Showing papers in "Radio Science in 1994"

Estimation of the transmitter and receiver differential biases and the ionospheric total electron content from Global Positioning System observations

Abstract: In the estimation of the ionospheric total electron content from the Global Positioning System (GPS) observables, various instrumental systematic effects such as the biases in the GPS satellites and receivers must be modeled. This paper describes a procedure, based on a Kalman filtering approach, for estimating these instrumental biases as well as the total electron content at each GPS station, using dual GPS data. The method is applied to six data sets, of 48 hours each, spanning one year, from the Deep Space Network with GPS stations in Australia, Spain, and the United States. The formal errors for the estimated satellite biases and for the total electron content at each station are about 0.07 ns and 0.2×1016 el/m2, respectively. The variation in time of the satellite biases (relative to the mean of all of them) estimated in different epochs during 1-year period, is below 1 ns.

Three-dimensional electromagnetic modeling using finite difference equations: The magnetotelluric example

Abstract: We have developed a robust and efficient finite difference algorithm for computing the magnetotelluric response of general three-dimensional (3-D) models using the minimum residual relaxation method. The difference equations that we solve are second order in H and are derived from the integral forms of Maxwell's equations on a staggered grid. The boundary H field values are obtained from two-dimensional transverse magnetic mode calculations for the vertical planes in the 3-D model. An incomplete Cholesky decomposition of the diagonal subblocks of the coefficient matrix is used as a preconditioner, and corrections are made to the H fields every few iterations to ensure there are no H divergences in the solution. For a plane wave source field, this algorithm reduces the errors in the H field for simple 3-D models to around the 0.01% level compared to their fully converged values in a modest number of iterations, taking only a few minutes of computation time on our desktop workstation. The E fields can then be determined from discretized versions of the curl of H equations.

Spectral approach to solving three-dimensional Maxwell's diffusion equations in the time and frequency domains

Abstract: We describe a new explicit three-dimensional solver for the diffusion of electromagnetic fields in arbitrarily heterogeneous conductive media The proposed method is based on a global Krylov subspace (Lanczos) approximation of the solution in the time and frequency domains We derive solutions stable to spurious curl-free modes and provide estimates of the computer complexity involved in the calculations Such estimates together with numerical experiments attest to a computationally efficient method suitable for large-scale problems Also included are modeling examples drawn from practical geophysical applications

Rapid 2.5‐dimensional forward modeling and inversion via a new nonlinear scattering approximation

Abstract: We introduce a novel approximation to numerically simulate the electromagnetic response of point or line sources in the presence of arbitrarily heterogeneous conductive media. The approximation is nonlinear with respect to the spatial variations of electrical conductivity and is implemented with a source-independent scattering tensor. By projecting the background electric field(i.e., the electric field excited in the absence of conductivity variations) onto the scattering tensor, we obtain an approximation to the electric field internal to the region of anomalous conductivity. It is shown that the scattering tensor adjusts the background electric field by way of amplitude, phase, and cross-polarization corrections that result from frequency-dependent mutual coupling effects among scatterers. In general, these three corrections are not possible with the more popular first-order Born approximation. Numerical simulations and comparisons with a 2.5-dimensional finite difference code show that the new approximation accurately estimates the scattered fields over a wide range of conductivity contrasts and scatterer sizes and within the frequency band of a subsurface electromagnetic experiment. Furthermore, the approximation has the efficiency of a linear scheme such as the Born approximation. For inversion, we employ a Gauss-Newton search technique to minimize a quadratic cost function with penalty on a spatial functional of the sought conductivity model. We derive an approximate form of the Jacobian matrix directly from the nonlinear scattering approximation. A conductivity model is rendered by repeated linear inversion steps within range constraints that help reduce nonuniqueness in the minimization of the cost function. Synthetic examples of inversion demonstrate that the nonlinear approximation reduces considerably the execution time required to invert a large number of unknowns using a large number of electromagnetic data.

