Scattering Of Radio Waves Research Articles

On the Necessity of Allowing for Antenna Heights in the Calculation of Tropospheric Radio Wave Scattering

On the Necessity of Allowing for Antenna Heights in the Calculation of Tropospheric Radio Wave Scattering

Radiowave Scattering from Surface and Subsurface Heterogeneities

Finite-difference time-domain (FD-TD) techniques allow practical numerical computation of radiowave scattering by a wide range of objects (e.g., rocks) on or in a planetary regolith. Numerical models are evaluated for two-dimensional (2-D) cases in which burial depthDand object sizeRof a single scatterer vary and for which the separation distanceLbetween two scatterers varies. A buried object typically has a significantly weaker scattering response than the same object resting on the surface, although the strongest response may occur when the scatterer is partially buried. Large objects scatter strongly and in complex ways, but the bounds on solutions appear to be well-defined; this should be useful in predicting aggregate behavior of ensembles of scatterers. The response of closely spaced objects differs significantly from the response of the objects calculated separately, but the coupling decreases rapidly with separation. Two-dimensional wavelength-scale objects (R≊ λ0) separated byL≥ 15Reffectively behave as independent scatterers. Computations in 2-D can be adapted for three- dimensions (3-D); preliminary results are consistent with measured radar backscatter cross sections for the Moon and Venus and with estimates of block population densities on the Moon. The FD-TD code generalizes to 3-D, permitting the set of case studies to be expanded for interpretation of data such as those from Magellan.

Angular scattering of radio waves: Implications for mode coupling in the solar corona

view Abstract Citations (22) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Angular Scattering of Radio Waves: Implications for Mode Coupling in the Solar Corona Bastian, T. S. Abstract The solar corona is highly inhomogeneous. The magnetionic modes may therefore couple under certain circumstances, and the polarization properties of solar radio sources may, as a consequence, be modified. An application of the simple theory of mode coupling to the propagation of radio waves in the solar corona leads to the expectation that mode coupling in quasi-transverse (QT) magnetic field regions should always be weak (coupling ratio Q much less than 1) at meter wavelengths. Observations to date suggest that, on the contrary, mode coupling is moderate (Q approximately 1) to strong (Q much greater than 1) in QT regions at meter wavelengths. I suggest that observations and theory can be reconciled by including the effect of turbulence on mode coupling. Specifically, angular scattering on turbulent inhomogeneities greatly reduces the spatial scale relevant to the theory at low frequencies. As a result, the coupling ratio is increased by a corresponding factor and mode coupling may be moderate to strong in QT regions at meter wavelengths. Publication: The Astrophysical Journal Pub Date: January 1995 DOI: 10.1086/175190 Bibcode: 1995ApJ...439..494B Keywords: Coupled Modes; Polarization (Waves); Solar Corona; Solar Magnetic Field; Solar Radio Emission; Computation; Electron Density (Concentration); Plasma Turbulence; Propagation; Wave Scattering; Solar Physics; RADIATION MECHANISMS: NONTHERMAL; SUN: CORONA; SUN: RADIO RADIATION; TURBULENCE full text sources ADS |

Scattering of an EM wave off a whistler wave in the ionosphere

Scattering of radio waves from the ionosphere has been a subject of extensive investigation in the past and is a powerful tool for plasma diagnostics. An obliquely propagating whistler wave of frequency {omega} with respect to ambient magnetic field in the ionosphere, disturbs the electron density of the plasma. When a high frequency electromagnetic (EM) wave of frequency {omega}{sub 0} is sent into the disturbed region, the EM wave gets scattered with a frequency shift {omega}. In this communication, we study the scattering of radio waves from the F-region of the ionosphere.

Scattering of radiowaves by a melting layer of precipitation in backward and forward directions

A melting layer of precipitation is composed of melting snowflakes (snow particles); the assumption of spherical particles along with mass conservation is used. The melting layer is studied by deriving the size distribution of the melting snow particles, the thickness of a melting layer, the density of a dry snow particle, and the average dielectric constant of a melting snow particle. Vertical profiles of radar reflectivity and specific attenuation are computed at 1-100 GHz by using the Mie theory for five raindrop size distributions at rain rates below 12.5 mm/h. The radar bright band is explained with computed radar reflectivities at 3-10 GHz. It is shown that the radar bright band can be absent in the melting layer at frequencies above 20 GHz. This agrees with radar observations at 35 and 94 GHz. The specific attenuation, as well as the average specific attenuation of the melting layer, is divided into absorption part and scattering part. The latter is increasingly significant with the increase of frequency. The total zenith attenuation due to stratiform rain is divided into the rain zenith attenuation and the additional zenith attenuation, which is the difference between zenith attenuation, due to the melting layer, and attenuation, due to the same path length of the resulting rain. The additional zenith attenuation increases with the increase of rain rate even at frequencies above 20 GHz. This should be taken into account in radar remote sensing and satellite-Earth communications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Scattering of radio waves by high atomic Rydberg states

The scattering coefficient of highly excited atomic levels formed by recombination from a plasma and energetically broadened by plasma field fluctuations is determined. For this purpose, the population of the levels is calculated from a detailed balance equation which is solved consistently together with the corresponding equation for the free electron spectrum. Numerical results are obtained for parameters appropriate for ionospheric conditions. The solution proves to be in no way related to LTE situations, therewith invalidating usual treatments of the problem, which are not consistently based on a detailed balance approach. The theoretical result for the equilibrium coefficient for resonant scattering of radio waves by Rydberg atoms is consistent with experimental data obtained from the ionosphere. This suggests that the considered mechanism is exclusively responsible for the scattering of radio waves in particular and electromagnetic radiation in general.

