Determination of the turbulent spectrum in the ionosphere by a tomographic method

A theory of tomographic reconstruction of the statistical properties of the random turbulent ionospheric plasma is presented. Derived integral equations for the coherence functions of the measured fields allow the determination of inhomogeneous layer coordinates and the reconstruction of cross-sections of the electron density correlation functions. For statistically homogeneous layers and a transmitter on board a moving satellite with a linear receiving array on the ground, we have the possibility of determining the three-dimensional correlation function structure or its spectrum using a set of two-dimensional cross-sections. One receiver allows the reconstruction of the spectrum of the two-dimensional cross-section of the correlation function. We also consider the solution of the inverse problem for non-homogeneous fluctuations. In this case the distribution of the electron density fluctuations, its variance and the correlation coefficient, characterizing the spatial structure of fluctuations may be reconstructed by a tomographic technique. Experimental results on the identification of the layer height of the irregularities and on the spectrum of the two-dimensional cross-sections of the correlation function measurements are presented.     

Electromagnetic wave propagation in a field-aligned-striated cold magnetoplasma with application to the ionosphere

The problem of wave propagation in a cold magneto-ionic medium with plane or circular cylinder small scale random density fluctuations is considered. By including second or terms in the perturbation it is possible to derive a refractive index for a “mean” wave in the medium. It is shown that the striations cause new cutoffs and resonances to occur. The validity of the theory for a warm plasma will depend on the plasma temperature and on the spatial spectrum of fluctuations. Possible applications are considered.     

Variations of the lower ionosphere parameters measured by the resonance scattering method

This paper presents the results of diagnostics of the lower ionospheric parameters by using the resonance scattering method (RSM) of radio waves on periodic artificial irregularities (PAI). The following ionospheric parameters have been measured by the heating facilities ‘Zimenki’ and ‘Sura’: electron density including the E-F interlayer valley and the lowest height of the ionosphere; the velocity of the vertical wind; the relaxation times of the PAI characterizing coefficients of ambipolar diffusion, and coefficients of attachment and detachment of electrons from negative ions. Variations of these parameters have been considered as a function of height, local time, season, solar activity as well as being induced by passing acoustic-gravity waves and by ionospheric disturbances.     

ELF and VLF wave generation by modulated HF heating of the current carrying lower ionosphere

This paper presents further experimental results on ionospheric current modulation, using powerful amplitude modulated HF waves produced by the new heating facility at Ramfjordmoen near Tromsø, Norway. As a result of the current modulation, waves in the ULF, ELF and VLF range can be efficiently generated. The experiments discussed here cover the range from low ELF up to 7 kHz. The observed signal strengths are of the order 1 pT. Decomposition of the received ELF/VLF waves into R- and L-mode shows that both modes are usually of comparable strength. The signal strength as a function of modulation frequency shows pronounced maxima at multiples of approximately 2 kHz. The paper also presents a brief theoretical discussion of the processes involved in the generation of ELF/VLF waves by HF induced current modulation.     

Diagnostics of the lower ionosphere by the method of resonance scattering of radio waves

Results are presented on measurements of lower ionosphere parameters (electron concentration, coefficient of ambipolar diffusion, etc.) by the method of resonance scattering of radio waves by periodic artificial irregularities. The method of resonance scattering is based on the generation of periodic irregularities in the ionospheric plasma by powerful radio emission and investigation of the characteristics of the back scattering of diagnostic radio waves by these irregularities.     

Passage of a powerful HF radio wave through the lower ionosphere as a function of initial electron density profiles

The results of numerical modelling of powerful HF radio wave propagation through the ionosphere plasma are presented. Comparison of the heating wave parameters with those of a low powerwave gives the possibility to study the self-action of the powerful HF wave. At low altitudes the ‘translucence’ of the ionosphere plasma takes place. At high altitudes the wave absorption sharply increases. Both self-action effects lead to the reduction of the altitude range of the heated region. The dependence of self-action effects and the resulting electron temperature profiles on the initial electron density profiles are studied.     