Two‐dimensional location and shape reconstruction

Abstract: A method for reconstructing the location and the shape of a bounded impenetrable object from measured scattered field data is presented. The algorithm is, in principle, the same as that used for reconstructing the conductivity of a penetrable object and uses the fact that for high conductivity the skin depth of the scatterer is small, in which case the only meaningful information produced by the algorithm is the boundary of the scatterer. A striking increase in efficiency is achieved by incorporating into the algorithm the fact that for large conductivity the contrast is dominated by a large positive imaginary part. This fact, together with the knowledge that the scatterer is constrained in some test domain, constitute the only a priori information about the scatterer that is used. There are no other implicit assumptions about the location, connectivity, convexity, or boundary conditions. Some refinements of the algorithm which reduce the number of points at which the unknown function is updated are incorporated to further increase efficiency. Results of a number of numerical examples are presented which demonstrate the effectiveness of the location and shape reconstruction algorithm.

On the origin of quasi‐periodic radar backscatter from midlatitude sporadic E

Abstract: We describe a newly discovered polarization process that appears to be induced by an atmospheric gravity wave (AGW) when it altitude-modulates a sporadic E (ES) layer in the nighttime, midlatitude ionosphere. This large-scale polarization process appears capable of accounting for three as yet unexplained features found in radar backscatter from field-aligned irregularities in ES layers: (1) kilometer-scale, wavelike variations in the mean Doppler velocity, (2) mean Doppler velocities much larger than background ionospheric motion, and (3) quasi-periodic patterns in backscatter power when plotted as a function of range and time. We show that the polarization electric field develops as a result of the altitude modulation and that its properties are similar to those of the AGW. The novel feature in this process is the spatial modulation that is produced in the field line-integrated Pedersen conductivity by the variation in the ion-neutral collision frequency associated with the varying altitude of the ES layer. The resultant electric field together with the altitude-modulated ES layer then drive the production of secondary plasma waves via the gradient-drift instability. The three puzzling features are shown to be associated with the characteristics of these secondary waves.

Simultaneous nonlinear reconstruction of two-dimensional permittivity and conductivity

Abstract: A new inversion algorithm for the simultaneous reconstruction of permittivity and conductivity recasts the nonlinear inversion as the solution of a coupled set of linear equations. The algorithm is iterative and proceeds through the minimization of two cost functions. At the initial step the data are matched through the reconstruction of the radiating or minimum norm scattering currents; subsequent steps refine the nonradiating scattering currents and the material properties inside the scatterer. Two types of basis functions are constructed for the nonradiating currents: “invisible” (global) basis functions, which are appropriate for discrete measurements and nonradiating (local) basis functions, which are useful in studying the limit of continuous measurements. Reconstructions of square cylinders from multiple source receiver measurements at a single frequency show that the method can handle large contrasts in material properties.

Polarimetric passive remote sensing of ocean wind vectors

Abstract: This paper investigates the theory of polarimetric passive remote sensing of wind-generated sea surfaces and the potential application of polarimetric radiometry to ocean wind remote sensing. Theoretical polarimetric emission coefficients of small-scale sea surfaces are evaluated using the small perturbation method (SPM). The SPM is derived to second order and applied to the Stokes vectors of thermal emission from random rough dielectric surfaces described by anisotropic directional spectra. To verify the accuracy of the SPM, a Monte Carlo simulation is performed to calculate the Stokes vectors of the emission from the simulated one-dimensional random rough surfaces with a power law spectrum for various observation angles and surface parameters. The theoretical results of the SPM for all four Stokes parameters are in excellent agreement with the numerical results obtained from the Monte Carlo simulation. Moreover, the second-order coherent fields are indispensable in the theoretical evaluation of the third and fourth Stokes parameters. Otherwise, the reflectivities of random rough surfaces would be significantly overestimated, and the signs of the third and fourth Stokes parameters would be incorrect. The SPM is then applied to small-scale sea surfaces described by an empirical sea surface spectrum. It is found that the azimuthal signatures of Stokes parameters agree qualitatively well with aircraft Ku-band radiometer data. Theoretical model functions of the Stokes parameters are illustrated. Advantages of wind direction retrieval using polarimetric Q and U measurements are discussed, and it is expected that a spaceborne polarimetric radiometer has a potential of providing wind vector measurements with uniform accuracy across all parts of swath.