Scattering of HF Radio Waves from the Sea: A Review of the Theory Based upon a Simplified Model

A simplified, physically intuitive model of diffuse scattering of radio waves from a rough surface is used to present a self-contained derivation of first- and second-order cross sections, essentially in agreement with the standard expressions. Incoherent addition of the second-order contributions (electromagnetic and hydrodynamic) leads to a cross section which is slightly different from the more rigorously derived cross sections of Barrick (1972b) and Johnstone (1975). A surface current v has been incorporated in this model, with the main change to the cross section being a frequency shift of the entire spectrum by an amount Llw = -2ko . v.

A comparison of H-alpha intensity and radio wave scattering on eight low-latitude lines of sight

view Abstract Citations (12) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Comparison of H alpha Intensity and Radio Wave Scattering on Eight Low-Latitude Lines of Sight Spangler, Steven R. ; Reynolds, Ronald J. Abstract We have measured the Hα emission in the directions of eight extragalactic radio sources for which measurements of interstellar scattering have been made. We find that the diameter of the radio scattering disk at 1 GHz (θ_1 GHz_, a measure of the strength of scattering) is correlated with emission measure as determined from the Hα measurements. We also find that strong radio scattering characterized by θ_1 GHz_, 30 milliarcseconds is produced in regions with emission measures EM <~ 10^2^ cm^-6^ pc. The slope of the θ_1 GHz_-EM correlation is steeper than would be the case if all lines of sight were characterized by the same turbulent outer scale and "modulation index" (σ_n_/n_0_). A plausible interpretation of the observations is as follows: The lines of sight to unscattered or lightly scattered sources traverse only a diffuse medium, which is revealed by Hα observations at high latitudes. Heavily scattered sources are viewed through an additional component of interstellar H II, which has turbulence of different properties as regards rms density variation, outer scale, or both. Our results are consistent with a recent suggestion by Anatharamaiah and Narayan that strong radio wave scattering originates in extended, relatively low density envelopes of H II regions. Publication: The Astrophysical Journal Pub Date: September 1990 DOI: 10.1086/169173 Bibcode: 1990ApJ...361..116S Keywords: Extragalactic Radio Sources; H Alpha Line; Interstellar Matter; Radio Scattering; H Ii Regions; Interstellar Extinction; Magnetohydrodynamic Turbulence; Astrophysics; INTERSTELLAR: MATTER; NEBULAE: H II REGIONS; TURBULENCE full text sources ADS | data products SIMBAD (9) NED (2)

Aspect sensitivity of stratospheric VHF radio wave scatterers, particularly above 15‐km altitude

The aspect sensitivity of VHF radar scatter in the stratosphere is investigated with particular emphasis being placed on the region above 15‐km altitude. It is found that the scatter is highly aspect sensitive below about 18 km in the stratosphere as is well known, but above this height the scatter becomes more isotropic again. The highest degree of aspect sensitivity occurs around 18‐km altitude, where the backscatter polar diagram has a 1/e half‐width θs of less than 1.5°, and the horizontal correlation length can exceed 30 m. At heights above 20 km, however, θs is in excess of 4° and increases with increasing height. At any one height there is a range of scatterer shapes, from scatterers with large length‐to‐depth ratios up to almost isotropic scatterers. These results are confirmed using studies of backscattered power as a function of angle, comparisons of radial velocities as a function of beam bore‐tilt angle, and studies of the spectral width compared to the expected beam‐broadened spectral width. The results complement and support the only other published study of scatterer anisotropy in the region of 15‐ to 30‐km altitude (Hocking et al., 1986).

Experimental studies and improvements on the accuracy of the indoor random field measurement method for obtaining the radiation efficiency of electrically small antennas

AbstractThe indoor random field measurement (RFM) method is useful for measuring the radiation efficiency of a small built‐in antenna operating in the vicinity of electric gear and the human body. In this paper we report on the first experimental check of the accuracy of the indoor RFM method. We introduce the DDD value as a figure of merit for the measurement site. A new method of using radiowave scatterers is also proposed to improve the accuracy of the measurement. We find that the new indoor RFM method is effective for measuring the radiation efficiency of a small antenna.

