Effects of Ionospheric Radio Wave Scattering in the Decameter Interferometry

We consider some theoretical issues concerning diffraction of radio waves in a randomly irregular ionosphere with application to the problems of long-wavelength interferometry of cosmic objects. The statistical characteristics of intensity fluctuations of the decameter radio emission from discrete sources in the case of ground-based observations by two-element interferometers with very long and small baselines are analyzed. Analytical expressions are obtained for the autocorrelation function of the radiation intensity and for the scintillation spectrum of a point source in the limiting cases of large and small phase increments in an irregular ionospheric plasma. We find that in the case of radio interferometric reception, the scintillation spectrum corresponding to observations of a source by a single antenna is transferred from the zero-frequency region to the region of the Doppler frequency of the interferometer. It is shown that decameter ground-based and space-borne radio interferometers can be used to study the angular distribution of the radio brightness of cosmic sources under conditions of both quiet and disturbed ionosphere.     

Radiophysical and radio astronomical diagnostics of ionospheric effects induced by a ground-based infrasonic transmitter (preliminary results)

We present results of preliminary studies aimed at detection of weak ionospheric disturbances induced by acoustic emission of a ground-based controlled transmitter. Radio astronomical and radiophysical facilities based on the decameter radio telescope URAN-3 were used in these experiments. Three methods were applied in this study: occultation of the disturbed region by radio emission from discrete cosmic sources, scattering of sounding radio waves from the perturbed ionospheric region, and weakly-oblique sounding of the ionosphere. It is shown that the parameters of transmitted, scattered, and reflected signals are well correlated with the parameters of the acoustic radiation, and that the weak ionospheric disturbances detected in our experiments are actually induced by the acoustic radiation. G. V. Karpenko Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, Khar'kov, Ukraine. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 8, pp. 785–798, August 1999.     

Experimental Studies of the Effects Observed During the Nonlinear Interaction of Two High-Power Radio Waves in a Magnetoplasma

We present the results of experiments on modification of the ionospheric F region by two high-power (P_eff 20 MW) O-mode electromagnetic waves. The experiments were performed at the Sura heating facility of the Radiophysical Research Institute (Nizhny Novgorod, Russia) in May 23–27, 1993 at the pump frequencies near the 4th, 5th, or 6th harmonics of the electron gyrofrequency. Ionospheric perturbations were diagnosed by measuring the stationary spectral characteristics of the stimulated electromagnetic emission (SEE) of the ionospheric plasma. We determine the features of variation in the spectral characteristics of particular SEE components during the simultaneous heating of the ionospheric plasma by two radio waves in comparison with the case of a monochromatic pump wave. We observed the effect of enhanced generation of the broad up-shifted maximum (BUM) by the higher-frequency pump wave. This is accompanied by strong suppression of the BUM induced by the lower-frequency pump wave. It is shown that the effects observed during the two-frequency heating of the ionosphere have well pronounced gyroharmonic properties, i.e., depend on both the electron-gyroharmonic number and the frequency detuning of the pump waves from a harmonic of the electron gyrofrequency. We also pointed out that a change in the properties of artificial small-scale ionospheric irregularities (striations) excited by high-power radio waves is not the cause of a change in the properties of the down-shifted maximum and the BUM during the two-frequency modification of the ionospheric plasma. Ways for the further development of these studies are discussed.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 2, pp. 110–133, February 2005.     

On the features of decameter radio astronomy

We consider some effects of the VHF radio wave propagation in randomly irregular plasma near the Earth. Applications of these effects to main problems of decameter radio astronomy are discussed. In particular, we show that significant measurement errors of about tens to hundreds of percent for the intensity of the VHF radio emission from an extra-terrestrial source may occur due to scattering and focusing/defocusing of the radiation in the ionosphere if VHF radio astronomical facilities operate at middle latitudes. We find that the angular resolution of discrete radio sources observed using radio interferometry and the well-known scintillation methods cannot be better than about a degree due to the effect of the developed ionospheric turbulence. We propose a modified scintillation method based on the spectral analysis of radio emission from discrete sources, which allows the useful high-frequency signal corresponding to diffraction of VHF emission at weak inhomogeneities of interplanetary plasma to be separated against strong, relatively low-frequency fluctuations of the received radiation due to the effect of the developed turbulent structure of the ionosphere. We show that such a method allows the angular resolution of extra-terrestrial radio sources to be improved up to arcsecond level. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 43, No. 2, pp. 95–105, February 2000.     