A full finite difference time domain implementation for radio wave propagation in a plasma

Abstract: A full finite difference time domain methodology is developed for electromagnetic wave propagation in a plasma The finite difference grid is consistent with central difference approximation of the curl, divergence and gradient operators that appear in the joint equations of Euler and Maxwell, and the coupling effects between the fluid velocity and the electric field To accomplish the time advancement, the central difference approximation is invoked for the time derivatives and leapfrog concepts are employed The resulting difference equations converge to the exact equations, provided that the developed stability requirement is satisfied Finally, numerical results are provided and compared with the inverse fast Fourier transform results of closed-form, frequency domain solutions for the half space problem; the agreement between solutions is shown to be excellent

Polarimetric scattering and emission properties of targets with reflection symmetry

Abstract: This paper investigates the symmetry of polametric scattering and emission coefficients of media with reflection symmetry.

Experimental evidence for the formation and entry of patches into the polar cap

Abstract: A mechanism for the formation of polar cap patches is proposed based on data collected by the Sondrestrom incoherent scatter radar, magnetometers located on the west and east coasts of Greenland and Digisondes, digital ionospheric sounders, operated at Qaanaaq and Sondrestrom. This instrumentation has served to identify the formation of patchlike structures and follow their temporal evolution and entry into the polar cap. Prior to the onset of the event, the background ionospheric plasma was moving in a poleward direction and formed part of a tongue of ionization directed toward and into the polar cap. The event starts with the appearance of a fast plasma jet containing eastward directed velocities in excess of 2 km s−1. This plasma jet consists of a channel extending 300 km in width, where the F region ion temperature reaches values in excess of 4000 K and the E layer Ti is enhanced above 5000 K. The existence of a region containing high electron temperatures associated with soft precipitation (likely the cusp) and located adjacent and equatorward of the fast plasma jet is very suggestive of the jet being the result of a tension force acting upon recently reconnected field lines. This view is supported by the fact that the fast plasma jet was seen in the prenoon sector under IMF By negative conditions. The elevated Ti values inside the plasma jet are exactly collocated with depleted F region densities. We suggest that the recombination loss of O+ is increased by a factor >10 due to the dependence of the O+ + N2 reaction on the ion temperature. The eroding action of the fast plasma jet proceeds until the poleward moving tongue of ionization is divided into regions containing high and low densities. Magnetic field perturbations associated with the fast plasma jet were observed by the different magnetometer stations. The large negative bays recorded at most of the sites have been used to trace the poleward motion of the jet. At the time that the fast plasma jet reaches Qaanaaq, the Digisonde there measured low ƒ0F2 values. A few minutes before and after this minimum, high ƒ0F2 values were observed. The ratio of the enhanced ƒ0F2 values on either side of the minimum, to the minimum ƒ0F2 is about 2 (or a density ratio of 4). This is the commonly accepted signature of a polar cap patch. The series of events leading to the formation of the patchlike density structure has been designated a density breakoff event.

Spatial structure of the E region field-aligned irregularities revealed by the MU radar

Abstract: The MU radar (34.9°N, 136.1°E, geomagnetic latitude 25.0°N) is utilized to observe the spatial structures of the field-aligned irregularities in the midlatitude E region. The multibeam observation in 12 directions has found that the “quasi-periodic” echoes have a monochromatic wave structure propagating toward SSW. Much smaller-scale structure of the irregularities has been observed by the interferometry techniques with three receivers. The results are consistent with a model in which a southward propagating gravity wave modulates the sporadic E layer to generate the quasi-periodic echoes of the irregularities.