Two years of continuous measurements of turbulence parameters in the upper mesosphere and lower thermosphere made with a 2‐MHz radar

Since February 1985, the 2‐MHz narrow beam radar near Adelaide, Australia, has been used to measure the short‐term root‐mean‐square fluctuating velocities of radio wave scatterers in the altitude range 80–100 km. Data have been recorded about every 2 to 3 hours. This is in addition to the routine measurement of winds in the region between 60 and 100 km altitude. Under certain assumptions, these fluctuating velocities can then be converted to turbulent energy dissipation rates and eddy diffusion coefficients. These measurements are presented, and their interpretation as turbulence is discussed. The possibility of contamination from high frequency, short wavelength gravity waves is considered and a correction procedure to allow for such contamination is developed. It is concluded that the corrected measurements represent a good indication of turbulence activity. Between 84 and 92 km, the turbulence intensities were approximately constant with increasing height, but between 80 and 84 km turbulence was relatively weak. Seasonal variations are discussed, and important interannual differences between 1985 and 1986 are noted.

Absolute reflectivities and aspect sensitivities of VHF radio wave scatterers measured with the SOUSY radar

By accurately calibrating the SOUSY radar in West Germany it has been made possible to measure absolute values of effective reflection coefficients and turbulence structure constants. Some typical values of these parameters as a function of altitude are presented. Such profiles are presented for both a vertically directed beam, and also for two beams directed 7° off-vertical. Comparisons of powers on the vertical and off-vertical beams show that scatter became more aspect sensitive at the tropopause and in the lower stratosphere, but, unexpectedly, scatter was observed to become considerably more isotropic in the higher regions of the stratosphere (above 15–18 km) on this occasion. An enhancement of signal from the tropopause occurred not only on the vertical beam, but also on the off-vertical beams. Comparisons of signal strengths scattered from the mesosphere and measured with the vertical and off-vertical beams showed that for the present observations mesospheric scatter was close to isotropic. The backscatter cross-sections at VHF have been compared with other measurements at medium and high frequencies at other locations, and these comparisons help set some limits on the scales of turbulent and specular scatterers in the mesosphere.

Scattering of elf radio waves on global inhomogeneities of the earth-ionosphere cavity

Using the Born approximation in the Stratton-Chu equation, a solution was obtained for the vertical electric component of an elf wave in the earth-ionosphere cavity with day-night and polar inhomogeneities. Calculations were made at a 62-GHz frequency using three models of a cavity inhomogeneity for different orientations of the beam path relative to the inhomgeneity. Relative perturbations of the field and diurnal variations in the observed elf field are obtained for a cavity with an inhomogeneity.

Scattering of radiowaves at quasiperiodic ionospheric array distorted by natural inhomogeneities

Scattering of radiowaves at quasiperiodic ionospheric array distorted by natural inhomogeneities

Interstellar Broadening of Compact Low Galactic Latitude Radio Sources

Scattering of radio waves off inhomogeneities in electron density in the interstellar medium can produce an apparent broadening in the angular diameter of an intrinsically compact background radio source. The magnitude and distribution of this effect at low galactic latitudes (|b|&lt;5°) is not well known, although several cases suggest substantial broadening in certain directions, such as the Cygnus X region (Anderson et al. 1972), and the galactic center (Davies, Walsh, and Booth 1976). Large scattering in the plane is consistent with the scintillation properties of pulsars seen through substantial thicknesses (≳ 1 kpc) of the galactic disk.

Scattering of Radio Waves in the Solar Corona

Radiation generated in the solar corona can be drastically altered by refraction and scattering as it escapes. This is especially true for sources having a frequency near the local plasma frequency, where the index of refraction approaches zero. Early investigations of the problem (e.g. Fokker 1965; Steinberg et al. 1971; Riddle 1972, 1974) have shown that the size, shape, location, intensity and time profile for the observed source can all be influenced.

Scattering of short radio waves on artificial magnetically oriented inhomogeneities exited in the ionosphere by intense radio radiation

Scattering of short radio waves on artificial magnetically oriented inhomogeneities exited in the ionosphere by intense radio radiation

Empirical model for general circulation of the atmosphere at ionospheric levels above 100 km

This paper presents an empirical model for space-time distribution of the basic parameters of the general circulation of the atmosphere at ionospheric levels ( E-and F-regions). The model is based on the results of a physico-statistical analysis of experimental data on the measurement of horizontal ionospheric drifts by close spaced receivers, carried out by the world network of stations in 1958–1970. This model allows an evaluation of the motion parameters at a given latitude, local time, season and the level of solar activity to be made. The limitations and shortcomings of the model are discussed, the results are compared with theoretical and semi-empirical schemes of the atmospheric general circulation, as well as with data of both rocket measurements of wind and drift measurements of plasma by the method of incoherent scatter of radio waves. The physics of the results obtained are stressed. The characteristics of the model are tested and defined using the materials af the coordinated program of drift measurements in the E-region from 8 stations of the northern hemisphere in 1971–1974. The characteristics of motions at higher latitudes and the longitudinal effect are discussed.

Scattering of radio waves by periodic artificial ionospheric irregularities

Scattering of radio waves by periodic artificial ionospheric irregularities

Interstellar Scattering and Scintillation of Radio Waves

A review is presented of interstellar scattering and scintillation observations in the light of modern theory for extended, spatially homogeneous, power law inhomogenieties. Formulas are presented for the experimentally observable quantities and are compared with data. A general discussion of the model is given. (GHT)