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.     

The properties of magnetoactive plasma generated by high microwave power with variable polarization

The paper deals with the measurement of the electron densityN and the transverse electron temperatureT _e of a magnetoactive plasma, generated in a circular waveguide by a strong microwave signal with variable polarization. The plasma is generated in a homogeneous magnetic field parallel to the axis of the circular waveguide. The results of the measurements show the great influence of wave polarization upon the properties of the plasma. The results of the penetration of the electromagnetic wave into the plasma as a function of the wave polarization are given, too.The authors wish to thank Dr. J. atlov for his helpful discussions and suggestions concerning the purity of electromagnetic wave polarization.     

Incidence of electromagnetic waves on a plasma-vacuum boundary

An electromagnetic wave polarized in a plane normal to the magnetostatic field, parallel to the plasma boundary, falls from a vacuum on a half-space filled with magnetoactive plasma and bounded by a wall. The coefficient of wave reflection is computed for plasmas with ₀ ² / _h ² 1.The authors express their sincere thanks to Mr. J. Václavík for his discussions of the study.     

On the role of large-scale ionospheric inhomogeneities in the formation of middle-latitude F-spread

We present the results of a special experiment carried out in the settlement of Zimenki, Nizhniy Novgorod province, on ionospheric vertical sounding by HF signals under F-spread using practically synchronous measurements of the multiple-ray nature of normal waves reflected from magnetoactive ionospheric plasma with ordinary and extraordinary polarizations at frequencies shifted by 0.7MHz. The experiment confirms the dominating role of large-scale electron-density inhomogeneities of the moving ionospheric disturbance type in the formation of middle-latitude F-spread.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 5, pp. 564–567, May, 1996.This work was supported by the Russian Foundation for Fundamental Research (project N. 95-02-03716 RFFI).     

Propagation of electromagnetic waves in hot magnetoactive plasma

The paper derives the general form of the tensor of dielectric permittivity? _ij(ω,k), Eq. (15), of non-relativistic hot magnetoactive collisionless plasma taking into consideration the influence of spatial dispersion. The general form of the tensor? _ij(ω,k) is used to express the tensorε _ij(ω,k) in the region of weak and strong spatial dispersion and in some special cases. A general dispersion equation (30) is derived and an analysis is made of the waves propagating in hot magnetoactive plasma. The expressions derived are used to investigate the damping of a right-handed circularly polarized wave propagating in hot magnetoactive plasma in the direction of the magnetic field.     

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.     

Interaction of RE<Superscript>3+</Superscript> impurity ions with inhomogeneous field of radiation

A scheme is proposed for computing the probability of the dipole-forbidden f–f transition at the interaction of the RE³⁺ impurity ion with the field of an inhomogeneous exciting radiation. It is shown that in the case under consideration, the prohibition on electric dipole transitions can be repealed without involving the Judd–Ofelt theory of induced electric dipole transitions. The quantitative estimation of the absorption cross-section of YAG:Er³⁺ crystal at the wavelength of 1530 nm (⁴I_15/2 → ⁴I_13/2) was carried out for the simplest model of the inhomogeneous exciting radiation.     

Excitation of high-frequency fluctuations by suprathermal electrons in a slowly varying magnetic field

Excitation of high-frequency fluctuations by suprathermal electrons in a dense magnetoactive plasma (_p > > _B) is considered. When the magnetic field is slowly varying, the initially stationary function of collisionless particle distribution rearranges considerably due to magnetic moment conservation. It is shown that in this case the system remains stationary relative to high-frequency potential (Langmuir) disturbances. However, anisotropy of the velocity distribution of fast electrons occurring as the magnetic field increases causes excitation of whistler mode electromagnetic waves in the plasma.Siberian Institute of Terrestrial Magnetism, Ionosphere, and Radiowave Propagation, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 8, pp. 879–883, August, 1991.     

Effect of the polarization of the electromagnetic wave on wave energy absorption caused by the linear transformation of waves

The paper deals with the experimental verification of the effect of the polarization of the electromagnetic wave on the linear transformation of waves in an inhomogeneous magnetoactive plasma. Experiments were carried out in magnetic field satisfying condition _ce/>1, where _ce, are the electron cyclotron frequency and the frequency of the electromagnetic wave. The expressive dependence of the electromagnetic wave energy absorption on its polarization found experimentally is in excellent agreement with the theory.The authors wish to thank the director of the Institute of Plasma Physics Dr. J. Váa for his interest in this work and to Drs. R. Klíma, V. Kopecký and J. atlov for many valuable discussions and comments.     

