Frequency dependence of anomalous absorption caused by high power radio waves

High power radio waves can modify the ionospheric electron density distribution to produce field aligned plasma irregularities which give rise to the anomalous absorption of HF radio waves. The coefficient of anomalous absorption of a vertically propagated radio wave due to scattering from field aligned irregularities has been calculated, taking into account the effects of the geomagnetic field on electron motions. These results are compared with those of other theoretical models. Furthermore, the scale lengths of field aligned irregularities produced by a high power radio wave during recent high latitude modification experiments have been determined from measurements of the anomalous absorption by means of this theory.     

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.     

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.     

Ionospheric plasma modification in the vicinity of a spacecraft by powerful radio pulses in topside sounding

A mechanism of strong turbulence is proposed for interpretation of the resonances observed by a wide-band receiver during topside sounding. The turbulence is created in the vicinity of the spacecraft due to the striction modulation instability. Experimental results obtained with the aid of a wide-band receiver on board the Intercosmos-19 satellite are discussed in terms of strong wave-wave and/or wave-particle interaction, namely electron acceleration in Langmuir cavitons and non-linear generation of the electron Langmuir plasma waves and Tonks-Dattner resonances.     

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.     

Ionospheric scintillation observations with radio interferometry

Radio astronomical interferometric observations are affected by atmospheric refraction, being particularly sensitive to inhomogeneities in the atmosphere. At frequencies below 2 GHz the influences of the ionosphere are significant in radio astronomy, especially for single dish observations and for connected element interferometry.Analytical expressions for the manifestations of weak ionospheric scintillation in radio interferometric observations, are derived. We indicate which ionospheric scintillation parameters can be derived from radio interferometric measurements. It is shown that the baseline dependence of the observed amplitude scintillation index implies a direct determination of the height of the region of random irregular electron distribution. Furthermore, the linear scale of the irregularities causing scintillation can be determined directly from the baseline dependence of the scintillation index S₄. From the mean square phase fluctuations as a function of interferometer baseline, the spatial scale of the irregularities responsible for this effect can also be determined. From a comparison with observational mid-latitude data we find indications that scintillation irregularities occur in the lower parts of the F2-layer. The spatial scale of irregularities causing amplitude scintillation is of the order of about 25 to about 500 metres. Phase scintillations are caused by irregularities with dimensions which are an order of magnitude larger.     

Ionospheric modification experiments in northern Scandinavia

The heating facility at Ramfjordmoen near Tromsø, Norway, is briefly described, and a survey is given of the experiments performed with this facility until now. These experiments comprise D-region modification, polar electrojet modulation at VLF, ELF and ULF, HF absorption and backscatter due to short-scale field-aligned irregularities, stimulated radio wave emission of the modified ionospheric plasma, short-time scale HF absorption due to the parametric decay instability, airglow modification, excitation of large-scale irregularities, and F-region cross modulation.     

The diffraction of VLF radio waves by the Antarctic ice cap

Measurements of the amplitude and phase of VLF radio signals from the Omega transmitters on La Reunion Island and in Argentina have been made on routine Antarctic re-supply nights from Christchurch, New Zealand. It has been found that when the propagation paths to the transmitters cross the Antarctic ice cap, the direct path signals are very rapidly attenuated below the receiver noise level, the dominant signal source then being provided by the radio waves diffracting around the edge of the ice cap. These results have been made possible by the simultaneous use of the phase and amplitude data in a synthetic aperture antenna type analysis.     

Modelling of transequatorial propagation of HF radio waves

In the geometrical optics approximation, a synthesis oblique ionogram of ionospheric and magnetospheric HF radio wave signals propagating between magnetic conjugate points has been carried out. The magnetospheric HF propagation is considered for a model of the waveguide formed by field-aligned irregularities with depleted electron density. The characteristic peculiarities of the magnetospheric mode have been determined: (i) strong disperion of the group delay with a frequency at 14–18 MHz, from − 1.4 to 0.6 ms/MHz for magnetically conjugate points at geomagnetic latitudes φ = 30°, 40° and 50°, respectively, (ii) spreading ∼ 1–2 ms, and (iii) a possibility of propagation between magnetic conjugates points at moderately low geomagnetic latitudes φ₀ ∼ 30–40° at frequencies exceeding 1.5 times the maximum usable frequency (MUF) of multi-hop ionospheric propagation.     

Re-radiation of VLF radio waves from mountain ranges

VLF signals transmitted from Hawaii, Japan and Australia (NPM, NDT, NWC) and scattered from large mountain ranges such as the Andes or Rockies are monitored in New Zealand in the presence of the much larger direct signals. Measurements of the amplitudes of these indirect signals are reported in the range 0.l-2.0μVm⁻¹.Sudden changes in the refractive index along the path, such as those caused by abrupt changes in ground conductivity, are shown to give rise to reflected amplitudes much smaller than those measured. However, the measured amplitudes are found to be comparable with reasonable estimates of the re-radiation from typical surface currents induced by the incident wave in mountains of suitable height, ground conductivity and steepness.     

