G. N. Boiko

Dynamic characteristics of stimulated radio emission from ionospheric plasma

The authors present results of an experimental investigation of stimulated radiation from ionospheric plasma. The results include the spectral characteristics, the intensity as a function of the power of the pump wave, the time evolution, and the relaxation.

On the study of artificial ionospheric turbulence by means of stimulated electromagnetic emission

Results of measurements of the characteristics of stimulated electromagnetic emission induced in ionospheric plasma by pulsed high-power radio waves (diagnostic emission) presented. The dependences of the properties of diagnostic emission on the parameters of the diagnostic wave, ionospheric conditions, and the level of development of artificial ionospheric turbulence are determined, and criteria are given for selection of optimal diagnostic conditions for ionospheric-plasma sounding. Results of experiments on the sounding of artificial ionospheric turbulence by means of diagnostic emission are provided. It is established that with transition from daytime to evening conditions, the characteristic times of emission decay are sharply increased. This is attributed to an increase in the natural perturbation of the F-region.

Results of investigation of the langmuir and upper-hybrid plasma turbulence evolution by means of stimulated ionospheric emission

The results of investigations of the temporal evolution of high-frequency plasma turbulence obtained by measurements of stimulated ionospheric emission (SIE) features for different stages of interaction of powerful radio emission, with an ionospheric F region plasma are presented. It is found that the relaxation time decreased up to 2–4 times with increase in the pump duration and pumping power under the conditions of striction parametric instability (excitation of Langmuir plasma turbulence) over times t≤2.00 ms. A similar decrease of the SIE relaxation time was also observed under the conditions of thermal parametric instability (excitation of upper-hybrid plasma turbulence) in long-term quasicontinuous heating of ionospheric plasma. The possible mechanisms of collisionless dissipation of plasma turbulence energy are discussed to explain the observed dependence of the plasma wave damping rate on the temperal stage of development and intensity of plasma turbulence.

Investigation of the dynamics of HF plasma turbulence by means of artificial ionospheric radio emission

The results of measurements of the growth and decay characteristics of artificial ionospheric radio emission and their dependence on the level of low-frequency artificial turbulence, time of day, and pump-wave frequency are presented. A time delay of the onset of the exponential nature of the decay process is detected, and its characteristics are studied. It is shown that the effect is determined by nonlinear pumping over the spectrum of high-frequency plasma turbulence. The experimental results demonstrate the possibilities of using artificial radio emission to study the properties of high-frequency plasma turbulence. Areas of future research are discussed.

Variations in the Characteristics of a HF Signal over an Oblique Sounding Path during the Solar Eclipse on August 11, 1999

We present the results of measurements of the HF-signal characteristics over the path Praha - Nizhny Novgorod during the solar eclipse on August 11, 1999. A 10-dB increase in the HF-signal amplitude and an increase in the frequency of quasiperiodic fading were observed during daytime in the course of the eclipse with phase about 68-85%. An interpretation of the effects observed is given. On the basis of simulations, we show that the increase in the amplitude is related to a decrease in absorption losses which is caused by the corresponding decrease in the electron density in D- and E-layers by about 30-45%. The increase in the fading frequency results from beating of the modes that have different Doppler shifts due to phase-path variations during the eclipse. An estimate for the rate of electron-density variation during the eclipse was obtained: dN/dt ∼ (2-2.5) ·102 cm-3s-1.

Peculiarities of the development and saturation of artificial ionospheric turbulence during a high-power disturbing transmission

The peculiarities of the wide-band anomalous attenuation of radio waves reflected in a region of the ionosphere disturbed by a high-power pumping wave are investigated experimentally. It is shown that saturation of the anomalous attenuation sets with an increase in the radiation power: its intensity and time of development are stabilized. This indicates the saturation of the magnitude and spectrum of small-scale artificial inhomogeneities responsible for the effect of anomalous attenuation. The weakening of this effect in the process of development of large-scale artificial inhomogeneities is described. The results of an investigation of the initial stage of self-action of a high-power radio wave under the conditions of cyclic operation of the high-power transmitter are given.

