Distributions of the irregularities which produce ionospheric scintillations

The distribution of nighttime irregularities which produce satellite scintillation has been examined for a midlatitude location using a large array of receivers. The irregularities are aligned along the earth's magnetic field and appear to extend from top to bottom of the F-region, being preferentially observed near the F-region ionization peak where they produce the strongest scintillations. A new method of mapping the horizontal distribution shows patches of various shapes and sizes but with no systematic structure.     

Power spectra of VHF intensity scintillations from F2- and E-region ionospheric irregularities

An experiment is described for the routine study of scintillations and ionospheric irregularities at high-latitudes using NNSS satellites with additional coordinated observations by means of the EISCAT ionospheric radar facility. Early results, obtained during the development phase of the experiment, are presented of the power spectra of intensity fluctuations at 150 MHz observed at the equatorwards edge of the high-latitude irregularity zone. The spectra of 165 samples of night-time scintillation recorded during October 1982 to May 1983 show a spectral index with a mean value of −3.58 and a steepening of the spectral slope with increasing S₄. Some examples of scintillation arising from irregularities at E-layer height show spectral indices of magnitude generally smaller than for F-region cases. A few spectra have been found with a clear break in spectral slope at around 10 Hz, suggesting two regimes for irregularities of different scale sizes.     

Simultaneous generation of electron temperature and density fluctuations by high power HF radio waves

The parametric interaction between right-hand circularly polarized electron cyclotron waves as well as non-resonant density and temperature perturbations is considered by taking into account the radiation pressure and the differential Joule heating nonlinearities. A nonlinear dispersion relation, which admits a new class of thermal parametric instabilities for the case in which Joule heating nonlinearity far exceeds the radiation pressure, is derived. It is found that the temperature and density fluctuations are rapidly driven when the pump frequency is close to the electron gyrofrequency. The relevance of our investigation to enhanced density and temperature fluctuations due to the action of high power HF radio waves in the Earth's ionosphere is pointed out.     

Regimes of ionospheric turbulence from fractal analysis of satellite radio signal scintillations

Samples of amplitude scintillations of the radio signal from a geostationary satellite obtained at a midlatitude station near Irkutsk were processed. For calculating the fractal dimensionalities the Grassberger and Procaccia [(1983) Physica D9, 189] algorithm was used. Results of the data processing tend to divide into two groups. One group includes those realizations for which it was possible to obtain reliable estimates of dimensionality. Three of the seven realizations considered were in this group, and the fractal dimensionalities were found to be low (3.12 4.5). The other data fall within the second group; a reliable estimate of dimensionality for them is unobtainable in terms of the method used. We suppose that this is attributable to the high dimensionality of the process. Power spectra of the signals of this group are close to those with an exponent of −2. The spectra of the signals of the first group are markedly steeper. On the basis of the data analyzed it is supposed that there exist two modes of ionospheric turbulence in midlatitudes, namely the mode with low dimensionality typical of localized turbulent processes, and the mode with high dimensionality typical of homogeneous turbulence that covers an extensive region of the ionosphere.     

EISCAT observations of large scale electron temperture and electron density perturbations caused by high power HF radio waves

In this paper EISCAT observations of the effect of artificial modification on the F-region electron temperature and electron density during several heating experiments at Tromsø are reported. During O-mode heating at full power (ERP = 240 MW) the electron temperature is increased by up to 55% of its ambient value at altitudes close to the heater interaction height. Measurements of the electron density have revealed both enhancements and depletions in the vicinity of the heater reflection height. These differences are indicative of variations in the balance between the transport and chemical effects. These results are compared with a time dependent numerical model developed from the perturbation equations of Vas'kov and Gurevich [(1975) Geomagn. Aeron.15, 51]. The results of numerical modelling of the electron temperature are in good agreement with the EISCAT observations, whereas there is less good agreement with regard to electron density.     

Mid-latitude ionospheric scintillations of VHF radio signals associated with peculiar fluctuations of Faraday rotation

Simultaneous measurements of the scintillation and Faraday rotation of 136 MHz radio waves radiated by the Japanese geostationary satellite ETS-2 have been studied at Kokubunji Japan during April to May, 1977.These preliminary results show that there is a close association between scintillations and the peculiar fluctuations of the Faraday rotation due to ionospheric spread F, and an empirical formula relating them is presented.Furthermore, the successive daily values of the two parameters show a close correspondence with a cross correlation coefficient of about 0.8. An unexpected result is that cross-correlation between the incidence of Es and the Faraday fluctuations on a daily bases is found to be about 0.65.     

