Y. M. Yampolski

Bistatic HF radar diagnostics induced field‐aligned irregularities

In late March and early April of 1995, an experimental campaign involving the modification of the F2 ionospheric layer took place in Russia and Ukraine. HF pulsed and CW Doppler radar observations of field-aligned irregularities within the region over the Sura heater were made with the UTR-2 radio telescope serving as the receiver. In this paper, some preliminary results of our bistatic radar observations of the modified region are presented. These results include measurements of the drift, evolution, and decay of inhomogeneities and turbulence contained within the modified region and evidence of interactions between the ionospheric plasma, Alfven waves, and free geomagnetic pulsations.

Traveling ionospheric disturbance diagnostics using HF signal trajectory parameter variations

In the model of a perfectly reflecting surface the inverse problem of restoring dynamic and statistical parameters of traveling ionospheric disturbances (TIDs) from variations of angles of arrival and Doppler frequency shift of HF radio signals on one-hop oblique radio paths is solved. The technique for determining dispersion law, fluctuation spectrum, geometry, direction, and velocity of TID motion is described.

HF radar observations of decaying artificial field‐aligned irregularities

Bistatic HF radar scatter from magnetic field-aligned irregularities created by the Sura heater near Nizhny Novgorod, Russia, has been detected and analyzed at the UTR 2 decametric radio telescope near Kharkov, Ukraine. The decay phase of the irregularities exhibits two decay time constants in general. The shorter time constant, which is only sometimes seen, is consistent with classical diffusive dissipation theory for small-scale, field-aligned waves. The longer time constant, which is almost always present, is much too long to be accounted for by diffusion. It is more likely associated with the decay of large- and intermediate-scale plasma turbulence. Three-wave nonlinear interaction with turbulent modes is shown to be capable of generating the field-aligned irregularities.

Non-linear interaction between Schumann resonances and HF signals

Abstract The effect of cross-modulation between Schumann resonances (SR) and HF (15 MHz) roundthe-world signals (RWS) has been established experimentally. The peaks near the frequencies 8, 14, 20 Hz were discovered in the spectra of the RWS. These maxima correspond to three first modes of SR. A theoretical model which explains the effect of cross-modulation is proposed. The interaction of the signals is due to the electron heating by SR fields in the D-layer of the ionosphere. This leads in its turn to the modulation of the electron collision frequency and to the attenuation of RWS. It is essential that the fair weather electric field in the ionosphere is taken into account. The influence of this field leads to a linear dependence of the electron temperature variations on the field strength of SR.

Application of polarimetric sounding to HF ionospheric remote sensing

It is shown that the scattering of electromagnetic waves by refractive index inhomogeneities in a continuous random medium is accompanied by a certain polarimetric phenomenon that is analogous to the Brewster effect of classic electrodynamics. In this paper, an observational method and results are discussed for the case of bistatic HF radar sounding of small-scale ionospheric inhomogeneities at frequencies above MUF. The height of the scattering layer, its thickness, and characteristic plasma drift velocity in the ionosphere are estimated as a result of statistical processing.

HF radar probing of the lower magnetosphere

The first observations of ion acoustic waves at altitudes of about 4000 km in the auroral zone were carried out using the powerful SURA HF radar in 1991. Here we report additional observations made with the Russian SURA radar and the Ukrainian UTR 2 receiving system in December 1992. Although echoes similar to the earlier observations were detected, we performed a number of diagnostics and have concluded that no unambiguous magnetospheric echoes were seen at SURA during the 1992 campaign. (There is some indication that the more sensitive UTR 2 system received magnetospheric echoes.) Some aspects of the coding scheme used in the experiments that might lead to spectral artifacts are discussed. A new antenna under construction at SURA will allow transmission at twice the frequency used to date and should permit more definitive observations in the future.

Interaction between artificial ionospheric turbulence and geomagnetic pulsations

Abstract Quasiperiodical variations in the Doppler shift of HF signals scattered by artificial ionospheric turbulence have been investigated experimentally. It is assumed that a possible cause for these variations might be natural ionospheric MHD-waves. The amplitude of the magnetic component of such waves resulting in the observed Doppler shift magnitudes has been estimated as 1 γ.

Effects of Energetic Solar Emissions on the Earth–Ionosphere Cavity of Schumann Resonances

Schumann resonances (SR) are the electromagnetic oscillations of the spherical cavity bounded by the electrically conductive Earth and the conductive but dissipative lower ionosphere (Schumann in Z Naturforsch A 7:6627–6628, 1952). Energetic emissions from the Sun can exert a varied influence on the various parameters of the Earth’s SR: modal frequencies, amplitudes and dissipation parameters. The SR response at multiple receiving stations is considered for two extraordinary solar events from Solar Cycle 23: the Bastille Day event (July 14, 2000) and the Halloween event (October/November 2003). Distinct differences are noted in the ionospheric depths of penetration for X-radiation and solar protons with correspondingly distinct signs of the frequency response. The preferential impact of the protons in the magnetically unshielded polar regions leads to a marked anisotropic frequency response in the two magnetic field components. The general immunity of SR amplitudes to these extreme external perturbations serves to remind us that the amplitude parameter is largely controlled by lightning activity within the Earth–ionosphere cavity.

Ionospheric diagnostics using wave field diffraction near the caustic

Results of investigating the spatial and temporal HF field structure near the skip distance are presented. The data were obtained on midlatitude radio paths less than 1000 km in length, using broadcasting stations as sources of the probe signals. The signals reflected by the ionosphere were received and processed with a multichannel coherent receiver and the largest phased array of the decameter radio telescope UTR-2. The strong dependence of the probe signal parameters on the ionospheric plasma conditions has allowed formulating and solving a number of model inverse problems of ionospheric diagnostics. It proves possible to determine parameters of the parabolic ionospheric layer, and the effective electron collision frequency, to evaluate horizontal gradients of the electron density and some parameters of the medium-scale-size ionospheric irregularities. A number of tests made both within the method and by means of comparison of the restored parameters against the data of other diagnostic techniques demonstrate the high potential and accuracy of the approach described in the paper.