Measuring ionospheric movements using totally reflected radio waves

It is shown that for radio waves of a particular frequency reflected totally from the ionosphere the effect of refraction as well as reflection can be simulated by an effective reflecting surface. This mirrorlike surface will give the correct angle of arrival and Doppler shift for all radars operating at this frequency. It is theoretically possible for the effective reflecting surface to be folded back on itself, but this is unlikely except for F-region echoes refracted by sporadic E-clouds. If the surface is not folded and exists everywhere, it is always possible to describe its motion and change in terms of wave undulations. Experimental data for F-region echoes show that these wave undulations are very dispersive. However, the matching between the best fitting model and the experimental data is worse than expected for reasons we do not understand.     

Reflection of VLF radio waves from distant mountain ranges

A computer cross-correlation technique is being used to determine the group delays and directions of arrival of man-made subionospheric VLF signals which have reached the receiver by paths other than the direct great circle path. The 200 baud MSK signals transmitted by NWC, NPM and NLK allow time resolution to at least 5 ms and, with 15 min of integration, the sensitivity can be as low as about 0.1 μV m⁻¹ in quiet conditions. Reflections from the Andes, the Rockies and the mountains of S.E. Asia have now been identified at Dunedin, New Zealand. Round-the-world and round-the-world-the-other-way signals have also been observed.     

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.     

Direction of arrival of radio waves reflected from the E-region

A simple system for measuring the direction of arrival of continuous high frequency radio waves reflected obliquely from the ionosphere is described. Results obtained over a near vertical path are used to evaluate changes in the electron density gradients (tilts) in the E-region. Wave interference on the transmission due to multiple hop or ground wave propagation is discussed.     

General formulae for absorption of radio waves in the ionosphere

The relationship between group path, phase path and absorption of radio waves is discussed and new approximations relating these quantities are presented. The new relationships include dependence on the angle between the wave normal and magnetic field directions and so, in contrast to other approximations they are not restricted to quasi-longitudinal or quasi-transverse situations. For deviative absorption it is found that the ordinary mode quasi-transverse approximation introduces errors of less than 5% except in the case of purely longitudinal propagation. For non-deviative absorption, use of the quasi-longitudinal approximation can introduce significant errors which particularly affect the determination of latitudinal variations in absorption.     

Iterative methods for treating the multiple scattering of radio waves

An iterative method is developed for treating multiple scattering in an extended random medium. The basis of the method is to use the single-scatter theory to derive a recurrence relation for the complex field. Since no restriction is placed on the incremental layer thickness, the single scatter theory can always be applied in this manner, subject only to the narrow-angle-scatter restriction. Indeed, in the differential limit of zero layer thickness, the recurrence relation, which is a difference equation, converges to the parabolic wave equation.The recurrence relation for the complex field is then used in conjunction with the Markov assumption to derive recurrence relations for the complex field moments of all orders. The recurrence relations for the general complex field moments converge to well known differential equations in the limit of zero layer thickness. The result is derived without constraints on the stastics of the index of refraction fluctuations. Moreover, the method of development suggests that the conditions for narrow-angle scattering alone are sufficient to guarantee the validity of the Markov approximation.Finally, a local phase-screen approximation is used to develop alternative recurrence relations for the complex field and the general complex field moments that are conceptually simpler and more efficient for numerical computation. Throughout the development, a formulation is used that retains an explicit dependence on the propagation angles relative to an arbituary but fixed co-ordinate system.     

A simple sounder to measure the properties of ionospherically reflected radio waves

A system which measures the direction of arrival, amplitude, group path and phase path of high frequency radio waves reflected from the ionosphere is described. A CW double sideband modulated signal was used and the measurements were made over a slightly oblique path. Comparisons between the group height determined using this ionospheric sounder and an ionosonde located near the midpoint of the transmission are given and results of fast fluctuations in the measurements are presented.     

The angular distribution of radio waves partially reflected from the lower ionosphere

Measurements of radio waves partially reflected from the D-region made using two antennae of very different beamwidth are reported. The arrays are composed of 40 and 4 dipoles respectively. It is shown that the gain of the larger array over the smaller is often variable—both in height and time. These results can be used to estimate the off-vertical angles from which significant energy is returned. For altitudes less than 80 km angles less than 10° seem to be usual but at higher altitudes the angles increase to values of the order of 15°–20°. Other important properties of the echoes, such as the probability distribution of the amplitude were also measured. The results are discussed with particular reference to the differential absorption method of measuring electron densities and also to the nature of the irregularities responsible for the partial reflections.     

Non-linear interaction of decametre radio waves at close frequencies in oblique propagation

Observations of ionospherically reflected signals received from a powerful modifying and a weak pulsed diagnostic transmitter over the same oblique path are described. The observed changes in signal strength and in angle of arrival of the diagnostic wave, caused by the modifying wave, are interpreted in terms of the stratification resulting from the standing wave structure of the modifying wave; the effects of this stratification are calculated numerically by solving a model problem.     

The effect of wind velocity on the amplitude scintillations of millimetre radio waves

The effect of transverse wind velocity on the amplitude scintillations of millimetre radio waves is studied. Scintillation data obtained on two line-of-sight microwave links at 36 GHz and 110 GHz on a common 4.1 km path are used to estimate the wind velocity perpendicular to the propagation path. The estimated wind velocity is within 20% of the value obtained from direct measurements using a conventional anemometer.