L. M. Erukhimov

Temporal behaviour of artificial small-scale ionospheric irregularities: Review of experimental results

Abstract Features of artificial small-scale ionospheric irregularities (ASI) induced at F-region heights by powerful HF waves are discussed. The investigations presented here were performed during the past two decades at the Zimenki and Sura heating facilities, located at middle latitudes near Nizhniy Novgorod, Russia, as well as at the Gissar facility located at a lower latitude near Dushanbe, Tadzhikistan. The measurements were made by a variety of diagnostic methods employing artificial field-aligned scattering of HF and VHF radio waves, sounding of the disturbed region by means of low-power probing waves and testing of the artificial turbulence by means of stimulated electromagnetic emission (SEE). The dependence of ASI on such parameters as transverse scale length of the artificial irregularities, their location in the disturbed region, the power of the heating wave, the duration of HF radiation, geophysical conditions, aftereffects of the preceding modification, and the schedule of heater operation is considered for both the development stage after pump turn-on and the decay stage after pump turn-off. The temporal evolution of ASI spectral characteristics during the heater period is discussed. An empirical model for the ASI, based on the data available, was elaborated and verified by computer simulation of SEE generation.

Artificial ionospheric turbulence (review)

This study is an analysis of artificial ionospheric turbulence (AIT) arising near the level at which a powerful wave is reflected with ordinary polarization. AIT is an inhomogeneous structure in the ionosphere with a size on the order of centimeters or tens of kilometers and with characteristic frequencies from a fraction of a hertz (aperiodic inhomogeneity) to several megahertz (plasma waves). The authors are primarily concerned with small-scale artificial ionospheric turbulence (SAIT), i.e., with inhomogeneities that are greatly extended along the geomagnetic field with transverse dimensions that are less than the wavelengths of the perturbing waves - the pumping waves (PW) - in a vacuum.

A combined parametric and conversion mechanism for upshifted stimulated electromagnetic emissions

We study the combined process of a three-wave parametric decay of an oscillating electric pump field into an electron Bernstein and a lower hybrid wave, and a conversion of the pump wave off slowly growing field aligned irregularities into an upper hybrid wave which merges with the lower hybrid wave. It is found that slowly growing, upshifted stimulated electromagnetic emissions, observed experimentally for pump frequencies slightly above electron cyclotron harmonics, are well explained within this model. In the radiation spectrum there is a lower cutoff frequency which increases linearly with the cyclotron harmonic.

PECULIARITIES IN THE EVOLUTION OF THE BUM OF STIMULATED RADIO EMISSION OF THE IONOSPHERE

We present measured spectral and dynamic characteristics of the broad upshifted maximum (BUM) in the spectrum of artificial ionospheric radio emission (AIRE). It is shown that BUM is already formed starting from pump-wave frequencies that are smaller by at least 10 kHz than the values of electron gyrofrequency harmonic nfHe determined by the disappearance of the principal downshifted maximum (DM) in AIRE spectrum. The characteristics of additional maxima in BUM spectrum, which are a multiple of its principal maximum, are studied. It is shown that the frequency of BUM intensity maximum does not depend on the pump-wave frequency and the characteristic times of its evolution increase sharply for fPW close to nfHe. We discuss the possibility of existence of the “fast“ and “slow“ components in BUM spectrum, which have significantly different evolution times. Experimental results on BUM behavior for different regimes of ionosphere modification are presented.

A new upshifted spectral stimulated electromagnetic emission structure, observed between electron cyclotron harmonics

Observations are reported of a new upshifted stimulated electromagnetic emission (SEE) feature, termed broad upshifted structure (BUS). This wideband emission is found to be upshifted by up to 170 kHz above the pump frequency if the pump frequency lies in a frequency range far from a gyroharmonic frequency. The dependence of the BUS feature on the pump frequency and pump power, as well as its temporal evolution, are investigated. The experiments were carried out at the Sura heating facility in Russia by modification of the ionospheric F region, using a HF ordinary mode pump wave.

Hysteresis effect in the artificial excitation of inhomogeneities in the ionospheric plasma

A hysteresis effect is discovered in the dependence of the scattering cross section of artificial ionospheric inhomogeneities on the intensity of the radio waves exciting them. The thresholds for the excitation and cutoff of the plasma instability are determined and the dependence of the magnitude of its fluctuations on the power of the exciting transmitter is established.

ON THE EFFECT OF BUM GENERATION ENHANCEMENT REVEALED USING THE SCHEME OF ADDITIONAL HEATING OF IONOSPHERIC PLASMA

We present measured characteristics of the artificial ionospheric radio emission (AIRE), which were obtained experimentally using additional heating of the ionospheric F-region by O-polarized waves. It is shown that the observed enhancement of intensity of the broad upshifted maximum (BUM) of the AIRE can result from the influence of electrons accelerated in the plasma: esonance region on its generation. An empirical model of the phenomenon observed is developed. It is concluded from experimental results that the BUM has a complex structure and only one of its components produces the above emission enhancement. We show the possibility of using the AIRE in additional heating of ionospheric plasma for diagnostics of artificial ionospheric turbulence and investigation of the features of perturbation propagation along the geomagnetic field lines.

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.

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.

Numerical simulation of the mutual conversion of ordinary and extraordinary waves in a magnetised plasma by artificial ionospheric turbulence

Abstract Some effects of normal mode coupling in weakly anisotropic inhomogeneous plasma are analysed on the basis of the numerical solution of transfer equations for the Stokes parameters. The numerical analysis of normal mode conversion on a set of isolated irregularities demonstrates the possibility of an effective polarisation transformation on such structures. It is shown that by the appropriate selection of discrete irregularities of the external magnetic field direction or plasma electron concentration resulting in radio-wave refraction, one can control electromagnetic radiation polarisation characteristics. Analytical expressions for mean Stokes parameters have been obtained for the rare isolated irregularities. The opportunity for the simulation of mutual wave conversion processes in the ionospheric heating experiments is discussed.