Propagation characteristics of the ELF emissions observed by the satellite Akebono in the magnetic equatorial region

Abstract: Emissions with their frequencies below 100 Hz are often observed by the Akebono satellite in the vicinity of the geomagnetic equatorial plane. These ELF emissions are classified into two types: One is an ion cyclotron wave below the local proton cyclotron frequency and the other is assumed to be magnetosonic wave observed not only below but also above the local proton cyclotron frequency. The wave normal directions of the latter type of emissions were estimated by using the wave distribution function method. It is found that the emissions are propagating with their wave normal direction nearly perpendicular to the meridian plane. The propagation characteristics of these emissions are also examined by ray tracing including the effects of ions. The ray tracing study clarified that the wave can propagate around the plasmapause because of the trapping effect of density gradient. In this paper we propose that these emissions are generated just outside the plasmapause and are propagating around the plasmapause with their wave normal nearly perpendicular to the geomagnetic field line and to the meridian plane.

A comparative study of two-dimensional multiple scattering techniques

Abstract: The scattering of an electromagnetic plane wave from an arbitrary configuration of parallel circular cylinders is investigated using four different techniques. The cylinders are made of perfectly conducting or homogeneous dielectric material. These techniques are a boundary value type of solution, an iterative scattering procedure, a hybrid approach based on a combination of exact and method of moments solution, and a high-frequency asymptotic approximation. The analysis is given in detail for the transverse magnetic (TM) polarization, and that for the transverse electric (TE) polarization is outlined. Numerical results are provided to show the major differences between these techniques and the validity of using circular cylinders in modeling composite two-dimensional scatterers.

Basic properties and gravity wave initiation of the midlatitude F region instability

Abstract: Observations of midlatitude spread F irregularities have shown that the most violent eruptions are often periodic and may be related to modulations of the F region bottomside. Similar aspects of equatorial spread F have been conclusively shown to be the result of seeding by atmospheric gravity waves. In this paper, we pursue a nonlinear description of the midlatitude F region instability first proposed by Perkins (1973) and show that, by itself, this instability saturates at approximately 1%. With gravity wave seeding, however, the growth of the instability is significantly enhanced and progresses until it is limited by third- or fourth-order nonlinearities. In the course of the analysis we expand the Perkins linear instability study, particularly with regard to the wave propagation angles suitable for stimulating linearly unstable, and eventually, nonlinear turbulent upwellings. Although gravity waves themselves have limited spatial extent, we also show that gravity waves which propagate perpendicular to the Earth's magnetic field create electric fields which map along the field lines to considerable distances. This transmission of electric fields is dependent on both the particular characteristics of the ionosphere along the field lines and the wavelength of the perturbation, but under certain circumstances, gravity wave-induced large-scale electric fields can map to the F region from either the E or conjugate F region and thus influence F region electrodynamics. Finally, we note that our results are consistent with observations of midlatitude spread F irregularities by the MU radar in Japan and large-scale perturbations of the F region over Arecibo.

The electromagnetic field of a vertical electric dipole in the presence of a three‐layered region

Abstract: The electromagnetic field generated by a vertical electric dipole in the air over the surface of a two-layered region is determined for continuous-wave excitation. The region of interest consists of a conductor or dielectric with high permittivity, coated with an electrically thin layer of a dielectric under a half-space of air. Simple explicit formulas are derived for the field at all points in all three regions including the surface wave. Typical applications are to microstrip circuits and antennas, communication over the earth when this is coated with a layer of asphalt or cement, and communication over the sea or a lake when this is under a layer of ice.

