摘要:

A uniform asymptotic expansion of the time‐harmonic antenna current with large parameter In [2kz/(ka)2] is given. As usual, the expansion is based on Hallén's filament integral and is restricted by the conditionz≫ a but is otherwise uniform inkz. The use of averaging leads to the vanishing of the second term of the asymptotic series. Two different averaging procedures are used to derive two accurate formulas: The first one, which is very simple, is useful for practical applications. The second formula is of theoretical interest. An improved asymptotic series for largekzis derived to relate the averaging results to existing asymptotic results. Comparison of these two formulas and the exact antenna integral demonstrates that these formulas are more accurate than any other existing formulas for infinite cylindrical antenna current. The Hallén filament integral is found to approximate closely the exact integral even when the conditionz≫ais violated. Discrepancies in the literature are di

ISSN:0048-6604    

DOI:10.1029/93RS01438

年代:1993

数据来源: WILEY

摘要:

The problem of transverse electric plane wave scattering from a rectangular channel which is engraved in a perfect conducting plane is investigated. A Fourier transform technique is employed to express the scattered field in the spectral domain in terms of parallel‐plate waveguide modes. The boundary conditions are enforced on the conducting surface and the channel aperture to obtain simultaneous equations for the transmitted field inside the channel. The simultaneous equations are solved to represent the transmitted field in a series form. By using the stationary phase approximation, the exact expression for the far‐zone scattered field is obtained and the echo width behavior is numerically studied in terms of the scattering angle, frequency, and channel size. It is found that the behavior of the echo width versus the channel depth exibits resonance irrespective of the size of the channel width. An approximate series expression (Kirchhoff solution) for the echo width is valid for high‐frequency scattering regimes (a>2 λ). We also present an approximate closed form solution for nadir backscattering which is valid when

ISSN:0048-6604    

DOI:10.1029/93RS01371

年代:1993

数据来源: WILEY

摘要:

Guided waves in a general class of waveguides with biisotropic filling are studied. The biisotropic medium is the most general isotropic linear medium, that is, nonreciprocal chiral medium. Field equations are derived first for arbitrary waveguide cross sections and then applied for a special circular cross section. The following special boundary impedance conditions have been covered: ideally conducting surface, isotropic, and anisotropic impedance surface and open surface. Some general properties of the eigenwaves are established, and the influence of nonreciprocity together with chirality and losses is discussed. Calculated curves are shown for biisotropic circular waveguides with ideally conducting and isotropic impedance surfaces.

ISSN:0048-6604    

DOI:10.1029/93RS00361

年代:1993

数据来源: WILEY

摘要:

This paper presents a model with ellipsoidal scatterers for applications to polarimetric remote sensing of anisotropic layered media at microwave frequencies. The physical configuration includes an isotropic layer covering an anisotropic layer above a homogeneous half space. The isotropic layer consists of randomly oriented spheroids. The anisotropic layer contains ellipsoidal scatterers with a preferential vertical alignment and random azimuthal orientations. Effective permittivities of the scattering media are calculated with the strong flucutation theory extended to account for the nonspherical shapes and the scatterer orientation distributions. On the basis of the analytic wave theory, dyadic Green's functions for layered media are used to derive polarimetric backscattering coefficients under the distorted Born approximation. The ellipsoidal shape of the scatterers gives rise to nonzero cross‐polarized returns from the untilted anisotropic medium in the first‐order approximation. Effects of rough interfaces are estimated by an incoherent addition method. Theoretical results and experimental data are matched at 9 GHz for thick first‐year sea ice with a bare surface and with a snow cover at Point Barrow, Alaska. The model is then used to study the sensitivity of polarimetric backscattering coefficients with respect to correlation lengths representing the geometry of brine inclusions. Polarimetric signatures of bare and snow‐covered sea ice are also simulated based on the model to investigate effects of different scattering mec

ISSN:0048-6604    

DOI:10.1029/93RS01605

年代:1993

数据来源: WILEY

摘要:

We report here for the first time the detection of a seasonal variation in the occurrence of peaks in the spectra of energetic ∼ 100‐keV electrons precipitating from the inner radiation belt as the result of cyclotron resonance between the waves and the electrons. The spectra reported here were observed from the low‐altitude polar orbiting satellite P78‐1 at midnight local time. The data, acquired over the 5‐year period from March 1979 to July 1984, compose the first multiyear data set used to study cyclotron resonance peaks. The geographical distribution of the events is strongly clustered around the 22.3‐kHz VLF transmitter NWC in Australia. Binning the data into seasons, we find that the events occur 2.5 times more often during the 6 months from day 80 to 263 than during the remainder of the year and that the seasonal difference is statistically significant at a high level. Since the observed change is in the frequency of occurrence of the peaks and not in the mean energy of the peaks, we attribute the effect to a seasonal change in ionospheric transmission of VLF waves rather than a seasonal change in the equatorial cold pla