Disturbances in the lower ionosphere caused by a low altitude nuclear explosion

Observations of the lower ionospheric disturbance caused by a low altitude nuclear explosion are presented. A forward scatter radar, frequency 41 MHz, power 2.5 kW, was used to study these disturbances. The first radar scattering signal consisting of three peaks appeared 40 s after the explosion. It was due to early ionization by delayed y-rays. The second kind of disturbance generated after 190 s was clearly different from the first. The scattering signal had a constant component which indicated a strong specular reflection. The field strength increased by more than 20 db. This disturbance was produced by the direct shock wave. The third kind of disturbance began after 8 min, lasted 5.0 min, and was probably dominated by the fireball/smoke cloud oscillation when it reached its stabilization altitude and approached hydrodynamical equilibrium with the ambient atmosphere. Using numerical computation techniques, we have explained the above results well.     

Small-scale fluctuations of magnetic and electric components of the ELF and VLF wave fields in the sub-auroral topside ionosphere : stochastic characteristics of the wave field

When the Interkosmos-14 and Interkosmos-19 satellites crossed the region of spatially varying electron concentration in the topside ionosphere adjacent to the high-latitude boundary of the main ionospheric trough, it was discovered that there were simultaneous fluctuations of plasma density, temperature and the amplitudes (H_x and E_y) of the ELF and VLF radio/plasma emissions. The probability characteristics of the naturally perpendicular H_x and E_y fluctuations are analysed. The correlation coefficient R(_H, E_y) turned out to be less than 0.6 at frequencies of F ⩽ 4.65 kHz, while at higher frequencies R increases, up to 0.9 at 15 kHz. The following interpretations are proposed:

1. 1.1. While measuring noise emissions, as a rule a mixture of numerous elementary waves is recorded.
2. 2.2. At frequencies exceeding the local lower hybrid resonance frequency (in our case f_LHR ≈ 5 kHz), a mixture of electromagnetic waves experiencing the influence of the inhomogeneous electron concentration N_e is registered.
3. 3.3. At frequencies which are lower than the local value f_LHR the mixture mainly consists of ELF waves. The wave field has a complicated structure, and the dynamical coherence between electric and magnetic field components is not as simple as at VLF frequencies (f ≈ 15 kHz).
4. 4.4. It is shown that the wave components for a mixture of electromagnetic and electrostatic waves (for instance a mixture of VLF and lower hybrid frequency waves) have a lower correlation coefficient because the electrostatic waves are unrelated to the electromagnetic waves.
5. 5.5. The correlation analysis offers an opportunity to detect the presence of waves of various types in the wave mixture.

     

Refraction distortions of transionospheric radio signais caused by changes in a regular ionosphere and by travelling ionospheric disturbances

This paper generalizes experimental data on variations of the angles of arrival of transionospheric radio signals caused by changes in a regular ionosphere and by effects of medium-scale travelling ionospheric disturbances (TIDs). The data are based on radio astronomical observations of discrete sources and compact active features on the Sun as well as on angular measurements of signals from artificial Earth satellites with geostationary and circular orbits.The experimental data are interpreted through calculations of refraction corrections using a Gaussian model of a regular ionosphere disturbed by a three-dimensional travelling wave (the TID model) as well as an adaptive model of a regular ionosphere. Some possibilities of correcting refraction distortions with the use of appropriate models and ionospheric diagnostic tools are discussed.     

Reflection of electromagnetic waves by density gradients in a magnetised plasma

The reflection of electromagnetic waves by density gradients in a plasma with a magnetic field derived from a scalar potential is studied. Conditions are found for reflection at grazing incidence at a surface S containing field lines. No restriction is placed on the angle between the wave-normal and the field, and account is taken of the curvature of the surface and the anisotropy of the medium. Particular reference is made to whistler-mode waves in the magnetosphere. The minimum electron density enhancement needed for reflection is calculated. Guidance by small-scale irregularities and ‘super whistlers’ (whistlers with upper cut-off above one-half the equatorial electron cyclotron frequency) are briefly discussed.     

Some aspects of change in flaked stone industries of the mesolithic and neolithic in Southern Britain

In a recent article, one of the writers suggested that the traditional bipartite division of the British Mesolithic, based on consideration of microlith types, was reflected in the shape of waste flakes (Pitts, 1978a). Four early assemblages were dominated by narrow flakes, in contrast to three later ones with a higher representation of broader flakes. In this paper, the originally tentative conclusions are explored in greater detail, and the next major boundary, the start of the Neolithic, is investigated. Related changes in the tool components are also examined and the outlines of an explanatory framework are proposed.     