Efficiency of the microwave energy absorption in a plasma at high magnetic fields

It was shown experimentally that the efficiency of the microwave energy absorption in an inhomogeneous magnetoactive plasma strongly depends on the polarization of electromagnetic waves entering the plasma. At high magnetic fields satisfying the condition _ce />1 the RHCP wave (right-hand circularly polarized wave) is absorbed most effectively; here _ce and are the electron cyclotron frequency and the angular frequency of an electromagnetic wave respectively. The experimental results are in a good qualitative agreement with theory.On leave fromA. F. Ioffe Physico-Technical Institute, Acad. Sci. USSR, Leningrad. The authors wish to express their appreciation to Drs. V. I.Fedorov and A. D.Pilija from A. F. Ioffe Physico-Technical Institute, Leningrad, for many valuable discussions.     

Field transformation in a multimode optical waveguide with random irregularities of the film surface

A self-consistent mathematical model for the transformation of the average intensity of the mode spectrum I(z) of a waveguide field in a multimode planar optical waveguide with a step profile and rough surface is developed. This model is based on the matrix model for multiple scattering of modes in an optical waveguide. The elements of the intermode scattering matrix are found, which describe the process of mutual transfer of the energy of modes along a waveguide and their transformation into radiation modes. The transformation of the I(z) modes in waveguides with large-and small-scale inhomogeneities is investigated. It is shown that the largest qualitative differences in the noted dependences manifest themselves only in the initial portions of the optical waveguide. The length z of these portions is much smaller than the characteristic scale length L _k at which the fundamental energy of the kth mode excited in the optical waveguide is renewed. The effect of self-filtration of the mode spectrum I(z) is described, as a result of which a stable (normalized), independent of distance z, distribution I* is formed. It is established that irregularities of the optical waveguide boundaries exert a depolarizing effect on a guided light beam. The specific features of the normalization of the radiative dissipation of a group of modes I_i(z) in an optical waveguide are investigated. It is ascertained that, in the case of small-scale irregularities, the attenuation coefficient is described by a nonlinear monotonic dependence α(z), which asymptotically converges to the value α*, characteristic of the normalized field I*. When the optical-waveguide film has large irregularities, the dependence α(z) is characterized by a pronounced maximum due to the formation of alternative channels of radiative dissipation of the energy of waveguide modes.     

Electeostatic equilibrium of a modulated electron beam in a magnetoactive plasma

The two-dimensional (axisymmetric) equilibrium of a modulated electron beam (sequence of bunches) in a magnetoactive plasma under resonance conditions, when the frequency of modulation of the beam _M is close to (less than) the plasma frequency _p, is studied. The field of the collective electrostatic wave, focusing the bunches, is compensated by the thermal repulsion of the beam electrons. Based on the solutions obtained, it is established that the external magnetic field has a twofold effect on the equilibrium beam: first, to a weakening of the radial component of the focusing field because of the appearance of anisotropy in the dielectric permittivity tensor of the plasma and, second, an additional radial focusing of the bunches when they are rotated by the Lorentz force. The regions of the beam and plasma parameters in which one or another of the indicated effects predominates are determined and the conditions for the predominance of magnetic over electrostatic focusing are found.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 3–8, March, 1985.The author thanks V. B. Krasovitskii for proposing the subject and for constant interest in this work.     

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.     

Study of large-scale irregularities generated in the ionosphericF-region by high-power HF waves

Experimental studies of the features of artificial ionospheric turbulence was performed at the “Sura” heating facility in August 1998 using numerous diagnostic tools, such as scintillation, chirp-sounding, backscattering, and stimulated electromagnetic emission (SEE) measurements, as well as sounding a HF-disturbed volume (DV) by probing waves. It has been found that generation of strong artificial large-scale irregularities (ALSIs), which manifest themselves through the F-spread on ionograms, scintillations of the satellite signal propagated through the DV, and amplitude fluctuations of the probing wave sounding the DV, is observed not only for an overdense heating, at fo≤foF₂, but also at higher frequencies fo>foF₂≥f _uh (here fo is the pump-wave frequency, foF₂ is the critical frequency of the F₂-layer for O-mode electromagnetic wave, and f _uh is the plasma frequency at the upper-hybrid resonance height). This means that transfer of the pump-wave energy in the plasma due to the development of thermal parametric (resonance) instability, rather than thermal self-focussing instability, plays the key role in the ALSI generation in the case where the O-mode HF wave is used for the overdense heating. This conclusion is also confirmed by the fact that the ALSI generation is suppressed in the gyroharmonic frequency range, which is similar to the well-studied quenching of the downshifted maximum (DM) in SEE spectra. In this paper, we discuss new ALSI features revealed by the measurements, as well as the limits by which one can control the ALSI spectrum using complex pumping schemes. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 43, No. 6, pp. 497–519, June, 2000.     

