ISSN:0048-6604    

DOI:10.1029/95RS00580

年代:1995

数据来源: WILEY

摘要:

A new algebraic approach is proposed to calculate the electrostatic potential distributed around a point source in isotropic Maxwellian plasma. The method derives a power series expansion of the radial distance from the source with frequency‐dependent coefficients. Distance and frequency are normalized to the Debye length and to the plasma frequency, respectively, so that the expression keeps its entire generality whatever the experimental conditions might be. The proposed method is based upon the Mittag‐Lefler expansion of the inverse of the plasma dispersion function for the infinite series of Landau poles. After mathematical clarification of the validity of this expansion, a significant correction of the previous works leads to a self‐consistent interpretation of the true contribution of the higher‐order poles at large distance from the source. The power series expansion is compared to the classical so‐called “Landau wave approximation” which is proved to include in reality the contribution of higher‐order poles independently from the plasma temperature. For practical use the power expansion is needed to obtain a precise result at distances from the source shorter than about 15 Debye lengths, while the Landau wave approximation gives correct results at larger distances. This work provides all necessary baselines for precise three‐dimensional modeling of mutual impedance devices to be used in space plasma experiments where the Debye length is comparable to th

ISSN:0048-6604    

DOI:10.1029/94RS03167

年代:1995

数据来源: WILEY

摘要:

We present observations with a 915‐MHz boundary layer profiler of reflective layers in the clear air that persist as light rain falls through them. In the three cases presented a clear‐air echo is initially present over several hours and confined to a horizontal layer a few hundred meters thick. Subsequently, light rain falls through the layer but does not eliminate the spatial pattern of refractive index irregularities which presumably accounts for the clear‐air echo. When the reflectivity of the rain is comparable with that of the clear air, Doppler spectra in the vertical profiler beam have separate modes arising from Rayleigh scattering by the rain and Bragg scattering by the clear air. The modes are distinguishable because of the fall velocity of the drops relative to the air. Using an algorithm to separate the clear air and precipitation components of these bimodal spectra, we can isolate the power and velocity of each mode. This allows us to see interactions of the two backscattering mechanisms. In one puzzling case the clear‐air reflectivity in a layer increases by an order of magnitude as rain falls through it, but in the other cases no such strong effect occurs. We discuss possible causes of the increase in reflectivity but find no convincing expl

ISSN:0048-6604    

DOI:10.1029/94RS03168

年代:1995

数据来源: WILEY

摘要:

A thin substrate approximation is used to derive closed form solutions for the Green's functionsGeandGmappearing in the mixed potential integral equation modeling of planar microstrip structures. The expression forGmis given in terms of the contributions from a source and its image, while that ofGeis derived in terms of the so‐called quasi‐dynamic contribution plus a “correction” term. Excellent agreements with the exact numerical integration of the corresponding Sommerfeld integrals for a wide range of parameters are obtained. In particular, it is demonstrated that the addition of the correction term is needed in order to accurately predict the sharp dip and its associated phase reversal that appear in the magnitude ofGein the nea

ISSN:0048-6604    

DOI:10.1029/94RS02927

年代:1995

数据来源: WILEY

摘要:

In this paper the characteristic mode theory is used for the computation of the receive and transmit mode aperture field of the parallel plate‐fed antenna. In the transmit case the input region has the form of a waveguide carrying a single mode wave with electromagnetic characteristics that are different from those of the output region. The output region is considered to be free space. In the receive case the structure is excited by an incident wave which can be either a transverse electric (TE) or a transverse magnetic (TM) plane wav

ISSN:0048-6604    

DOI:10.1029/94RS03158

年代:1995

数据来源: WILEY

摘要:

Measurements of low grazing angle radar sea backscatter and multipath in the frequency range from 30 to 300 MHz, along with the supporting theoretical modeling, have been lacking because of the practical difficulties associated with fielding radars in this band. An ultrawideband (UWB) radar that radiates waveforms encompassing this band has recently been installed at a seaside location near the Naval Research and Development facility, Pt. Loma, San Diego, California. Propagation path loss measurements were performed at 40, 90, 140, and 190 MHz. Broadband clutter measurements were performed over the frequency band 200 to 300 MHz. The theoretical model is based on an integral equation solution for the surface fields. The surface fields are obtained using a new mathematical derivation of the integral equation for the surface fields (Ott, 1992). This new integral equation extends the frequency range of the former algorithm, WAGNER, to the VHF band. The surface fields are then integrated to obtain either the forward scatter (multipath) or the backscatter (clutter).

