E. N. Myasnikov

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.

The spectral form of small-scale plasma turbulence in the auroral ionosphere

Abstract The spectra of amplitude fluctuations have been determined using 400 MHz transmissions from the Navy navigation satellite system (NNSS) observed at Spitsbergen. A spectrum model for electron density fluctuations is discussed. It is shown that a typical spectrum cut-off scale is l011 ∼5 + 15 km in the direction of the geomagnetic field.

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≥fuh (here fo is the pump-wave frequency, foF2 is the critical frequency of the F2-layer for O-mode electromagnetic wave, and fuh 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.

Observation of total electron content, amplitude and phase scintillations in the auroral ionosphere

The connection between the spatial gradient of the equivalent vertical total electron content (Ntot) of the ionosphere and scintillation patches are examined. It is shown that in the evening hours under quiet geomagnetic conditions, a steplike gradient of Ntot with latitude develops at the northern edge of the main ionospheric trough. Later in the evening, the above structure moves southward and the latitudinal gradient of Ntot becomes very pronounced and around it a scintillation patch is formed. During magnetic disturbances, the monotonic sharp gradient of Ntot is destroyed. A number of maxima of total electron content which become collocated with many scintillation patches are formed. Further, under disturbed conditions, the power law index of both the amplitude and phase scintillation spectra shows a marked variation with latitude which may be quasiperiodic.

Turbulence spectrum of the upper ionosphere

The problem of defining the spectral form of ionospheric irregularities with dimensions from hundreds to thousands of meters is considered. A generalized model is proposed for the ionospheric turbulence spectrum, taking into account both the anisotropic properties of the large-scale fraction of irregularities and the dependence of the anisotropy (elongation) of small-scale irregularities of the upper ionosphere along the Earth magnetic field direction on the transverse scale of those irregularities. Relations have been derived to determine the basic parameters of the irregularity spectrum of the uppers ionosphere (anisotropy indices for large-scale and small-scale fractions) and the depth of a thin ionospheric layer through measurement of the spectral characteristics of amplitude and phase fluctuations of orbital satellite signals. Using this model of the plasma irregularity spectrum, we can explain consistently many well-known experimental data on spectral characteristics of the phase and amplitude fluctuations of orbital satellite signals both in the high-latitude and midlatitude ionosphere.

Spectral structure of auroral F layer patches

Analysis of the power law spectral form for the differential phase and amplitude scintillation spectra has been performed for the orbital satellite beacons at 150–400 MHz receiving at the site Verkhnetulomsky (68.4°N, 31.85°E). It is shown that inside of the local scintillation patches, which are seen during ground-based observations in the vicinity of the east-west direction, a significant increase of power law slope occurs at the scales l⊥ > 0.5 km. A model of plasma density spectrum combining both the small-scale rod-like and large-scale sheet-like irregularities is proposed.

Determination of internal scale of artificial ionospheric turbulence in direction of geomagnetic field

A 150-MHz satellite beacon is used to determine the internal scale in the direction of the geomagnetic field I0‖ for the spectrum of artificial ionospheric turbulence created by the Yastreb heating facility located near Nizhny Novgorod in continuous operation at a frequency of 5.75 MHz (ordinary polarization) with effective power P·G≃100·150 kW. It is found that I0‖ ≃ 3−4 km for transverse inhomogeneity scales I⊥ ≃ 1−2 km and I‖ ≃ 0.7−0.9 km for I⊥ ≤ 0.5 km.

Spatial shape and development dynamics of auroral scintillation patches

Results of simultaneous measurements of altitude distribution of auroral small-scale irregularities and total electron content by the method of radio illumination of the ionosphere by signals from an orbital satellite at 150–400 MHz are presented. It is shown that under magnetically quiet conditions the small-scale irregularities (ℓ⊥∼1km) tend to occupy local regions, which extend over ≲ 100– 200 km along the geomagnetic fieldB with ≲ 5– 20 km from the North to South. Characteristic times of formation and collapse of such local structures under the conditions of weak geomagnetic activity are of the order of more than one hour.

Measurement of the anisotropy of inhomogeneities in the auroral ionosphere by means of signals from satellites

The authors show how it is possible to determine the shapes of inhomogeneities by using a method of correlation analysis of the fluctuations in signals from orbiting satellites. The authors show that when this method is used, the finite thickness of the layer containing the inhomogeneity must be taken into accout. It is established that the inhomogeneities in the auroral ionosphere which are responsible for the amplitude fluctuations in the signals are extended along the lines of force of the geomagnetic field and that they have a shape which is close to being axially symmetric in the plane orthogonal to the geomagnetic field and that the fluctuations in the signals may be concentrated in localized regions.