A diagnostic tool for the study of stochastic properties of density striations excited by powerful electromagnetic waves in the ionospheric plasma

Powerful electromagnetic waves illuminating the ionospheric plasma generate small-scale density irregularities elongated in the direction of the geomagnetic field. Stochastic motion of these irregularities results in fluctuations of HF radio signals backscattered by the illuminated region. Observations of the full wave form of the radio signals make it possible to reconstruct the distribution function of the irregularities over their velocities. An experiment with such a reconstruction is proposed.     

A new approach to the theory of Cherenkov radiation based on relativistic generalization of the Landau criterion

Relativistic generalization of the Landau criterion is obtained which, in contrast to the classical Tamm-Frank and Ginzburg theories, determines the primary energy mechanism of emission of nonbremsstrahlung Cherenkov radiation. It is shown that Cherenkov radiation may correspond to a threshold energetically favorable conversion of the condensate (ultimately long-wavelength) elementary Bose perturbations of a medium into transverse Cherenkov photons emitted by the medium proper during its interaction with a sufficiently fast charged particle. The threshold conditions of emission are determined for a medium with an arbitrary refractive index n, including the case of isotropic plasma with n<1 for which the classical theory of Cherenkov radiation prohibits such direct and effective nonbremsstrahlung emission of these particular transverse high-frequency electromagnetic waves. It is established that these conditions of emission agree with the data of well-known experiments on the threshold for observation of Cherenkov radiation, whereas the classical theory only corresponds to the conditions of observation of the interference maximum of this radiation. The possibility of direct effective emission of nonbremsstrahlung Cherenkov radiation, not taken into account in the classical theory, is considered for many observed astrophysical phenomena (type III solar radio bursts, particle acceleration by radiation, etc.).     

Kinematics of four-wave decay of high-frequency plasma oscillations into upper hybrid and electron-cyclotron plasma waves under multiple electron gyroresonance conditions

We consider the conditions for four-wave decay of two primary plasmons with wave vector and frequency ₀ close to the multiple gyroresonance frequency n_Be into two secondary plasmons with frequencies ₁ > ₀ and ₂ < ₀. The secondary plasmons belong to the upper hybrid and the electron cyclotron branches. It is shown that the main features of the broad upshifted maximum (BUM) in the SEE spectrum can be explained in the context of the proposed process. The BUM feature appears in the region of frequencies having a positive shift from the high-power radio wave frequency. In particular the broad band nature of the BUM can be a result of the broad spectrum of wave number k₀ of the primary plasma waves. In this case the observed cut-off frequency f_cutoff limiting the BUM spectrum on the lower side can result from the lower bound of k₀ (the increase in ₁ corresponds to decay of shorter wave plasmons). In our approach we assume that the generation of primary plasma oscillations by the high-power radio wave and the conversion of secondary plasma waves into the electromagnetic waves is due to coherent scattering of corresponding waves by small-scale magnetic-field-aligned artificial irregularities or to another nonlinear processes.Published from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 3, pp. 301–317, March, 1996.     

Pinning of vortices and penetration of the magnetic field into a periodically modulated long Josephson contact

The upper field of the Meissner regime, H _up, and overheat field H′_c1, above which vortices start penetrating into a Josephson contact, are calculated throughout the range of pinning parameter I. The stability of likely configurations is investigated. It is shown that H _up = H′_c1 at any I. The existence of a single vortex centered at the extreme cell in the contact is demonstrated to be a possibility. At I > 3.69, such a vortex may exist even in a zero magnetic field. At 1.48 < I < 3.69, this vortex can exist in an external field in the range from some H _v to H _up. At I < 1.48, the vortex cannot exist under any conditions. From the equality of H _up and H′_c1 at any I, the conclusion is drawn that penetration of vortices into any Josephson medium is conditioned by the need to satisfy flux quantization conditions. Here, not the forces of vortex pinning at defects in the medium but quantization requirements are of major importance, which are satisfied in specific quantum ways rather than by meeting equilibrium conditions for vortices, forces, etc.     

Altitude–time variations in electron number density in the ionospheric <Emphasis Type="Italic">E</Emphasis> layer

We present the results of measuring the electron number density in the ionospheric E layer by a method based on the creation of artificial periodic irregularities of the ionospheric plasma with two different spatial scales. Artificial periodic irregularities were created by the radiation of the Sura heating facility at frequencies 4.7 and 5.6 MHz. The electron number density was determined by the ratio of relaxation times of the signals backscattered by artificial periodic irregularities during their sounding by probing radio waves at the mentioned frequencies. Features of the electron-density profiles obtained in 2006 and 2007 in the altitude range 95–115 km are discussed and their altitude–time variations are analyzed.