ISSN:0048-6604    

DOI:10.1029/94RS03016

年代:1995

数据来源: WILEY

摘要:

A matrix formulation is performed for a linear current‐carrying source located within a cased borehole valid for any frequency. The surrounding formation could be any number of concentric homogeneous regions. It is shown that induced polarization phenomena at the casing‐formation interface can also be modeled as a limiting special c

ISSN:0048-6604    

DOI:10.1029/94RS03160

年代:1995

数据来源: WILEY

摘要:

We investigate observationally and theoretically the response of polarimetric backscattering at 24‐cm wavelength to the thickness of Arctic sea ice in leads and first‐year ice features. We employ backscattering data acquired by the Jet Propulsion Laboratory airborne synthetic aperture radar (SAR) during March 1988 in the Beaufort Sea, together with nearly simultaneous passive microwave imagery acquired by the U.S. Navy Kaband radiometric mapping system. We find that 24‐cm copolar ratios and copolar phases vary strongly with apparent ice thickness. We observe copolar phase shifts between −10° and −50° (relative to multiyear ice phases) for new ice features in the imagery, as well as positive copolar phases in a first‐year ice feature. Copolar ratios also vary with apparent thickness, from values larger than those expected theoretically for seawater to values slightly lower than those expected for thick ice. We derive a signature model based on scattering from a rough air/sea ice interface with realistic vertical profiles of brine volume and relative permittivity beneath. Model predictions for copolar ratios and phases show ice thickness‐dependent variations consistent with those observed. We present simulation results showing that plausible ice thickness variations between pixels in a multilook average diminish, but do not eliminate, the signature response to thickness. This suggests that direct thickness estimation of sea ice in leads may be possible using polarimetric SAR at wavelengths of

ISSN:0048-6604    

DOI:10.1029/94RS02313

年代:1995

数据来源: WILEY

摘要:

An analytical solution of Gaussian beam scattering by a conducting circular cylinder coated with an anisotropic material is formulated in terms of the wave functions for anisotropic media. This solution is formally similar to the solution of Gaussian beam scattering by a conducting elliptical cylinder with an isotropic dielectric coating. In the anisotropic dielectric coating and in the exterior free space region, the fields are expanded by the newly developed wave functions for anisotropic media and the well‐known wave functions for isotropic media, respectively. Numerical results are presented for the reference purpose. The effects of the spot size and the location of the waist of the beam, and the electrical size of the anisotropic‐coated conducting circular cylinder on the scattered field are discus

ISSN:0048-6604    

DOI:10.1029/94RS02925

年代:1995

数据来源: WILEY

摘要:

The algorithm developed to recover the zonal (east‐west) wind profile at Jupiter by Doppler tracking of the Galileo probe is well established in theory and verified by computer simulation. We extend the Doppler wind recovery algorithm to include detection and extraction of microscale wind structure. When the probe encounters regions of high wind shear, turbulence, atmospheric waves, or nonsteady aerodynamics (buffeting) a rapidly varying component is introduced into the probe telemetry Doppler data. These effects can be isolated by their respective Doppler signatures. Analysis of residuals allows fine structure in the wind profile to be located and retrieved. The overall wind recovery is a three‐step process: (1) The large‐scale zonal wind profile is recovered by converting the probe telemetry frequency residuals (as caused by the Doppler effect) to a velocity and back‐projecting it into the local horizontal east‐west direction at the probe location. The zonal winds are derived from the velocity residuals by a least squares algorithm. (2) Regions of high zonal wind shear are recovered numerically relative to the large‐scale wind structure and serve as a correction to the previously derived large‐scale profile. (3) The probe trajectory is updated to reflect both the large‐ and small‐scale wind structure, and the large‐scale wind profile is recalculated. The Galileo Doppler wind experiment described previously and the wind microstructure retrieval algorithm outlined here are the only reasonable means by which the vertical profile of the zonal winds at Jupiter can

ISSN:0048-6604    

DOI:10.1029/94RS03162

年代:1995

数据来源: WILEY