Temperature fluctuations near the mesopause inferred from meteor observations with the middle and upper atmosphere radar

Abstract: Using meteor echo measurements with the middle and upper atmosphere (MU) radar (35°N, 136°E), operated at 46.5 MHz, we examined time-height variation of the ambipolar diffusion coefficient D, determined from the decay rate of meteor echoes. The height of a meteor trail was determined with an accuracy of about 1 km, by using an interferometer for determination of the arrival angles of meteor echoes. Profiles of D were obtained about every hour owing to the high rate of usable meteor echoes (3000–5000 per day). The mean profile of D showed an exponential increase with a scale height of 5.7–6.5 km in spring, summer, and autumn, which was generally consistent with the density scale height, while the scale height was as much as 8.1 km in November and December. Using the Boussinesq approximation, we found that the normalized D fluctuations D′ can be related to those of temperature T′ as D′/D0 ∼ 3/2 · T′/T0. The observed D′/D0 involved a wavelike structure with periods shorter than the inertial period, which often showed downward phase propagation. Fluctuations of D′/D0 and the horizontal wind velocity showed phase relations consistent with a linear gravity wave theory. Furthermore, frequency spectra of D′/D0 were generally consistent with those of the horizontal wind velocity for wave components with periods longer than about 8 hours, while for short-period components the spectral amplitudes of D′/D0 were considerably enhanced.

Measurement of the electromagnetic properties of chiral composite materials in the 8-40 GHz range

Abstract: Materials described by the constitutive equations D = eE + βe ∇ × E and B = μH + βμ ∇ × H lack inversion symmetry due to chirality or handedness in their microstructure. In this paper, we describe the first direct measurements of the material properties e, μ, and β for microwave chiral composite materials prepared in our laboratory in the frequency range of 8–40 GHz. We have used both reflection and transmission measurements for normally incident, linearly polarized plane waves in a specially designed free-space facility using spot focusing antennas. Different concentrations of the chiral inclusions in the form of miniature metallic springs that are left-handed only, right-handed only or equally mixed (racemic) have been studied. It is shown that e and μ are comparable for all three samples at a given concentration, but β has equal but opposite values for the left- and right-handed samples, whereas it is nearly zero for the equichiral sample. The values of β thus obtained are compared to estimated values for suspensions of naturally occurring chiral molecules. The accuracy of the measurements is assessed by using the experimental procedure and inversion algorithm for standard materials like quartz. A new technique involving time domain repines has been used to remove the ambiguity that is usually encountered in the inversion of measured S parameters.

Irregularity structures in the cusp/cleft and polar cap regions

Abstract: A case study of the temporal behavior of ionospheric scintillations and their frequency spectra in the cusp/cleft and polar cap regions is presented. These measurements were made at Sondrestrom and Thule, Greenland, using the 243-MHz transmissions from quasi-stationary satellites during a coupling energetic and dynamics of atmospheric regions (CEDAR) high-latitude plasma structure (HLPS)/solar terrestrial energy program (STEP) global aspects of plasma structures (GAPS) campaign. During this campaign, the incoherent scatter radar (ISR) observations were also performed at Sondrestrom, which defined the dynamic ionospheric environment in the cusp/cleft region. The availability of the radar results has enhanced this case study. It is found that scintillations at Sondrestrom are abruptly enhanced about an hour before magnetic noon when the propagation path to the satellite entered the cusp/cleft region. Subsequently, a series of enhanced and reduced scintillation activity was detected. The enhanced scintillation structures were found to be asymmetric, with sharp leading edges and diffuse trailing edges. Spaced-antenna scintillation measurements at Sondrestrom detected considerable velocity shear, and the frequency spectra showed flat low-frequency portions, implying the presence of turbulent plasma flows. A comparison with the ISR observations indicates that the temporal variation of scintillation was caused by the poleward convection of alternate regions with high- and low-ionization density, the density depletions being caused by channels of high zonal flows associated with velocity shear. The level of scintillation observed in the low-density regions imply the presence of small-scale irregularities with considerable irregularity amplitude. In contrast to the above behavior, the polar cap scintillations exhibit deep minima between the transit of successive “patches” of ionization, and their frequency spectra imply the absence of turbulent plasma flows. It is postulated that in the cusp/cleft and polar cap regions, the gradient-drift instability mechanism generates the observed small-scale irregularities associated with discrete density enhancements, whereas a shear-driven instability, such as the nonlinear collisional Kelvin-Helmholtz (K-H) instability mechanism, may generate the irregularities in the intervening low-density regions.