Intensity of x-ray solar radiation and the value of the anomalous absorption of radio waves during periods of sudden ionospheric disturbances

An experimental statistical dependence is obtained between the intensity of the x-ray radiation of the sun in the range 1–8 å (I_1–8) and the value of the anomalous absorption of radio waves at a frequency of I=13 MHz during periods of sudden ionospheric disturbances. This dependence has the form ₁₃ I _1–8 ^0.8 cos , and can be used for the operative classification of bursts of x-ray radiation. Its character is explained by a decrease in the coefficient of the losses of electrons with a rise in the intensity of the x-ray radiation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 21, No. 11, pp. 1558–1562, November, 1978.I wish to thank E. A. Benediktov and V. V. Belikovich for their interest in the work and their critical evaluation of its results.     

Analysis of the Dispersion Characteristic Features of Radio Channels Using a Chirp Ionosonde

We show that stationary points of the dispersion characteristics of ionospheric HF radio channels can appear due to the effect of irregularities with scales l 2-5 km and relative variations N/N 10^-3 of electron density. These stationary points can give rise to peaks of the impulse characteristics of wideband HF radio channels. Analytical formulas for estimation of the position, height, and duration of a peak are obtained. We develop and test experimentally methods for determination of the impulse characteristics of ionospheric HF radio channels with various operating frequencies and passbands using chirp-ionosonde signals as well as methods for estimation of the parameters l and N/N of ionospheric irregularities.     

Plasma waves near the double resonance layer in the ionosphere

Dispersion properties of plasma waves with frequencies close to the upper hybrid frequency _u and the multiple electron cyclotron frequency n_Be (double resonance) are considered for an inhomogeneous plasma with opposite gradients of the electron density and magnetic field magnitude. We show that a region of possible solutions of the dispersion relation decreases in real space as well as in wave vector space as of the wave frequency approaches double resonance. The results are applied to an interpretation of experiments on ionospheric modification by high-power radio waves with frequencies close to n_Be.Radiophysical Research Institute, Nizhny Novgorod, Russia. Swedish Institute of Space Physics, Uppsala Division, Sweden. Swedish Institute of Space Physics, Kiruna Division, Sweden. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 5, pp. 617–633, May, 1994.     

Radar observations of artificial ionospheric turbulence during a magnetic storm

We present the results of radar observations of artificial ionospheric turbulence (AIT) created due to modification of the ionosphere by high-power radio emission from the Sura heating facility (Nizhny Novgorod region, Russia). Measurements were carried out in August 18–22, 2003 in the evening time (16:00–20:00 UT) with the use of over-the-horizon chirp HF radars on the Khabarovsk-Rostov-on-Don, Irkutsk-Rostov-on-Don, Inskip (England)-Rostov-on-Don paths, and also on the Moscow-Rostov-on-Don path for which reference signals of the standard-time RVM station were received. It is found that conditions for propagation of HF signals through the upper ionosphere at frequencies exceeding the maximum usable frequency for standard hop propagation through the F region were realized on the Irkutsk-Sura path in the presence of the powerful sporadic E_s layer. The presence of such signals was revealed at the Rostov-on-Don station by receiving radio waves which escape from the altitudes of the ionospheric F region due to scattering by artificial small-scale magnetic-field-aligned irregularities. We studied the ionospheric effects of a magnetic storm occurring during the experiment by using the measurement data of the Doppler frequency shift of signals scattered by artificial ionospheric turbulence. It is shown that during a magnetic storm, the electric field and the drift velocity of irregularities at the altitudes of the F layer over the Sura facility reach values of 8.6 mV/m and 186 m/s, respectively, which are typical of the high-latitude ionosphere. We consider the relation between quasi-periodic oscillations of the Doppler frequency shift of the scattered signal and propagation of magnetohydrodynamic waves excited during the magnetospheric storm.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 47, No. 9, pp. 722–738, September, 2004.     

Amplitude Profiles of Solar Type IIId Radio Bursts with Echo Components and the Global Dislocation of Their Elementary Quasi-Monochromatic Sources