TID,electron density and ionospheric sounding

For the approximation of a parabolic ionospheric layer, the relation between the fluctuation amplitude of radio phase path in HF vertical sounding and sinusoidal disturbance parameters has been found. Horizontal disturbance propagation has not been considered. The determining factors are the relation of the vertical wavelength of the disturbance to the half-thickness of the layer and between operating and critical frequency.     

Travelling ionospheric disturbances and the effectiveness of powerful HF transmitters in ionospheric modification and radio location of the Moon

Using ray tracing we investigate, on a qualitative level and in the linear approximation, the effects of medium-scale travelling ionospheric disturbances (MS TIDs) arising when powerful HF radio transmitters are operated in conjunction with antenna arrays designed for ionospheric modification (heating) and for radio location of the Moon. It is shown that the HF radio wave focusing effect, arising during the movement of the MS TIDs, can give rise to a strong inhomogeneous and nonstationary modulation of the space-time distribution of the field intensity of a powerful radio transmitter both at heights near the reflection region (in heating experiments) and at the exit from the ionosphere (in radio location of the Moon). The excess of intensity over an unperturbed value for typical parameters of MS TIDs in experiments on ionospheric modification can reach values of hundreds of percent: a ‘spot’ of increased intensity of the wave field can have the size of about 1–10 km, and can move with a velocity close to the MS TID phase velocity.In the case of lunar radio location, the inhomogeneity and nonstationarity of the wave field intensity distribution at the exit from the ionosphere substantially complicates the evaluation of the corresponding distribution on the Moon's surface and the interpretation of the Moon-reflected radio signal characteristics.     

Electric fields and electron density irregularities in the equatorial electrojet

Two Centaure rockets were launched from Thumba (0 47′S dip). India, with a new arrangement of double probe sensors for the simultaneous measurements of the irregularities in the electron density and the electric field along and perpendicular to the spin axis of the rocket. These experiments were carried out during the period when type I irregularities were observed with the VHF backscatter radar at Thumba. Irregularities with scale sizes ranging from a few meters to a few kilometers in the electron density and in the electric field components both in the east-west and the vertical direction could be studied with these experiments. Irregularities in the electric field in the medium scale size range (30–300 m) were observed with peak to peak amplitudes up to 20 mV m⁻¹ and in the small scale (⩽ 15 m) with peak to peak amplitudes up to 5 mV m⁻¹. Horizontally propagating waves with horizontal scale sizes up to 2.5 km were observed in the region below 105.5 km. Using linear theory for the electrojet irregularities, it was found that for 5 % perturbations in the electron density, the amplitude of the electric field can be as large as 20–30 mV m⁻¹. The spectrum of the irregularities in the vertical electric field in the rocket frame of reference was calculated and it was found that for the range of scale sizes between 10 and 70 m, the mean spectral index was −2.7 and −2.6. while in the scale size range 2–10 m it was −4.0 and −5.1 for the flights C-77 and C-73, respectively.     

Spatial and temporal distributions of midlatitude ionospheric scintillations observed by low-altitude satellites

Spatial and temporal distributions of ionospheric scintillations have been observed at Kashima (36.0°N, 140.7°E) using VHF and UHF signals from low-altitude satellites. From these observations, three different types of prevailing ionospheric scintillations seen from Japan are identified. Scintillations of type I are rather weak scintillations, occur most frequently during the daytime in summer and are primarily associated with the sporadic E-layer. However, considerable occurrences of type I scintillations are also observed during the night in summer and autumn, not necessarily due to the sporadic E-layer but occasionally due to F-layer irregularities which originate from localized midlatitude processes. Type II scintillations are much stronger than type I and occur near the equatorward horizon during spring, summer and autumn. Their occurrences start after sunset, reach a maximum before midnight and decrease subsequently, with a tendency for negative and positive correlations with the magnetic and solar activities, respectively. It is concluded that type II scintillations are the midlatitude aftermath of equatorial plume-associated irregularities and cause trans-equatorial propagation of VHF waves. From observations of type I and II scintillations, the boundary between midlatitude and equatorial scintillations is clearly identified. Type III scintillations are as strong as type II and appear only during magnetically active periods. They can be regarded as another aspect of the severe scintillation events observed on gigahertz waves from geostationary satellites as reported by Tanaka (1981).     

