摘要:

Simultaneous UHF and VHF radar observations of the 60–90 km mesosphere at Arecibo were made during the occurrence of a January 1981 type 4 solar X ray flare. The observations involved use of the UHF incoherent scatter technique at 430 MHz and the VHF turbulent scatter technique at 46.8 MHz. These observations provided a unique opportunity to test the basic premises of the turbulent gradient mixing hypothesis. UHF measurements show that enhanced electron concentration gradients were established with the flare onset. Two turbulent layers were observed with the 46.8 MHz radar before, during and after the flare. Enhanced scattering from both layers peaked within 3–6 min of the beginning of the flare. Only a slight variability in Doppler width of VHF returns from both layers was observed over the pre‐ to post flare period indicating that the power dissipation associated with turbulence remained essentially constant. We find that almost all the enhancement of VHF signals can be attributed to the flare induced increase in electron concentration gradients. We also conclude that the observed delay of 3–6 min between flare onset and the peak of VHF returns is related to an eddy overturning time required for mixing‐in of gradients to the Br

ISSN:0048-6604    

DOI:10.1029/RS023i002p00097

年代:1988

数据来源: WILEY

摘要:

Recent concern over possible long term stratospheric changes caused by the introduction of man‐made compounds has increased the need for instrumentation that can accurately measure stratospheric minor constituents. The technique of radio spectroscopy at millimeter wavelengths was first used to observe rotational transitions of stratospheric ozone nearly two decades ago, but has not been highly developed until recently. We report here on a ground‐based observing technique which employs a millimeter‐wave superheterodyne receiver and multichannel filter spectrometer for measurements of stratospheric constituents that have peak volume mixing ratios that are<10−9, more than 3 orders of magnitude less than that for ozone. We have used the technique for an extensive program of observations of stratospheric chlorine monoxide and also for observations of other stratospheric trace gases such as16O3, vibrationally excited16O3,18O216O, N2O, HO2, and HCN. In the present paper, analysis of the observing technique is given, including the method of calibration and analysis of sources of error. The technique is found to be a reliable means of observing and monitoring important stratospheric trace const

ISSN:0048-6604    

DOI:10.1029/RS023i002p00106

年代:1988

数据来源: WILEY

摘要:

A formulation providing time variations of the horizontal electric field in the ionosphere from HF Doppler data is presented. The relationship between the electric field and frequency deviations of ionospherically reflected HF radio waves has been extended from the theory given by Jacobs and Watanabe in 1966. In the present formula, the effect of electron decay due to attachment and/or recombination processes is taken into account for long‐lasting variations in the electric field. The developed formula can provide time variations of the horizontal field vector even from data observed at only one station. The formula has been applied to an isolated substorm event. In examining the validity of the deduced results, equivalent ionospheric current systems have been used. These have been deduced from worldwide magnetometer data. With these current systems, we can roughly estimate the horizontal electric field in the ionosphere under some assumptions. The time variation of the electric field deduced from the HF Doppler data agrees with that deduced from current systems and high latitude electrojet activity. The results indicate that the HF Doppler method is a sensitive and useful tool for estimating the electric field in the ionospher

ISSN:0048-6604    

DOI:10.1029/RS023i002p00119

年代:1988

数据来源: WILEY

摘要:

The apparent velocity calculated by the dispersion analysis is interpreted in terms of the fundamental parameters of the cross‐correlation analysis of spaced‐receiver data, without any assumptions in addition to those already present in the latter technique. The dispersion analysis is then generalized, by assuming a realistic (power law) functional form for the power spectral density of the irregularities in the ionosphericFregion. Next, an algorithm is proposed to estimate the anisotropy and the true drift velocity of the ground diffraction pattern directly from the cross spectra of the received signals. This technique is applied to HiLat spaced‐receiver data recorded at Tromsø, Norway (69.7°N, 18.9°N), in two different modes. In the first, it is applied on a relatively broad frequency band and its results compared with those from the cross‐correlation analysis. A good general agreement is found. In the second mode, the technique is repeatedly applied on narrow and adjacent frequency bands, to detect possible frequency variations of the anisotropy of the diffraction pattern. A behavior analogous to that of the apparent velocity, previously reported in the literature, is the

ISSN:0048-6604    

DOI:10.1029/RS023i002p00129

年代:1988

数据来源: WILEY

摘要:

Two algorithms which provide a statistical treatment to the estimation of the parameters of the cross‐correlation analysis of spaced‐receiver data are reviewed. Their results are compared using signals: (1) transmitted by a quasi‐stationary polar beacon and received at Goose Bay, Labrador (53.3°N, 60.3°W); and (2) transmitted by the orbiting Hilat satellite and received at Tromsø, Norway (69.7°N, 18.9°E). A good general agreement is displayed in this comparison. The former experiment indicates the possibility of extreme daily variations in the anisotropy of the ground diffraction pattern and in the true drift velocity of the in situ irregularities. The latter experiment displays geometrical enhancements in the intensity scintillation indexS4, in the rms phase fluctuation σϕand in the axial ratio of the ellipse which characterizes the anisotropy of the ground diffraction pattern, around the region of localLshell alignment of the ray paths. Increases of these parameters also are observed northward of Tromsø. These observations are thus consistent with a morphological model for anisotropy of high‐latitude nighttimeFregion irregularities proposed in the literature. Next, a possible dependence of the results of the spaced‐receiver measurements on the receiver baselines is discussed. It is argued that this mechanism could be responsible for the relatively small values of the anisotropy of the diffraction pattern obtained from the Hilat measurements at Tromsø. Finally, a procedure which combines a simple propagation model of the spaced‐receiver experiment with a nonlinear minimization algorithm is proposed to estimate the anisotropy of the in situ irregularities from that of the

ISSN:0048-6604    

DOI:10.1029/RS023i002p00141

年代:1988

数据来源: WILEY

摘要:

The regions of validity of the Kirchhoff and the first‐order small‐perturbation models in rough surface scattering are examined by numerical simulation. The procedure is to generate a one dimensional perfectly conducting random surface on the digital computer, compute the induced current on the surface due to an impinging plane wave by the moment method, and then calculate the far zone backscattered field and power. This procedure is repeated at least forty times and the results averaged to obtain the average scattered power. It is important to note that many so called Kirchhoff models in the literature involve simplifying assumptions in addition to the Kirchhoff approximation. To avoid confusion the Kirchhoff model in this study uses only the Kirchhoff approximation for the surface current. The far zone scattered field and power are evaluated numerically without further simplifying assumptions. Comparisons between the numerical calculations and the models are made for various values of the surface rms height and correlation length both normalized to the incident wave number (denoted by kσ and kL, respectively). By using these two parameters to form a two‐dimensional space the approximate regions of validity are then established. It is found that due to the inclusion of the coherent scattering component the Kirchhoff model continues to provide good agreement with numerical calculations over small angles of incidence when kσ is less than 0.2 and kL<2.0, a region where the Kirchhoff method was expected to fail. It is also found that the usual requirements of small height and small slope are inadequate to guarantee the validity of the first‐order perturbation model. It is necessary that kL also

ISSN:0048-6604    

DOI:10.1029/RS023i002p00163

年代:1988

数据来源: WILEY

摘要:

A method for calculating focusing in the stationary phase approximation is presented. By considering variations of Hamilton's equations a set of equations is obtained which, by integration along a single ray, provides information on the convergence or divergence of infinitesimally separated rays. The method applies to three‐dimensional ionospheric structure and is shown to be much more accurate than conventional finite flux tube approache

ISSN:0048-6604    

DOI:10.1029/RS023i002p00171

年代:1988

数据来源: WILEY

摘要:

New transmission properties of a magnetized ferrite slab are investigated theoretically for an oblique incidentHplane wave. When the direction of magnetization is normal to the plane of incidence, there is single refraction. For magnetization parallel to the plane of incidence, then double refraction leads to a double beam of microwaves at the exit surface. If the electromagnetic field vectors of these two beams can be made orthogonal then after the beams exit the ferrite slab they can be independently modulated and subsequently analyzed and demodulated at a remote receiver. It was found that the two transmitted beams are contrarotating and elliptically polarized. The required interrelationship of the transmission coefficients was determined for cross polarization. The analysis not only explains the double refraction effect but also defines new transmission coefficients that can be used to evaluate the components of the transmitted waves.

ISSN:0048-6604    

DOI:10.1029/RS023i002p00183

年代:1988

数据来源: WILEY

摘要:

Passive microwave instruments for remote sensing of the Earth's surface from space can measure many meteorological parameters in the absence of daylight and in the presence of clouds. Because of the large apertures that are required to achieve good spatial resolution (1–5 km) at microwave frequencies, these radiometers should be arrays of small antenna elements that are arranged so that the collecting area of the radiometer is much smaller than that of a conventional, solid scanning antenna with the same spatial resolution. Despite the various methods that might be used for combining the signals from the different array elements and the different ways the elements might be arranged, the sensitivity depends almost exclusively on the collecting area, integration time, RF bandwidth, and the system temperature. Interferometers and phased arrays have limitations on their bandwidths that depend on the size of the array; beam‐forming arrays are not so limited. A cross antenna is considered in some detail as an example of a sparse‐aperture radiometer. It has 5‐km resolution from an 800‐km orbit and significantly better sensitivity than the equivalent conically scanning parabolic antenna, while it has only 10% of the collecting area and no mov

ISSN:0048-6604    

DOI:10.1029/RS023i002p00193

年代:1988

数据来源: WILEY