ISSN:0048-6604    

DOI:10.1029/93RS01375

年代:1993

数据来源: WILEY

摘要:

The relationship between rain attenuation and depolarization over a 7‐km path was observed in about 30 separate rain events during the spring and summer of 1990. The experimental equipment was capable of measuring attenuation and cross‐polarization discrimination (XPD) directly at about one measurement per second. The relationship between these two variables was found to be of the nature of a deterministic relationship but with time‐varying parameters. Parameter variations appeared to be associated with rising and falling phases of rain attenuation. When successive measurements of XPD were plotted against the logarithm of attenuation, each rising or falling segment was found to follow a straight line relationship but a slightly different line for each segment. Further, in any given rain event, the slopes and intercepts of the extended straight lines were themselves highly correlated, indicating a tendency for all the lines to pass through a single point in the XPD‐log attenuation plane. A simple model in which the behavior is controlled by the parameters of rain density (attenuation) and the standard deviation of canting angle of the raindrops, varying independently, appears to be consistent with the observ

ISSN:0048-6604    

DOI:10.1029/93RS00567

年代:1993

数据来源: WILEY

摘要:

We analyzed the wind variances of gravity waves during 10–100‐min periods observed in the mesosphere at 67–80 km altitude with the MU radar at Kyoto (35°N, 136°E) and the MF radar at Saskatoon (52°N, 107°W) in 1989. In order to compare the results quantitatively we applied similar data analysis procedures to the data sets which were obtained with different observation techniques. We first calculated frequency spectra of wind velocity, then estimated variances, ,, and by integrating the spectra. The frequency spectra of the horizontal wind u′ and υ′ generally showed a spectral slope of about −5/3 at both locations. The spectral slope of w′ at Saskatoon ranged from −2/3 to −1 in winter, and −1/3 to −1/2 in other seasons, while it became nearly zero in May and September at both locations. The slope for the MU radar observations was less steep and sometimes became +0.15–0.3 in the frequency region lower than 50–70 day−1(periods more than 20–30 min). The variance, ,, and , showed semiannual variation with a large maximum in summer, a secondary peak in winter, and minima in September and April. The annual average of the ratios of, , andbetween the MU and Saskatoon MF radars was 1.6, 1.5, and 1.6. This result is interpreted as suggesting that the variance of gravity waves with periods of 10–100 min at 35°N is stronger than that at 52°N, even if the possible biases are taken into account. The variances due to gravity waves are more conspicuously enhance

ISSN:0048-6604    

DOI:10.1029/93RS01513

年代:1993

数据来源: WILEY

摘要:

Analysis of data from recent experiments leads to the observation that the distribution of underdense meteor trail durations differs from classical predictions. A new theory for the distribution of underdense meteor trail durations has been developed based upon the assumption of a normal height distribution and lognormal amplitude distribution which leads to a lognormal distribution of the trail durations. It is shown that this model provides a better fit to observed data than the classic model. This model, applied in conjunction with previously reported work, can be used to provide improved predictions of the performance of meteor scatter communications systems.

ISSN:0048-6604    

DOI:10.1029/92RS02879

年代:1993

数据来源: WILEY

摘要:

To develop the theory of FM CW radio acoustic sounding system (RASS), in this paper an analysis of influences from temperature gradient and radial range decay is presented. A nonconstant temperature gradient will significantly modify the spectrum of scattered signals and also cause a deviation of the maximum position of amplitude spectrum from the Bragg resonance, which should involve using any retrieval algorithm that will correct for this temperature bias. Without considering this effect, the error of 0.5–1 K will probably be introduced in the RASS temperature profile. An approximate retrieval algorithm is provided in text. Moreover, for the application of RASS using FM CW radar in lowest 100‐ to 200‐m atmosphere, the far‐field aspect sometimes will not be the case, and radial range decay will play a certain role, which is investigated also in thi

ISSN:0048-6604    

DOI:10.1029/93RS01514

年代:1993

数据来源: WILEY

摘要:

We have collected 136‐MHz Faraday rotation and scintillation data of signals transmitted by the satellite ETS‐2 and received at 13 ground stations in the Pacific‐Asia region. Using this data base, a statistical analysis has been carried out by correlating the nocturnal TEC disturbances with the amplitude scintillations. Also presented are several typical events observed simultaneously over this network. An important result in the present paper is that the occurrence of amplitude scintillations is closely associated with the wavelike disturbances in TEC with periods below 2 hours. These periods agree with the TIDs produced by atmospheric gravity waves in the ionosphere. Furthermore, simultaneous recordings at several spaced stations indicate the disturbances are traveling. It is therefore deduced that wavelike disturbances in TEC observed are manifested as the ionospheric response to gravity waves in the neutral atmos

ISSN:0048-6604    

DOI:10.1029/93RS01077

年代:1993

数据来源: WILEY