Angular characteristics of radio pulses reflected from the subpolar ionosphere

This paper presents the results derived by measuring angular spectra of HF-radio pulses reflected from the subpolar ionospheric F2-region (62°N) using vertical-incidence soundings and a phase direction finder with Doppler filtering. The results correspond to three main types. One is the classical mirror reflection from the undisturbed ionospheric F2-region, typical of mid-latitudes (deviations from zenith do not exceed 3°; the angular spectrum width is less than 1°). The second type includes oblique diffuse reflections with a deviation from zenith of from 10 to 45°. The azimuth of arrival of these reflections is distributed in the range from 0 to 360°, the angular spectrum width is from 5 to 10°, and the range varies from 400 to 600 km. The third type includes anomalous mirror reflections with small deviations from zenith (not greater than 3°) but with substantially larger detection ranges (for example, 500km) as compared with the main reflections (250–300 km).     

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.     

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.     

Stationary large-scale irregularities of the ionosphere

Ionospheric absorption measurements (Al method) made in the course of eight voyages by Soviet and other research vessels indicate that the global distribution of absorption shows a distinctive regional structure. Areas of abnormally high absorption in the neighbourhood of the equator have been located in the Pacific near the west coast of South America and in the Indian Ocean. The west Mediterranean area also shows abnormally high absorption. In some cases these areas of high absorption appear to coincide with areas of low nocturnal F-region electron density.     

The maintenance of the night-time ionosphere at mid-latitudes. I. The ionosphere above Malvern

The total rate of recombination in the night-time ionosphere above Malvern (at L = 2.6) was estimated using a model atmosphere, and the results were compared with the observed rate of change of total electron content to determine the net influx of plasma. Horizontal transport under the influence of electric fields was an important factor on a time-scale of an hour or less but when averaged throughout the night made little contribution. The main influx of plasma was a downward diffusion from the protonosphere, especially before midnight. The average downward flux increased steadily as the protonosphere filled after a magnetic storm, with a saturation time of at least 8 days.     

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.     

Topside irregularities in the equatorial ionosphere

This paper is a brief review of ionospheric irregularities in the equatorial topside ionosphere. Results from topside sounders, direct measurement satellites, and the Jicamarca incoherent scatter radar are discussed. Scintillation observations and theories of irregularities are not discussed in detail as these are the subject of other review papers. Many of the phenomena detected in the topside ionosphere are related to bottomside irregularities, commonly known as spread-F. These include aspect-sensitive scattering observed on topside sounders, significant concentrations of Fe⁺, electrostatic turbulence and the topside irregularities detected by the Jicamarca radar. Satellite measurements show that the irregularities in electron concentration have amplitudes which increase almost linearly with wave-length over the range 70m to 3km. Duct irregularities detected by the topside sounders and some wavelike irregularity structures detected occasionally by direct measurement satellites may be separate from the general spread-F phenomenon although this has not definitely been established.     

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.     

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.     

The maintenance of the night-time ionosphere at mid-latitudes. II. The ionosphere above St. Santin

The total rate of recombination in the night-time ionosphere above St. Santin (at L = 1.8) was estimated using a model atmosphere and the results were compared with the observed rate of change of total electron content to determine the net influx of plasma. Horizontal transport under the influence of electric fields was measured but at the latitude of St. Santin this was always small and averaged over the night as a whole the contribution was negligible. Downward diffusion provided the main source of plasma and the flux predicted was compared with the flux measured at 450 km. The comparison was good provided the model atmosphere was modified to use exospheric temperatures based on actual measurements by the incoherent scatter radar. A comparison with the results obtained at Malvern (Paper I) confirmed that the saturation time for the protonosphere at L = 1.8 is far less than at L = 2.6 and that the downward flux from the saturated protonosphere was also less.     

Amplitude statistics of signals reflected from the ionosphere

Statistical analysis methods used to define the amplitude distributions of signals returned from the ionosphere are discussed in this paper. Emphasis is placed on determining accurately the parameter B, which is the ratio of steady to random components present in a signal. Thus B > 1 if the signal is dominated by the steady component, and B < 1 when the random components dominate. This study investigates the characteristics of B for F-region and E-region ionospheric echoes, as well as some types of spread-F, observed at the southern mid-latitude station Beveridge (37.3 S and 144.6 E). The results indicate that amplitude measurements obtained in approximately 100 s are adequate for determining B. The results also illustrate some effects that the E-region can have on F-region echoes.It is found that frequency spreading, the most common type of spreading observed at Beveridge, displays strong specular reflections and some signal variation due to interference at the leading edge of the F-region echo (i.e. B > 2). Within the spread echo B fluctuates between 0 and about 1.5 but is typically less than 1. The autocorrelation function of signal amplitude has a relatively large coherence interval, suggesting that this type of spread-F is due to interference of specular reflections from coherent irregularity structures with horizontal scale sizes of tens of kilometres rather than scattering from small scale irregularities. A second form of spread-F which would generally be classified as frequency spreading on standard ionoerams is actually due to off-vertical reflections from patches ol irregularities which originate south (poleward) of Beveridge. Echoes within this oblique spread-F (OS-F) do not exhibit coherence indicating that the irregularities responsible are of a smaller scale than those producing normal frequency spread. Finally, the phenomenon of spreading occurring on the second hop, but not the first hop trace is studied. It is shown that the form of the second hop echoes can be reproduced using a simple geometric model of ground scatter. The interpretation is supported by the fact that B for spread second hop echoes is less than 1 whereas it is much greater than 1 for the corresponding first hop echoes.     