Polarized microwave emission from water waves

Abstract: Partially polarimetric measurements of thermal emission from a striated water surface at 91.65 GHz illustrate the potential for remote sensing of water wave direction by passive microwave radiometry. The three Stokes parameter measurements were made using a precision polarimetric radiometer trained on a rotatable water wave tank at several elevation angles from near nadir to near grazing. The polarimetric measurements are well corroborated by calculations using a tilted-facet geometrical optics model for the water surface emission and scattering. Multiple scattering of the incident background radiation is incorporated for observation angles approaching grazing. The downwelling background brightness is computed by using an atmospheric radiative transfer model. We show that azimuthal brightness variations in the third Stokes parameter are in phase quadrature with the first and second modified Stokes parameters. For observation angles near ∼60°–70° from nadir the first three parameters have particularly large azimuthal brightness variations and thus have significant potential for measuring ocean wave direction. Moreover, the azimuthal brightness variations caused by water waves are not negligible for many passive microwave atmospheric sounding and surface remote sensing purposes, even at nadir. A range of elevation angles resulting in minimal azimuthal variations is identified.

Modeling daytime F layer patches over Sondrestrom

Abstract: A comprehensive, time-dependent, high-latitude, one-species F region model has been developed to study the various physical processes which are believed to affect the polar cap plasma density distributions as a function of altitude, latitude, longitude, and local time. These processes include production of ionization by solar extreme ultraviolet radiation and particle precipitation; loss through charge exchange with N 2 and O 2 ; and transport by diffusion, neutral winds, and convection E×B drifts. In our initial calculations we have modeled highly structured plasma densities characterized by digisonde observations at Sondrestrom using both a time-dependent global convection pattern and spatially localized regions of transient high-speed flow

Inversion of diffusive transient electromagnetic data by a conjugate-gradient method

Abstract: Inversion of three-dimensional transient electromagnetic (TEM) data to obtain electrical conductivity and permeability can be done by a time-domain algorithm that extends to diffusive electromagnetic (EM) fields the imaging methods originally developed for seismic wavefields (Claerbout, 1971; Tarantola, 1984). The algorithm uses a conjugate-gradient search for the minimum of an error functional involving EM measurements governed by Maxwell's equations without displacement currents. The connection with wavefield imaging comes from showing that the gradient of the error functional can be computed by propagating the errors back into the model in reverse time and correlating the field generated by the backpropagation with the incident field at each point. These two steps (backpropagation and cross correlation) are the same ones used in seismic migration. The backpropagated TEM fields satisfy the adjoint Maxwell's equations, which are stable in reverse time. With magnetic field measurements the gradient of the error functional with respect to conductivity is the cross correlation of the backpropagated electric field with the incident electric field, whereas the gradient with respect to permeability is the cross correlation of the backpropagated magnetic field with the time derivative of the incident magnetic field. Tests on two-dimensional models simulating crosswell TEM surveys produce good images of a conductive block scatterer, with both exact and noisy data, and of a dipping conductive layer. Convergence, however, is slow.