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.     

Characteristics of ionospheric ELF radiation generated by HF heating

This paper describes new observations of the characteristics of ELF generation produced by modulation of the dynamo current system from HF heating of the ionospheric D-region. A model of the ELF antenna structure embedded in the D-region is described and stepped ELF frequency observations are shown to support the model assumptions. Presented are data on the phase height of the ELF ionospheric antenna versus ELF frequency, polarization of the downgoing wave and relationship to the dynamo current direction, correlation of ELF field strength with per cent cross-modulation, power linearity tests and duty cycle results. All observations used the high power heater facility of the Arecibo Observatory.     

A case study of a high latitude ionospheric electron density depletion

Incoherent scatter measurements inside and outside an ionospheric electron density depletion are described. The density depletion is probably caused by an enhancement of NO⁺-ions and subsequent dissociative recombination. The NO⁺-ions are increased because high electric fields present at the geographical location of the density depletion speed up the reaction O⁺ + N₂ → NO⁺ + N. The electron as well as the ion temperature within the density depletion are strongly enhanced, the latter due to Joule heating, also caused by the electric field.     

A study of ionospheric electron density deviations during two great storms

A large foF2 data base has been collected for the two great storms occurring in March and October 1989. Plots of foF2 deviations versus time and latitude show the large depressions cannot be caused by a composition change alone; the storm induced dynamo electric field must also play an important role, especially for low latitudes. The observed ionospheric response is hemispherically asymmetric, with the autumnal hemisphere suffering a longer lasting, more latitudinally extended, and deeper depression.     

The effects of ionospheric horizontal electron density gradients on whistler mode signals

Whistler mode group delays observed at Faraday, Antarctica (65° S, 64° W) and Dunedin, New Zealand (46° S, 171° E) show sudden increases of the order of hundreds of milliseconds within 15 minutes. These events (‘discontinuities’) are observed during sunrise or sunset at the duct entry regions, close to the receiver's conjugate point. The sudden increase in group delay can be explained as a tilting of the up-going wave towards the sun by horizontal electron density gradients associated with the passage of the dawn/dusk terminator. The waves become trapped into higher L-shell ducts. The majority of the events are seen during June-August and can be understood in terms of the orientation of the terminator with respect to the field aligned ducts. The position of the source VLF transmitter relative to the duct entry region is found to be important in determining the contribution of ionospheric electron density gradients to the L-shell distribution of the whistler mode signals.     

HF scattering induced by field-aligned irregularities in the equatorial E-region during total solar eclipses

In this paper, results and analyses of solar eclipse effects on the lower ionosphere are presented. After the first contact of the total eclipse on 16 February 1980, an absorption increment of 12 dB was observed. At the same time, the frequency of amplitude fading increased largely and Doppler frequency shift disturbances appeared. The calculation of signal strength is carried out by means of Booker's scattering theory, supposing an outer scale T_o = 1000 m and an inner scale T_i = 5 m, of space scale spectrum of field-aligned irregularities in the equatorial E-region. The calculated results agree fairly well with observations. Results showed that, because of the formation of lower ionospheric field-aligned irregularities in the course of the obscuration of solar local ionization source, radio wave scattering was strengthened.     

Magnetic pulsation generation by a powerful ground-based modulated HF radio transmitter

The Hall and Pedersen ionospheric conductivity disturbances produced by high power amplitude-modulated radio waves as a function of altitude for different initial electron density profiles are calculated. Disturbances of the integral Hall and Pedersen conductivities as well as the artificial magnetic pulsation amplitudes for different initial electron density profiles and different magnetic pulsation periods are calculated too. Some recommendations for carrying out heating experiments for artificial magnetic pulsation generation are proposed.     

Small scale irregularities associated with a high latitude electron density gradient: scintillation and EISCAT observations

A coordinated experiment involving scintillation observations using NNSS satellites and special program measurements with the EISCAT ionospheric radar facility is described. The results reveal the presence of sub-kilometre scale irregularities in the vicinity of a long lived steep equatorwards gradient in electron density. Evidence is presented of a southwards plasma flow which would cause the gradient to be unstable to the E Λ B gradient-drift mechanism. An instability growth time of about 4 min has been estimated from the observations. Cooler electron temperatures associated with enhanced densities rules out soft particle precipitation as an irregularity source in this case.