The Sura-EISCAT-WIND Experiments: Ionospheric Influence on the Response of a Decameter Interferometer with a Superlong Baseline

Results of the 1999 experiments on receiving radio-frequency signals from the SURA and EISCAT facilities on the WIND spacecraft at 5475 kHz are presented. Power and frequency distortions imposed by near-Earth plasmas on the response of SURA-EISCAT, an active decameter radio interferometer with a superlong baseline of ∼2000 km, are studied. Quasi-oscillatory variations in the intensity of the received radiation with a period of several tenths of a second and the corresponding maximum in the intensity fluctuation spectrum are observed during synchronized operation of the facilities under both quiet and perturbed geophysical conditions including the occurrence of mid-latitude F spread. Variations in the mean frequency of the spectral line both because of the motion of the spacecraft and the large-scale traveling ionospheric disturbances are detected. The obtained results are compared with the modern theoretical concept of propagation of short radio waves in a randomly irregular ionospheric plasma. The possibility to realize the limiting angular resolution of a ground-based decameter interferometer with a superlong baseline for observations of discrete space radio sources is discussed.

Sura-wind radar: Studies of nonlinear effects using VHF-wave occultion of the ionosphere

We present experimental results on reception of VHF signals of the transmitting facility “Sura” of the Radiophysical Research Institute by the NASA spacecraft WIND. The experiments were performed during daytime during the summer seasons 1997–1998. The dependence of power and spectral characteristics of the VHF radio waves on the power of the sounding radiation is analyzed. We find that, in a wide range of effective powers of the VHF radiation from about 40 kW to 160 MW, the phenomena observed in the radiation received onboard WIND (such as focusing, scintillations, or frequency deviations of signals) does not usually depend on the operation mode of the “Sura” facility. At the same time, broadening of the radiation pattern of the facility and decrease of the mean level of the received signal by about 6 dB toward the direction of maximum of the radiation pattern were observed at the peak radiation power of the facility. The experimental results are compared to present concepts on effects of self-focusing instability and nonlinear defocusing of VHF radio waves in the ionospheric plasma.

Anisotropy of the hectometer cosmic radio background

The angular anisotropy of cosmic radio emission at frequencies of 260, 512, 772, and 1028 kHz was studied on the WIND spacecraft when it moved to a distance of ~1.5×106 km from the Earth at the deepest phase of the last solar minimum. The modulation index of the temperature of the spacecraft dipole antenna rotating in the plane of the ecliptic was found to have an indistinct frequency dependence with a maximum of 17% near 500 kHz. This result modifies significantly the conclusions of the only similar experiment on the IMP-6 spacecraft carried out under less favorable observing conditions. Existing ideas about the causes of the radio-background anisotropy in the opacity region of the Galactic gas disk are briefly reviewed.

GROUND-BASED AND SPACE-BORNE STUDIES OF MIDLATITUDE F-SPREAD

We present the results of the first space-borne SURA-WIND experiment aimed at studying the features of short-radiowave propagation in the case of radio occultation of the near-Earth plasma in the presence of a developed midlatitude F-spread. We discover the effect of screening of a signal at frequency 4525 kHz that is 0.8 MHz higher than the minimum observed frequency for the SURA-WIND radio path determined using the ionograms obtained by the ground-based vertical-sounding station. The screening effect is explained by the effect of large-scale irregularities of the number density of ionospheric electrons of sizes about a few tens of kilometers, which lead to the appearance of electron-density depletion regions in the ionosphere stretched along the geomagnetic field. These depletion regions are separated by the ordinary plane-parallel distribution of the electron number density. The features of the frequency spectra of intensity fluctuations of the HF radiation over the SURA-WIND radio path in the presence of a developed midlatitude F-spread are considered.