The dark polar ionosphere: Progress and future challenges

Abstract: Since the end of the 1970s, we have seen enormous progress in our understanding of the polar ionosphere and its structuring. With this benchmark issue of Radio Science it is appropriate to reflect briefly on that passage and some key questions that lie ahead. The discussion here will concentrate on the winter hemisphere, in keeping with the conditions under which most of the data studied to date have been gathered. The polar ionosphere alternates between two states, depending on whether the interplanetary magnetic field (IMF) is southward or northward. The former state is characterized by ∼100–1000 km islands of enhanced F region plasma, originating in sunlit upper midlatitudes, entering and traversing the polar cap. They become highly structured and produce severe scintillation. Despite much progress on the source, evolution, and ultimate fate of this polar plasma, we remain challenged by the process(es) which chop entering plasma into such islands. For northward IMF we have learned much about the near-Earth processes determining the character of polar cap arcs, velocity structure and electrodynamics, and energetics. A remaining challenge is to relate these structures to the topology and driving physical processes in the magnetosphere and solar wind. Here we sketch the principles behind the progress and the context of several key problem areas ahead.

Ionospheric refraction effects in slant range profiles of auroral HF coherent echoes

Abstract: The theory of auroral coherent echoes developed for VHF scattering by Uspensky et al. (1988, 1989) is applied to the interpretation of intensity and Doppler velocity slant range profiles of HF radar aurora. The theoretical model includes the effects of irregularity aspect sensitivity, ionospheric refraction of the radar beam, and the reception of signals from different heights. The predicted profiles of HF radar aurora are compared with Schefferville HF radar observations in the frequency interval of 9–18 MHz. Satisfactory agreement is found between theory and experiment for the intensity profiles. However, there are significant discrepancies for the Doppler velocity profiles. We discuss this lack of agreement in light of other recent observations.

Application of the measured equation of invariance to solve scattering problems involving a penetrable medium

Abstract: The measured equation of invariance (MEI) is a simple technique used to derive finite difference type local equations at mesh boundaries, where the conventional finite difference approach fails (Mei et al., 1992, 1994). Conventionally, finite difference or finite element meshes span from boundary to boundary, or to any surface where an absorbing boundary condition can be simulated. It is demonstrated that the MEI technique can be used to terminate meshes very close to the object boundary and still strictly preserves the sparsity of the finite difference equations. It results in dramatic savings in computing time and memory needs. In an earlier paper (Mei et al., 1992) it was shown that this new method can be applied to general boundary geometries including both convex and concave metal surfaces. In this paper we shall show that the MEI can be applied also to material discontinuities. The method of MEI is based on the postulate that the boundary equations, which govern the scattered fields, are independent of the incident fields. That postulate has been shown to be true for metal scatterers. In a penetrable medium the situation is quite different; in fact, inside the medium the separation of the incident and scattered fields is not a simple matter. Normally, the total field has to be calculated inside the medium, thus violating the postulate. Therefore we cannot use the MEI inside a penetrable medium. The space inside the medium has to be filled by finite difference or finite element meshes. This paper shows how to apply the MEI in this situation and concludes that the CPU time required for the MEI method is comparable to that of the method of moments (MOM), when MOM is based on the boundary integral method, i.e., if the scatterer is homogeneous. However, for inhomogeneous penetrable scatterer the MEI method definitely is preferable to the MOM.

Convection of polar cap patches observed at Qaanaaq, Greenland during the winter of 1989–1990

Abstract: During weak Bz or Bz < 0 conditions, antisunward convection dominates the central polar cap as shown by Digisonde drift measurements. During these conditions, polar cap F layer patches are observed routinely to drift antisunward within the overall plasma convection. A study is presented which compares patch motion derived from 630.0-nm all-sky intensified camera (ASIC) images taken at Qaanaaq, Greenland (87° CGL) with simultaneously obtained Digisonde drift measurements. During four periods of the winter 1989/1990 43 630.0-nm patches were identified and followed in their motion across the ASIC field of view. The shape, velocity, and drift direction of the individual patches were determined. The patch motion was then compared with the Digisonde drift measurements. The drift direction is generally antisunward (±25°); both data sets are in excellent agreement, with a typical ±20° scatter around a common central value. The velocity magnitudes from both measurements show considerable variability, but both measurements generally cover the same velocity range. The variations and variability of the velocity magnitude are discussed in the context of simultaneous IMF measurements. Rapid changes in velocity were traced to changes in Bz. Deviations from antisunward of the azimuths of the optical patch drift and the Digisonde drift measurements were controlled by IMF By in agreement with published By control of convection.