We prove experimentally the possibility of direct observations of natural radio echoes on the Sun. The generally simple echo hypothesis on the origin of the time splitting in solar type IIId radio bursts sustained the crucial experimental test. These conclusions are based on both the new positional data and previous information obtained using the UTR-2 antenna system in different years since 1973.In summer 1992, we were able to register for the first time a large number of decameter type IIId bursts with pronounced echo-like components using a radioheliograph at frequencies of 20 and 25 MHz. Elementary (quasi-monochromatic) sources of the short-lived bimodal type IIId bursts were observed especially often on July 6. In this case, the time delay of the second peak, which depends on the heliolongitude of the active region, reached a maximum of about 7 s.All these double-peak bursts were localized in the central region of the solar corona, and, similar to the case of single-pulse limb type IIId bursts at a given radio frequency f, the observed elementary sources of these events appeared to be nonstationary. Such a dynamic pulsating source can radically change, at least once, its location in the celestial sphere in the field of view of a two-dimensional radio heliograph during the burst lifetime . While decaying, an echo-like burst formed rather slowly may abruptly jump from one location to another. As far as its short (3- to 4-s) steep-front precursor is concerned, its visible source behaved usually as an ephemeral sub- or ultrarelativistic object moving at a subluminal velocity over a global-scale trajectory.To all appearances, a twice-pulsing, narrow-band ( f/f 0.01) radio burst with an intensity-time profile of type IIId is generally a rather long-term ( 10-20 s) transient response of the solar atmosphere to the short-term flare of harmonic radio emission generated in the real (probably motionless and compact) source. This complex double-peak profile is due not only to the existence of a reflecting surface deep in the corona near the corresponding plasma level but also, to a considerable extent, to some opaque or semi-transparent structures located at heights of the middle corona. In particular, due to the strong influence of such structures, a coronal primary source of a bimodal burst can be seen on the Earth in some indirect rays before the solar radio echo and has the form of an additional delayed pulse.     

Study of the Anglular Structure of Emission from the Radio Galaxy 3C295 in the Decameter Wavelength Range

We present the results of observations of the angular structure of the radio source 3C295 at decameter waves using the URAN-1 and URAN-2 interferometers. It is shown that the radio image of 3C295 at these frequencies differs drastically from the structure of this object observed at higher frequencies. The simplest model of the structure of this radio galaxy, comprising one component with a Gaussian brightness distribution of angular size (7.4 ± 0.4)× (8.7± 0.4) at 25 MHz and (9.1± 0.5)× (10.6 ± 0.5) at 20 MHz, is determined. A decrease in the spectral density of the emission with decreasing frequency, observed below 50 MHz, is explained.     

Form of the Frequency Spectrum of Fluctuating Radiation in the Case of Interferometric Reception During Experiments on Radio Sounding of the Solar Corona

We study theoretically the profile of a spectral line of radiation which is received by an interferometer after propagation through a medium with developed turbulent structure. We derive a formula describing the resulting frequency spectrum of an initially monochromatic radiation after its propagation through a multi-scale medium with strong large-scale and weak small-scale irregularities. It is shown that the frequency spectrum observed in this case should comprise the main spectral line with Gaussian profile and wings decreasing according to a power law with index dependent on the orientation of the interferometer baseline with respect to the direction of regular drift of frozen-in irregularities of the medium. We analyze distortions of the spectral response of a very-long-baseline interferometer due to the effect of large-scale irregularities of a medium with developed turbulent structure on wideband radiation from extraterrestrial discrete radio sources. It is shown that if the amplitude fluctuations of the received radiation are weak, then the measured frequency spectrum contains information on the drift velocity and the index of the spatial spectrum of interplanetary plasma irregularities with scales from hundreds to thousands of kilometers.     

Heterodyne interferometer with two spatial-separated polarization beams for nanometrology

An interferometer having accuracy in displacement measurement of <1 nm is necessary in nanometrology. To meet the requirement, the periodic nonlinearity mainly caused by polarization and frequency mixings should be less than deep sub-nanometer. In this paper, two spatial-separated polarization beams are used to avoid mixings and then the periodic nonlinearity. The developed interferometer demonstrates a periodic nonlinearity of about 25 pm and a 2 pm/Hz in displacement noise level.     

On the Neutron Yield in the Two-Beam Scheme of Laser Heating and Compression of Spherical Shell Targets with a Low‐Density Coating

At the initial stage of development of large-scale multiple-beam laser facilities for the ignition of the fusion reaction, facilities with a small number of beams is expected to be created. Such facilities are characterized by poor symmetry of irradiation of spherical targets. We have shown that with appropriate design of the target and distribution of the radiation intensity in laser beams the attainment of a neutron yield of 10¹⁵-10¹⁶ is possible even in twoside irradiation of the spherical targets.     

Testing general relativity: Progress,problems, and prospects

Recent results from radar and radio gravity experiments are:

1. Retardation of radar signals: 1.02 +?0.05 (planetary radar), 1.00 +? 0.04 (spacecraft);
2. Deflection of radio waves: 0.99 +? 0.12 (short baseline interferometry), +0.15 1.04 (short baseline interferometry), ?0.10. 0.90 +? 0.05 (short baseline interferometry);
3. Relativistic perihelion advance of Mercury: 0.99 +? 0.03 (planetary radar), 1.00 +? 0.01 (radar plus optical);
4. Time variation of the gravitational constant: (G/G) < 4 ×10^?10/yr (planetary radar). The first three results are expressed as fractions of the corresponding effects predicted by general relativity. In terms of the Eddington-Robertson parameters, the first two are approximately equal to (1+γ)/2 and the third to (2 + 2γ-β)/3. The third result depends on the assumption that the solar gravitational quadrupole moment vanishes; its effects cannot be separated usefully from those of general relativity with the present data set. The uncertainties are as given by the individual experimenters; in some cases they represent formal standard errors, in others an allowance for possible systematic errors is included.

