Pulse propagation through a plane stratified ionosphere

The paper concerns the propagation of pulses or wave packets through an ionospheric plasma. The rise time, which appears in the ionospheric weighting function, is shown to be a realistic measure of the magnitude of the pulse shape distortion and the shortest usable pulse length in single-path, single-mode communication. Although rather general, smooth models of the medium can be dealt with, we consider in particular the case of a horizontally stratified, unmagnetized and collisionless ionosphere. Two such simple models are studied, which describe the lower part of the layer and the region of the maximum of electron density, respectively. These models allow a completely analytic treatment, and hence one may obtain explicit expressions, in terms of the parameters of the profiles, for the rise time. The illustrations show that the rise time may be less than 1 μs for a steep profile typical for sporadic E-layers, but in the range of one to a few tens of μs for propagation through a normal layer. Results of preliminary estimates indicate, however, that pulse mixing due to the differences in group path between the O- and X-mode rays dominates over pulse shape distortion in many cases.     

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.     

Application of a scattering theory of VHF transequatorial propagation

Numerical application is made of the theory of scattering by long, curved, field-aligned irregularities of ionization density in the F-region developed by Ferguson and Booker. Using an intermediate-scale regime of irregularities with an outer scale equal to the scale height of the F-region and an inner scale equal to the ion gyroradius, combined with a small-scale regime with an outer scale equal to the ionic gyroradius and an inner scale equal to the electron gyroradius, calculations are made corresponding to (i) equatorial spread-F in the VHF and UHF bands, (ii) long-range transequatorial propagation of the type observed by Nielson and (iii) short-range transequatorial propagation of the type observed by Cohen and Bowles. The same ionospheric model yields field-strengths of the right order of magnitude in all three cases. The theory also predicts a focusing phenomenon that should be looked for experimentally.     

ELF emission in the outer ionosphere stimulated by short fractional hop whistlers

Results of a detailed study by digital methods of the energy spectra and the frequency-time characteristics of electromagnetic radiation stimulated by short fractional hop whistlers (S.W.) observed on Intercosmos 14 are presented. The phenomenon was observed at frequencies below I kHz in the ionosphere at altitudes from 360 to 450 km during summer night. It has been shown that the intensity of the stimulated emission is greater by an order of magnitude or more than that of the natural noise background and of the same order of magnitude or a little lower than the intensity of the triggering S.W. The duration of the stimulated emission varies from 0.27 to 0.64 s. The energy spectra of this emission have two maxima at frequencies of 730+ 35 Hz and 940+ 35 Hz, with a bandwidth ΔF ≃ 150 to 300 Hz.     

ELF/VLF propagation measurements in the Atlantic during 1989

The vertical electric field component was measured by a group of the Ukrainian Institute of Radio Astronomy on board the Professor Zubov scientific vessel during April 1989 at latitudes from 30°S to 50°N. Results of the amplitude measurements in the Atlantic of natural ELF radio signals and those from the VLF navigation system “Omega” at its lowest frequency of 10.2 kHz are given. Characteristics were obtained of the moving ship as the field-site for the ELF observations. Variations in the ELF radio noise amplitude recorded at tropical latitudes agree with the computed data for the model of three continental centres of lightning activity. The VLF results were obtained by the “beat” technique providing the simplest narrow-band amplitude registration. Range dependencies of the field amplitudes from A (Norway), B (Liberia) and F (Argentina) stations have been analysed. The VLF attenuation factor was estimated for the ambient day conditions along the four cardinal directions. This allowed the detection of a statistically significant attenuation difference between the east-west and west-east propagation paths. The VLF radio signal was also used as a probe to evaluate the effective height of the vertical electric antenna and to calibrate the ELF noise amplitudes.     

Application of refractive scintillation theory to radio transmission through the ionosphere and the solar wind,and to reflection from a rough ocean

The results of Booker and Majidiahi (1981) concerning refractive scattering by large-scale irregularities in a phase-changing screen are combined with the theory of diffractive scattering by small-scale irregularities in order to study three intensity scintillation phenomena. The first is the reflection of radio and optical waves from an ocean surface disturbed by a spectrum of water waves. The second is the scintillation of VHP, UHF and SHF radio waves traversing the ionospheric F-region. The third is the scintillation of VHF, UHF and SHF radio waves traversing the solar wind. In each case appropriate values are chosen for the mean square fluctuation of phase, for the outer scale, for the inner scale and for the spectral index. Spectral diagrams are drawn to show how the outer scale, the inner scale, the Fresnel scale, the focal scale, the lens scale and the peak scale vary with a relevant parameter (electromagnetic wave-frequency for the ocean, RMS fractional fluctuation of ionization density for the ionosphere, and distance of closest approach to the Sun for the solar wind). For the ionosphere and the solar wind, multiple refractive scattering by weak irregularities occurs in practice whereas it is strong single scattering that is assumed in the thin-screen theory ; potential consequences of this are discussed qualitatively.     