Copolarized and cross‐polarized enhanced backscattering from two‐dimensional very rough surfaces at millimeter wave frequencies

Abstract: Wideband millimeter wave experiments from 75–100 GHz on the scattering from two-dimensional very rough conducting surfaces are presented. The two-dimensional very rough surfaces are manufactured using a computer-numerical-controlled milling machine so that the surface statistics are precisely controlled. The surfaces have both Gaussian roughness statistics and Gaussian surface correlation functions. Bistatic scattering experiments on surfaces with either isotropic or anisotropic correlation functions are performed. Copolarized and cross-polarized bistatic scattering cross sections are measured for both transverse electric and transverse magnetic incidence at 20°. For isotropic surfaces, backscattering enhancement exists for both copolarized and cross-polarized returns and is found to be a function of the surface rms slope. In addition, a strong frequency dependence is observed across the 25-GHz bandwidth in the cross-polarized returns. To investigate the effect of surface correlation anisotropy, scattering experiments on anisotropic rough surfaces are also performed. It is found that the bistatic scattering cross section for an anisotropic surface is a function of the effective correlation length projected along the plane of scattering. Results on the bistatic scattering experiments presented here serve as a motivation to further pursue more elaborate and complete scattering experiments in order to advance research on scattering from very rough surfaces.

Applications of the conjugate gradient fast Fourier Hankel transfer method with an improved fast Hankel transform algorithm

Abstract: The conjugate gradient fast Fourier-Hankel transforms (CG-FFHT) method was recently proposed to solve the problems of electromagnetic wave propagation and scattering in axisymmetric inhomogeneous media. This new technique uses the CG method together with the FFHT to solve the wave equation iteratively. Each iteration of the CG method requires O(N log2 N) complex multiplications (N is the number of unknowns). For the application of low-frequency induction logging, the number of iterations is very small (less than eight). Furthermore, the CG-FFHT method only requires the storage of several vectors of dimension N. In this paper we present an improved fast Hankel transform (FHT) algorithm as well as some applications of the CG-FFHT method. It is shown that the improved FHT algorithm results in better accuracy and is more efficient than the other FHT algorithms. Moreover, with this FHT algorithm there is no need to pad the function to be transformed with zeros. Several numerical examples will be shown to illustrate the use of the improved FHT algorithm as well as the applications of the CG-FFHT method.

An iterative method for inverse scattering problems based on an exact gradient search

Abstract: A new iterative method based on an exact gradient search for inverse scattering problems is developed. In this scheme the two unknowns (the electric field and the electrical parameter within the object) are solved simultaneously. The bilinear property of the coupled integral equations is employed to drive the gradient of the residual ∇Φ. The exact gradient formulation allows one to perform an efficient line search for the solution. Some numerical results based on two-dimensional inverse scattering problems are presented to demonstrate the efficiency of the algorithm.

Propagation effects caused by a rough ocean surface on the electromagnetic fields generated by lightning return strokes

Abstract: In this paper we have analyzed the effects of propagation caused by a curved and rough ocean surface on the lightning generated radiation fields. In the analysis the electromagnetic fields and the return stroke spectrum was computed from a return stroke model recently introduced by Cooray [1993]. The results show that for frequencies higher than about 10 MHz the attenuation caused by the rough ocean surface is significant and, in the worst cases, the peak of the derivative of the radiation field can be attenuated by about 35% in propagating 50–100 km distance over a rough ocean surface. Comparison of the calculated spectrum with the experimentally observed spectrum show that some of the features in the measured spectrum can be accounted for by the propagation effects.