and a broadband one observed in a band of up to 20 Hz, were found in the sporadic E _s layer during the AFAI excitation. Analysis of the SEE spectra shows that the behavior of the DM component in time is irregular, which is possibly due to strong variations in the critical frequency of the F ₂ layer from 3.5 to 4.6 MHz. An interesting feature observed in the SPEAR heating experiments is that the generation of the DM component was similar to the excitation of AFAIs when the heater frequency was up to 0.5 MHz higher than the critical frequency. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 11, pp. 939–950, November 2008.     

Backscattering at the <Emphasis Type="Italic">E</Emphasis> and <Emphasis Type="Italic">F</Emphasis> layers for the vertical incidence of radio waves on the ionosphere

We present the results of the vertical ionosphere sounding at a frequency of 9.02 MHz using the “Sura” facility. Intense backscatter signals from meteor trails were observed at altitudes 100–130 km. Increased background of the scattered signal, which was about − 100 dB with respect to the mirror-reflected signal, was observed at altitudes of about 190–200 and 270–280 km. According to the Doppler-shift measurements of the scattered-signal frequency, the wind velocity was more than 30 m/s at altitudes 100–130 and 270–280 km and was significantly smaller in the altitude range 190–200 km. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 1, pp. 23–27, January 2009.     

Studying Specific Features of the Resonance Structure of the Background Noise Spectrum in the Frequency Range 1–10 Hz with Allowance for the Slope of the Earth’s Magnetic Field

We present new data on midlatitude features of the resonance structure of the magnetic ultralow-frequency noise spectrum in the frequency range 0.1–10 Hz, namely, the different frequency scale and frequency shift of the resonance-structure maxima for the East-West and North-South components. Resonance spectra of the magnetic components and polarization parameter of magnetic noise for the plane-stratified ionosphere model are numerically simulated with allowance for the inclination of the Earth's magnetic field on the basis of the International Reference Ionosphere IRI-2001 standard. Dependence of the calculated parameters of the resonance structure in the magnetic-component spectra on the source direction and inclination angle of the Earth's magnetic field is explored. The calculations make it possible to explain the different behavior of resonant oscillations in the linear-component spectra and the features of the resonance structure in the background-noise polarization parameter. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 7, pp. 575–584, July 2008.     

The wideband attenuation of short radio waves on mid-latitude paths during X-ray flares in January 2005

We present the results of the ionosphere oblique chirp sounding on the Cyprus—Nizhny Novgorod, Cyprus—Rostov-on-Don, and Moscow—Rostov-on-Don mid-latitude paths during X-ray flares in January 17, 19, and 20, 2005. It is found that during strong flares the blackout of short radio waves was observed over the entire frequency range of chirp sounding on the Cyprus—Nizhny Novgorod and Cyprus—Rostov-on-Don paths. Modeling of the electron-density profiles in the lower ionosphere based on absorption of short radio waves on the Moscow—Rostov-on-Don path at different stages of the decay of the X-ray radiation intensity is carried out. It is shown that at the instant corresponding to the maximum value of the flare radiation flux, the electron density in the lower ionosphere at altitudes 60–80 km increased by a factor of about 10 and 100 for flares with radiation flux densities 5·10⁻² and 3·10⁻¹ erg/(cm ²·s) in the wavelength range 0.5–4.0 Å which took place in January 19 and 20, respectively. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 1, pp. 1–8, January 2007.     

Frequency jitter and spectral width of an injection-seeded Q-switched Nd:YAG laser for a Doppler wind lidar

The design of a 50 Hz single longitudinal mode, diode-pumped and frequency-tripled Nd:YAG master oscillator power amplifier is described, and the first measurements of output parameters are presented. The laser oscillator is injection-seeded by a tuneable monolithic Nd:YAG ring laser and frequency stabilized by minimising the Q-switch build-up time. The laser system will be an integral part of an airborne instrument demonstrator for a first satellite based Doppler wind lidar to measure vertical profiles of one component of the atmospheric wind vector. This paper focuses on the investigation of the frequency jitter and the linewidth of the laser, which are measured on a pulse-to-pulse basis. For this purpose a compact, high accuracy beat frequency monitoring system has been developed at DLR. By operating the amplifier stage at half the repetition rate (50 Hz) of the oscillator, we could reduce the frequency stability from 10 MHz (rms) to 1.3 MHz (rms) (over a 14 s period). We have determined a mean linewidth of 15 MHz (FWHM) at 1064 nm. These measured laser parameters enable wind velocity measurements in the atmosphere (0–15 km) at an accuracy of 1 to 2 m/s. PACS 42.55.Xi; 42.60.Lh; 42.68.Wt     

First observations of stimulated electromagnetic emission in the ionosphere modified by the spear heating facility on Spitsbergen

We present the first results of observations of the stimulated electromagnetic emission (SEE) in the ionosphere modified by the Space Plasma Exploration by Active Radar (SPEAR) heating facility. Observation of the SEE is the key method of ground-based diagnostics of the ionospheric plasma disturbances due to high-power HF radiation. The presented results were obtained during the heating campaign performed at the SPEAR facility in February–March 2007. Prominent SEE special features were observed in periods in which the critical frequency of the F ₂ layer was higher than the pump-wave frequency (4.45 MHz). As an example, such special features as the downshifted maximum and the broad continuum in the region of negative detunings from the pump-wave frequency are presented. Observations clearly demonstrate that the ionosphere was efficiently excited by the SPEAR heating facility despite the comparatively low pump-wave power. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 11, pp. 951–955, November 2008.     