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.     

Interplanetary magnetic field structure and the variations of the ELF and VLF emissions in the topside ionosphere

The Intercosmos-13 data obtained when measuring ELF and VLF emission amplitudes during the vernal equinox of 1975 at auroral latitudes and over the polar caps are compared with certain IMF parameters [the polarity of the sector structure, the signs and magnitudes of the B_y, and B_z, components of the IMF as defined in the Solar Ecliptic coordinate system (Nishida, 1978)]. The comparison shows that:

• 1.(i) the positive polarity of the IMF sector structure (when IMF vector is directed toward the Earth) involves an enhanced probability of the detection of larger emission field intensities (>25–30dB);
• 2.(ii) the emission median intensity is ~20dB higher at B_y > 0 compared with B_y < 0;
• 3.(iii) the 0.72 kHz emission median intensity in the polar caps and at night-side auroral latitudes is lower when B_z > 0 as compared with B_z < 0;
• 4.(iv) at vernal equinox there is no north-south asymmetry in the dependence of ELF and VLF emission intensity on the IMF parameters.

     

The theory of radio windows in the ionosphere and magnetosphere

Radio waves in a stratified plasma can sometimes penetrate through a region where, according to a simple ray theory, they would be evanescent. They emerge on the far side in a different magnetoionic mode. This occurs when the incident wave normal is within a small cone of angles, called a radio window. The best known example is the Ellis window, used to explain the Z-trace in ionosonde records. Other phenomena where windows may be important have recently been studied. Simple approximate formulae are given for the transmission coefficient of a window and for its angular widths. These show the dependence on frequency, electron concentration gradient and direction of the ambient magnetic field. Comparison with more accurate calculations shows that these formulae are likely to be reliable in practical applications. The tracing of rays near a window is discussed, and the properties of a second kind of window are described.     

The relation between ionospheric profiles and ELF propagation in the Earth-ionosphere transmission line

Based on an approximate wave solution, it is shown that, for reflexion of ELF waves from a given ionosphere described by a simple profile of ionization density, the phase-integral method may be used above a certain level, and the ionosphere may be abolished below this level. The height of ELF reflexion thus determined is independent of angle incidence, but not of frequency. The level is the one where the ionosphere passes from rapidly varying to slowly varying behavior, judged in relation to the local wavelength.An approximate solution is obtained to the mode problem in the Earth-ionosphere transmission line in terms of four crossing plane waves, one pair having O wave polarization and the other X wave polarization. The velocity of phase propagation is calculated, and also the rate of attenuation due to leakage of energy into the region above the level of reflexion. The attenuation rate due to collisional absorption below the level of reflexion is also calculated using a method similar to that employed for dielectric loss in an engineering transmission line.As the frequency descends through the ELF band, penetration of the D-region occurs in succession for the O and X waves, leading to reflexion from the E-region at the Schumann resonant frequency and penetration of the ionosphere at micropulsation frequencies. Under quiet day-time ionospheric conditions the penetration frequency-band for the D-region is around 20–60 Hz in middle and high latitudes, but around 75–100 Hz at the equator. At a frequency low enough to be reflected primarily from the E-region under quiet ionospheric conditions, an increase in D-region ionization that is just sufficient to transfer primary reflexion from the E-region to the D-region results in an increase in the rate of attenuation. On the other hand, when once reflexion is firmly established at the lower level, further increase of ionization in the D-region causes a reduction in the rate of attenuation. Similar effects are expected to occur at night in association with a sub-E-region ledge of ionization. Small variations in the ionization profile of such a ledge are the likely cause of night-time fluctuations of transmission at 45 and 75 Hz.     

Role of the plasmapause and ionosphere in the generation and propagation of pearl pulsations

The source of Pc 1 (pearl) pulsations observed in the course of the local morning hours on 7 December 1977 has been determined by the amplitude and group delay methods. The frequency of pulsations exhibit the typical diurnal variation with the maximum frequency during dawn hours. The source location of pearls during every 1-h interval is compared with the position of the plasmapause inferred from the GEOS I measurements and from previous statistical analysis. It is shown that the source of high-frequency pulsations (f > 1 Hz) is well inside the plasmapause whereas low-frequency pulsations (f < 1 Hz) occur near the plasmapause. The source of pulsations is displaced to higher L-values in the course of the local morning hours and this displacement is associated with the decrease of the frequency of pulsations. The source displacement is much more pronounced than the simultaneous movement of the plasmapause position. These observations imply that the model of the Pc1 generation which locates the source only at the plasmapause has serious shortcomings. A model is discussed which takes into account the generation of Pc1 pulsations also well inside the plasmapause and the properties of the waveguide propagation of waves in the ionspheric duct.     

