Global propagation of atmospheric gravity waves: A review

In this paper we review the theory of acoustic-gravity waves, the interaction of such waves with the ionosphere, the experimental support for the existence of such waves in the upper atmosphere, and the role played by acoustic-gravity waves in thermospheric dynamics. After a thorough discussion on the properties of acoustic-gravity waves in an ideal isothermal atmosphere, the effects produced by horizontal winds, sharp boundary discontinuities, and dissipative processes are discussed. The generation of these waves by stationary or moving sources is then treated. It is shown that the atmospheric response to a stationary impulse source can be described by the emission of three waves: acoustic, buoyancy, and gravity. These discussions are then followed by reviewing propagation effects in a realistic atmosphere for both free waves and guided waves. Recent numerical results are given. When acoustic-gravity waves propagate through the ionosphere, interaction between the wave and the ionosphere will take place. The physical processes involved in such an interaction are examined.

Acoustic‐gravity waves in the upper atmosphere

Electrical capacitance tomography (ECT) is a new technology that is used to visualise the internal behaviour of industrial processes comprising dielectric components. A particular difficulty with capacitance measurement for tomography is that the stray capacitance to earth of an ECT sensor is large (/spl sim/150 pF) compared with the interelectrode capacitance to be measured (usually <0.5 pF), and the measurement range is wide (0.01-2 pF). The authors present a highly sensitive AC-based capacitance measuring circuit, which has been specially designed for this purpose, and a PC-based ECT system with this circuit. Experimental results show that this ECT system can measure as small as 0.01 pF capacitance with 0.0002 pF error, and could collect tomographic image data at more than 100 frames per second.

New AC-based capacitance tomography system

Finite-element numerical modeling and analysis of electromagnetic waveguides and resonators used in terahertz quantum cascade lasers are presented. Both metal-metal and semi-insulating (SI) surface-plasmon ridge waveguide geometries were investigated. Simulations and analysis of two types were performed: two-dimensional waveguides (eigenmode calculation), and two- and three-dimensional resonators (facet reflectivity calculation for infinite width and finite width waveguides, respectively). Waveguide simulations extend previous transverse one-dimensional analyses to two dimensions (for the lateral and transverse dimensions), and quantify the breakdown of the one-dimensional approximation as the ridge width is reduced. Resonator simulations in two and three dimensions are presented and are used to obtain facet reflectivities and output radiation patterns. For the metal-metal waveguide structures, these resonator simulations quantitatively show strong deviations for terahertz facet reflectivities from those ...

Electromagnetic modeling of terahertz quantum cascade laser waveguides and resonators

Nanotechnology enables the fabrication of wire-grid polarizers (WGP) in the visible optical region. At oblique angles of incidence, WGP can be used as polarizing beam splitters (PBS). As such, they have the advantages of large numerical aperture and high-extinction ratios in both transmission and reflection. Because of these properties, WGP is being explored as PBS replacement in projectors. In this article, we present a complete theoretical investigation of the WGP. Rigorous diffraction theory, exact lowest-order eigenmode effective-media theory, and form birefringence theory are discussed. These theories are compared with experimental measurement of T(θ) and R(θ) as a function of the polarization state of the input light and as a function of the incident angle θ. It is shown that only the rigorous diffraction theory can fit the data for all incident angles. Using diffraction theory we provide a calculation relating the optical properties of the WGP to the physical dimensions of the wire grids. Thus, a framework for optimizing the optical properties of the WGP for various applications and requirements is provided.

Optical wire-grid polarizers at oblique angles of incidence

Emphasizing practical applications, this approach integrates IBM PC BASIC programs and numerical techniques with the principles of engineering electromagnetics. This book discusses on-line parameters by numerical techniques and inserts a section on capacitance, conductance and inductance.

Elements of engineering electromagnetics

A method is presented for the derivation of quasiparabolic ionospheric layer parameters from a set of three data points on the leading edge of a sweep-frequency sky-wave backscatter ionogram. The method is illustrated with a numerical example. Possible extensions of the method are discussed.

Inversion of sweep‐frequency sky‐wave backscatter leading edge for quasiparabolic ionospheric layer parameters

Acoustic waves in the ionosphere

It is well known that various diurnal and morphological features of the ionosphere reveal substantial departures from the normally simplified assumption of spherical symmetry at certain times of the day or at certain geographic locations. A radio ray passing through such an ionosphere must bear information about its horizontal gradients. The leading edge of a backscatter ionogram is formed by obliquely propagated radio rays of minimum time delay and hence is useful in deducing information about the ionospheric horizontal gradients. In this regard, the ionospheric electron density distribution is modeled by a locally quasi-parabolic layer with six parameters. This six-parameter space is known as the ‘ionostate.’ Our object is to seek the ‘best’ ionostate in the sense that the corresponding mean square error in group delay is a minimum. A computer program has been written to carry out the specified procedure. A number of sample calculations are presented and discussed.

Backscatter inversion in spherically asymmetric ionosphere

A comprehensive, user-interactive software package for the IBM PC or compatible, resulting from an ongoing effort by the author to enhance the teaching and learning processes in the instruction of introductory electromagnetics courses is described. The associated topics are distributed throughout the realm of introductory electromagnetics, ranging from coordinate systems to antenna fundamentals. The PC-assisted instruction involves the use of the software in the classroom integrated with the lecture-discussion of the subject matter, as well as outside the classroom by the student at his/her own pace and leisure. Following brief descriptions of the 15 programs of the software package, four examples are presented in detail to illustrate the teaching and learning nature of the software. Student responses to the PC-assisted instruction are included. >

N. Narayana Rao

Papers

Elements of engineering electromagnetics

Inversion of sweep‐frequency sky‐wave backscatter leading edge for quasiparabolic ionospheric layer parameters

Acoustic waves in the ionosphere

Backscatter inversion in spherically asymmetric ionosphere

PC-assisted instruction of introductory electromagnetics