Parametric interactions of whistler mode waves and lower-hybrid resonance waves in the upper ionosphere and magnetosphere

We review the theory, as well as satellite and ground-based data testifying to the high efficiency of parametric interactions of whistler mode waves and lower-hybrid resonance (LHR) and ion-acoustic or ion-cyclotron waves. Some applications of the parametric effects to diagnostics of the ionosphere and the magnetosphere are discussed. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 8, pp. 736–739, August 1999.     

Variations of the multiple scattering contribution to the transmitted acoustic radiation intensity

A problem of acoustic radiation propagation through a 500-m layer of the plane-stratified turbulent atmosphere is solved by the Monte Carlo method. Quantitative estimates of variations of the multiple scattering contribution to the transmitted radiation intensity are presented. Calculations were performed for mid-latitude regional models of the atmosphere in winter and summer in clear and cloudy days for a point-sized omnidirectional source placed at altitudes H_s = 5–35 m above the Earth’s surface. Results of calculations for sound frequencies in range F = 500–4000 Hz with a step of 500 Hz and for outer scale of turbulence L₀ = 1–5 m with a step of 1 m and 10–100 m with a step of 10 m demonstrate significant regional and seasonal variability of the multiple scattering contribution. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 86–90, October, 2007.     

A parametric antenna for hydrophysical research on long-distance paths

This work considers the capabilities of a hydroacoustic antenna based on principles of nonlinear acoustics to perform hydrophysical studies on long-distance paths. The antenna is described and results from initial experiments at sea are reported. The antenna is installed at a depth of 40 m at the site of investigations by the Hydrophysical Institute, Abkhazia Academy of Sciences. At a pumping frequency of 20 kHz, it allows the medium to be probed in the 300–3000 Hz range of parametric radiation on long-distance routes. The sharp directional pattern (2° angular resolution) of the broadband parametric radiation ensures single-mode excitation of the marine waveguide throughout the frequency band. Generation of a specific signal by a parametric antenna provides conditions for using the frequency dispersion of the velocity of acoustic signal propagation in the marine waveguide to compress the signal as it travels along a long-distance path.     

On specific features of diurnal variations in characteristics of the diagnostic stimulated electromagnetic emission and their relation to the evolution of artificial ionospheric irregularities

We analyze variations in characteristics of the diagnostic stimulated electromagnetic emission of the ionosphere in the evening hours including the times of sunset both on the Earth’s surface at the observation point and in the ionosphere over it. It is found that an increase in typical times of evolution of the diagnostic emission begins to be recorded just before the sunset on the Earth’s surface when the ionosphere is illuminated and its parameters are not significantly changed yet. We state that the typical times of evolution of the diagnostic emission increase when the pump-wave frequency approaches the critical frequency of the ionospheric F₂ layer, but such an effect is not as significant as when passing from the illuminated to the unilluminated ionosphere. It is established that at the stage of diagnostic sounding the pump-wave pulse power does not exert any notable influence on the first (fast) stage of relaxation of small-scale artificial ionospheric irregularities, but can increase the decay time of the irregularities at the second (slow) stage of relaxation. Capabilities of the method for a study of artificial plasma turbulence using the diagnostic stimulated electromagnetic emission are discussed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 4, pp. 273–286, April 2008.     

First Experiments on Generating and Receiving Artificial ULF (0.3–12 Hz) Emissions at a Distance of 1500 km

We present first results on generation and reception of an artificial ULF signal at distances of 800 and 1500 km in the frequency range 0.3–12 Hz. The high-sensitivity receiving equipment and a stable current in the antenna exciting the ULF field in the Earth-ionosphere cavity allowed us to detect the signal with high accuracy. It was found that the amplitude-frequency dependences and the polarization of an artificial signal are strongly different in the daytime and at night, which is related to the influence of the near-Earth waveguide and resonance structures on the detected signal. We calculate the signal amplitude for different models of the ionosphere. It is shown that the model of an anisotropic, vertically inhomogeneous ionosphere describes adequately the amplitude-frequency dependences of the measured ULF signal. We calculate the apparent resistance of the underlying surface at the reception point using two measured orthogonal components H_x and E_y of an electromagnetic field. It is shown that the calculation accuracy of the apparent resistance obtained using a controlled ULF source exceeds significantly the accuracy achieved using natural electromagnetic fields.     

Wide-band measurement of gravitational waves: the VIRGO project

Summary To detect gravitational waves (GW) in a frequency range between 10 Hz and few kHz a ground-based antenna was designed by the French-Italian collaboration VIRGO. The antenna is a 3 km long interferometer with special suspensions for the optical components, planned to reduce the seismic noise in order to reach the low-frequency region. Results on the seismic isolation devices are given together with some preliminary results on the use of permanent magnets on the VIRGO suspensions. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Experimental study of the horizontal correlation of an acoustic field in an inhomogeneous shallow-water waveguide

We present the results of experiments on measuring the characteristics of a wideband low frequency (10–200 Hz) acoustic field in an inhomogeneous shallow-water waveguide in the Barents Sea area. Correlation functions of the signals received by hydrophones of autonomous bottom stations with independent reception, which are horizontally spaced 18 and 112 km apart, are measured. A pneumoacoustic radiator with an overpressure of 10.5 MPa and a volume of 7.5 l was used as a sound source. During the measurements, the signal source was shifted from the line of receivers to distances 10–225 km. The dependences of the correlation coefficient of the received signals on the radiation frequency and the interferometer-baseline length and orientation are studied. It is shown that pronounced local minima in the interval 10–40 Hz due to the difference of the inhomogeneities in the propagation channels and, therefore, the frequency characteristics of the medium are present in the frequency dependences of the correlation coefficient against the background of a monotonic decrease with increasing frequency. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 3, pp. 208–215, March 2009.