     

Experimental test of the effective-photon hypothesis

The effective-photon hypothesis postulates a photon energy variable with light intensity according to a relation of the form =hv/[1– ₀ f(I)]=hv ₁ . This hypothesis has been subjected to direct experimental test. A Mach-Zehnder interferometer is illuminated with aQ-spoiled Nd:glass laser and the time history of the interference fringes is recorded with an image converter camera. The experimental results show that indeed the laser pulse is not monochromatic and that a large frequency change occurs with light intensity. In addition, the analysis of the results indicates that the principle of conservation of energy is not in contradiction with the hypothesis of effective photon, provided photons can exchange energy among themselves.     

Manifestations of the 5-Min Photospheric Oscillations in the Solar Microwave Emission

Dynamic spectra of low-frequency (LF) modulation of microwave emission during solar flares are obtained. Observation data for five radio bursts at frequency 37 GHz recorded by a 14-m Metsäahovi observatory (Finland) radiotelescope in the period from 1990 to 1993 were used. Frequency modulation of the radio emission intensity with the average period 296±37 (1) s, which is close to the period of well-known photospheric oscillations, was observed. Possible mechanisms of photospheric oscillation influence on the regions of radio burst generation are discussed.     

A simple high-sensitivity interferometric position sensor for test mass control on an advanced LIGO interferometer

A small Michelson interferometer has been configured as a tracking mirror displacement sensor in order to achieve both large dynamic range (2.1 mm) and excellent sensitivity across a broad frequency range (6Hz–3 kHz). The interferometer is illuminated by a simple LED, uses broadband, non-polarising beamsplitters and contains no lensing optics. A DC-coupled balanced detector provides an error signal that is used to position the tracking mirror of the Michelson interferometer so as to maintain an interferometer operating position close to the centre of a particular fringe. The total interferometric sensor provides a small, simple and cost-effective means of achieving high-resolution displacement measurements.     

Experimental determination of physical processes in space,leading to deviations of radio synchrotron radiation spectra from the power law

We present universal formulas for spectral characteristics of cosmic radio sources of synchrotron radiation upon the presence of spectral density maxima at certain frequencies (spectra with negative curvature) taking into account most typical physical processes observed in space. On the basis of long-term observations of angular radiation structure of cosmic radio sources in the decameter wavelength range by the URAN radio interferometer system, we determine most probable physical processes resulting in spectra with extremum values for several quasars, radio galaxies, and their separate components. On the basis of these data, we estimate some parameters of cosmic medium, magnetic field, and angular sizes of compact radio sources and their components. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 2, pp. 91–110, February 2008.     

Study of the Excitation Conditions and Characteristics of Ionospheric Plasma Turbulence at the Development Stage of the Ponderomotive Parametric Instability

We present the results of studying the development features of nonlinear effects at the initial stage of interaction of powerful HF radio waves with the plasma in the ionospheric F region. Experimental measurements were performed at the Sura heating facility for a wide pump frequency range (4.5–9.0 MHz) and a variety of pulse durations (0.3–100 ms) and effective radiated powers (1–30 MW) at various times of a day. The performed measurements allow us to study the excitation thresholds and time–amplitude characteristics of the ponderomotive self-action of a pump wave as well as the relaxation characteristics of the stimulated electromagnetic emission of the ionosphere as functions of the pump parameters and ionospheric conditions. The measured development features of the ponderomotive parametric instability in the ionospheric plasma are compared with the calculation results. The instability threshold fields (E_th 220 mV/m) and the damping rates (_e 450 s^-1) of plasma waves, measured under evening-time conditions, are close to the estimates obtained on the assumption of collisional damping of Langmuir turbulence. A significant increase in the threshold field and the damping rate (by factors of up to 3 and 6, respectively) was observed under daytime conditions. In this case, the minimum values of these quantities (E_th 350 mV/m and _e 600 s^-1) were observed for pump-wave reflection heights of about z 230 km. The measurement and simulation results are indicative of the dominant effect of photoelectrons on the development features of ionospheric plasma turbulence under daytime conditions. We discuss the possibilities of using the developed method for comprehensive monitoring of the parameters of Langmuir turbulence and the background ionospheric plasma.