Production and control of ion-cyclotron instabilities in the high latitude ionosphere by high power radio waves

The possible generation and suppression of ion-cyclotron waves in a collisional plasma by external high power electromagnetic (EM) waves with frequency close to the local upper-hybrid frequency is considered. It is shown that the ion cyclotron instability can be destabilized (stabilized) for ω₀ <ω_UH (ω₀ > ω_UH), where ω₀ is the pump frequency of the EM wave. The results are applied to naturally occurring ion-cyclotron instabilities in the high latitude ionosphere.     

Shortcomings in our understanding of the lower ionosphere as revealed by an analysis of radiowave absorption measurements

Empirical relationships derived from an extensive series of equatorial ground-based radiowave absorption measurements are analysed in terms of our current understanding of the basic processes that control the formation and behaviour of the lower ionosphere. Multi-frequency absorption and virtual height measurements are combined with the results of a rocket borne experiment to construct an equatorial noontime reference electron density profile corresponding to the conditions of a non-flaring sun at solar cycle maximum. The enhancement in this electron density profile, and hence in the absorption of radio waves, as the solar 1–8 Å flux increases by moderate amounts is then calculated from our knowledge of the ion production and loss processes. In order to bring the calculated increase of absorption into agreement with the empirically established relationship (Gnanalingam, 1974), we find it necessary to reduce by a factor of about 5 the Meira (1971) nitric oxide densities below 90 km.Attention is drawn to the wide disparity between the effective recombination coefficient and the average dissociative recombination coefficient for the known ion composition in both the molecular and cluster ion regions of the lower ionosphere. The factor of 5 reduction in the nitric oxide profile required to explain the solar control of absorption, however, largely eliminates this disparity in the cluster ion region.Our analysis of the diurnal variation of radiowave absorption and virtual height also reveals a gross discrepancy between the calculated and measured variations. In order to resolve this discrepancy, a mechanism is needed which would cause the electron density in the 95–105 km region to decrease with increasing solar zenith angle faster than is predicted by the present theory of ion production and loss processes in this region.Also presented in this work is an ad hoc model of the diurnal variation of the electron density profile which is consistent with the measured diurnal variation of radiowave absorption and virtual height.     

Observational technique and parameter estimation in plasma line spectrum observations of the ionosphere by chirped incoherent scatter radar

With the 430 MHz incoherent scatter radar facility at the Arecibo Observatory, the plasma line spectrum has been measured with a linearly frequency modulated or ‘chirped’ pulse. On reception the signal is demodulated (‘dechirped’). The paper describes the experimental set-up and the parameter estimation for day-time ionosphere observations. Using a non-linear least squares fit it is possible to determine the linear and quadratic coefficients of a locally parabolic plasma line frequency versus height profile. The strength of the plasma line and the point of tangency of the locally parabolic plasma line frequency profile can be determined very accurately. In measurements of the day-time ionosphere the plasma line was found to be enhanced by about 70 times over the thermal equilibrium level.     

Some aspects of Doppler radar measurements of the mean and fluctuating components of the wind field in the upper middle atmosphere

The basic assumptions made when a Doppler radar is used to measure the mean and fluctuating components of the wind field in the middle atmosphere with various beam configurations are examined. Particular reference is made to the measurement of the various components of the Reynolds stress tensor associated with short period internal gravity waves. It is shown that it is not generally possible to measure the upward flux of horizontal momentum with the conventional Doppler radar beam configuration in the upper middle atmosphere and that an optimum beam configuration is that in which beams are directed at +θ,0 and − θ to the zenith in both the zonal and meridional planes. This allows five of the six components of the Reynolds stress tensor (all those except the horizontal transport of momentum) to be obtained directly from the mean square radial velocities. In addition, the mean wind components and, in principle, the horizontal divergence and stretching deformations may be obtained. The power spectrum of the horizontal velocity may also be calculated using only the assumption that the statistics of the motions are horizontally homogeneous.