On the IRI model predictions for the low-latitude ionosphere

Measurements of ionospheric electron density vertical profiles, carried out at a magnetic equatorial station located at Fortaleza (4°S, 38°W; dip latitude 2°S) in Brazil, are analyzed and compared with low-latitude electron density profiles predicted by the International Reference Ionosphere (IRI) model. The analysis performed here covers periods of high (1979/1980) and low (1986) solar activities, considering data obtained under magnetically quiet conditions representative of the summer, winter and equinox seasons. Some discrepancies are found to exist between the observed and the IRI model-predicted ionospheric electron density profiles. For high solar activity conditions the most remarkable one is the observed fast upward motion of the F-layer just after sunset, not considered in the IRI model and which precedes the occurrence of nighttime ionospheric plasma irregularities. These discrepancies are attributed mainly to dynamical effects associated with the low latitude E × B electromagnetic plasma drifts and the thermospheric neutral winds, which are not satisfactorily reproduced either in the CCIR numerical maps or in the IRI profile shapes. In particular, the pre-reversal enhancement in the vertical E × B plasma drifts around sunset hours has a great influence on the nighttime spatial distribution of the low-latitude ionospheric plasma. Also, the dynamical control exerted by the electromagnetic plasma drifts and by the thermospheric neutral winds on the low-latitude ionospheric plasma is strongly dependent on the magnetic declination angle at a given longitude. These important longitudinal and latitudinal dependences must be considered for improvement of IRI model predictions at low latitudes.     

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.

     

Comparisons between EISCAT observations and model calculations of the high latitude ionosphere

Calculations using a numerical model of the convection dominated high latitude ionosphere are compared with observations made by EISCAT as part of the UK-POLAR Special Programme. The data used were for 24–25 October 1984, which was characterized by an unusually steady IMF, with Bz < 0 and By > 0; in the calculations it was assumed that a steady IMF implies steady convection conditions. Using the electric field models of Heppner and Maynard (1983) appropriate to By > 0 and precipitation data taken from Spiroet al. (1982), we calculated the velocities and electron densities appropriate to the EISCAT observations. Many of the general features of the velocity data were reproduced by the model. In particular, the phasing of the change from eastward to westward flow in the vicinity of the Harang discontinuity, flows near the dayside throat and a region of slow flow at higher latitudes near dusk were well reproduced. In the afternoon sector modelled velocity values were significantly less than those observed. Electron density calculations showed good agreement with EISCAT observations near the F-peak, but compared poorly with observations near 211 km. In both cases, the greatest disagreement occurred in the early part of the observations, where the convection pattern was poorly known and showed some evidence of long term temporal change. Possible causes for the disagreement between observations and calculations are discussed and shown to raise interesting and, as yet, unresolved questions concerning the interpretation of the data. For the data set used, the late afternoon dip in electron density observed near the F-peak and interpreted as the signature of the mid-latitude trough is well reproduced by the calculations. Calculations indicate that it does not arise from long residence times of plasma on the nightside, but is the signature of a gap between two major ionization sources, viz. photoionization and particle precipitation.     

An improved model of the variation of electron concentration with height in the ionosphere

An empirical model of the variation of electron concentration with height is described which overcomes some limitations found in practice with a previous widely used model (Bradley and Dudeney, 1973). In particular, the new model will generate more realistic variations of electron concentration with real height and virtual height, both including and excluding an F1-ledge. The model has no gradient discontinuities and will reproduce cases in which the F1-ledge does not have a true turning point. The model should prove very valuable for a wide range of propagation problems and for certain aeronomical applications.     

A modified technique to locate the sources of ELF transient events

An algorithm is proposed for the location of the sources of ELF transient events which occur in the Schumann resonance range. The procedure yields a uniformly rotating vector in the frequency domain. The vector is formed out of the recorded vertical electric and horizontal magnetic field components. The changes of this vector with frequency provide a pair of equations for the evaluation of two of the three unknown parameters: these are the distance from the source, phase and attenuation constants of the waveguide.     

Differential Doppler measurements of the ionosphere during a solar eclipse

The effects of the solar eclipse of 26 February 1979 on the ionosphere were measured using differential Doppler techniques. Nayy navigation satellite passes were monitored at 12 sites located across the North American continent. These data yield a measurement of the vertical columnar electron content along a north-south line. Different sites monitoring the same pass provide simultaneous observations of ionospheric variations along different longitude lines. Two satellite passes occurred during or just after the eclipse. These data show a shoulderjust northward of the umbra region and a trough just behind the umbra containing large horizontal gradients. This sharp trough recovered quickly with a half-life of about 10 min.     

Nonlinear disturbances in the ionosphere due to acoustic gravity waves

The influence of the higher harmonics of an internal gravity wave on the formation of nonlinear quasi-periodic disturbances in the F-region of the Earth's ionosphere is considered. It is shown that the Boussinesq approximation cannot be used in describing a plane nonlinear gravity wave as nonlinearities associated with the compressibility of the atmosphere have to be taken into account.     

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.