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.     

Effect of severe auroral disturbances on the equatorial ionosphere

It is now an established fact that during extremely strong magnetic storms a sudden anomalous decrease in the F-layer critical frequency foF2 is sometimes noticed at the equator around noon-time and the duration of this effect is known to be anywhere between some tens of minutes to several hours. As an extension of earlier work by Turunen and Rao, 1980, seven severe auroral storm events based on AE index have been selected during the period July 1958–June 1960 and their effects on the equatorial ionosphere have been investigated utilizing the published ionospheric data for the chain of Indian stations starting from equatorial latitudes and extending up to the mid-latitudes. From this study, it is noted that at the equator around noontime the foF2 values decrease and the noon bite-out phenomena are enhanced. However, as one goes towards mid-latitudes this trend is reversed. Because of this, the Appleton anomaly is also enhanced during disturbed days. Besides, the fF_s values at the magnetic equator show an increase during disturbed days indicating thereby that the eastward equatorial electrojet current is enhanced on disturbed days. This suggests that the auroral electrojet current is coupled to the equatorial electrojet current possibly via the magnetosphere.     

Vertical penetration of planetary waves into the lower ionosphere

Earlier work which provided evidence for coupling between pressure variations in the stratosphere and lower ionosphere in winter has been extended. Day-to-day changes in the height of fixed electron density isopleths in the E-region at a middle latitude often exhibit quasi-oscillations with amplitudes between 2 and 10km and periods between 5 and 30 days. It is found that the correlation between these oscillations and corresponding variations in the height of winter isobaric surfaces in the stratosphere, resulting from the presence of planetary-scale waves, is sometimes good and sometimes poor. Examination of the type of wave disturbance in the stratosphere and of the stratospheric zonal wind profiles suggests that the conditions for stratosphere-ionosphere coupling are met only when well-defined planetary waves of increasing amplitude with height are seen in the lower stratosphere and when the stratospheric zonal wind pattern is favourable to the vertical propagation of such waves.     

Modelling the penetration of thundercloud electric fields into the ionosphere

In a previous paper, we considered the penetration of DC thundercloud electric fields E into the ionosphere and also into the region between the ionosphere and the ground (Velinov and Tonev, 1994). In the present paper, we extend the analysis by making a more precise approximation of the electric conductivity profiles by 5–10 piecewise exponential functions of altitude instead of the two functions used up to now. This allows a much more realistic representation of the atmospheric conductivity profile. Besides, Maxwell's equations are solved for more general boundary conditions, taking into account that the electrosphere is not a perfect conductor. This leads to the appearance not only of the transverse E_r (as had been assumed until now), but also of the geomagnetic field-aligned E_z component of the penetrating thundercloud electric fields. The computations show that both E_r and E_z cause significant variations of the electron density profiles N(z) in the ionosphere.     

On the global and regional behaviour of the mid-latitude ionosphere

Data from a chain of seven ionosondes in the range of 56–38 N and 1–38° E geographic coordinates were analysed to illustrate the global and regional behaviour of the mid-latitude F-region for some selected geomagnetic storms that occurred during the solar cycle 21. It was found that there are different spatial scales in the response of the mid-latitude ionosphere to the disturbance in the magnetosphere-ionosphere thermosphere system. The physical mechanisms and processes are discussed in relation to the relevance of various theories in the understanding of the dynamics of ionospheric storms.     

Absolute electron density measurements in the equatorial ionosphere

Accurate measurement of the electron density profile and its variations is crucial to further progress in understanding the physics of the disturbed equatorial ionosphere. To accomplish this, a plasma frequency probe was included in the payload complement of two rockets flown during the CONDOR rocket campaign conducted from Peru in March 1983. In this paper we present density profiles of the disturbed equatorial ionosphere from a night-time flight in which spread-F conditions were present and from a day-time flight during strong electrojet conditions. Results from both flights are in excellent agreement with simultaneous radar data in that the regions of highly disturbed plasma coincide with the radar signatures. The spread-F rocket penetrated a topside depletion during both the upleg and downleg. The electrojet measurements showed a profile peaking at 1.3 × 10⁵cm⁻³ at 106 km, with large scale fluctuations having amplitudes of roughly 10 % seen only on the upward gradient in electron density. This is in agreement with plasma instability theory. We further show that simultaneous measurements by fixed-bias Langmuir probes, when normalized at a single point to the altitude profile of electron density, are inadequate to correctly parameterize the observed enhancements and depletions.     

Climatic trends of the mid-latitude upper atmosphere and ionosphere

Long-term variations of electron concentration in the mid-latitude ionosphere, independent of heliogeophysical conditions and the meteorological characteristics of the atmosphere, have been determined. It is suggested that a long-term decrease of atomic and molecular oxygen concentrations in the mid-latitude thermosphere is the most possible reason for the trends found in ionospheric parameters.