Howard Chang

Bio: Howard Chang is an academic researcher. The author has contributed to research in topics: Ionosphere & Ionospheric heater. The author has an hindex of 1, co-authored 1 publications receiving 1156 citations.

Electromagnetic Waves in Stratified Media

Abstract: This book [1] was written at an important point in the development of applications of electromagnetic (radio) waves to communications, navigation, and remote sensing. Such applications require accurate propagation predictions for a variety of path conditions, and this book provides the theoretical basis for such predictions. The book is based on fundamental research in electromagnetic wave propagation that James R. Wait performed in the Central Radio Propagation Laboratory (CRPL) of NBS from 1956 to 1962. The mathematical theory in the book is very general, and the “stratified media” models are applicable to the earth crust, the troposphere, and the ionosphere. The frequencies of the communication, navigation, and remote sensing applications treated in this book range all the way from extremely low frequencies (ELF) to microwaves. The mathematical theory of electromagnetic wave propagation is based on Maxwell’s equations [2], formulated by James Clerk Maxwell in the 1860s. Experimental propagation studies in free space [3] and over the earth [4] also go back over 100 years. Research in radio science, standards, and measurements began in NBS in the early 1900s, and the long history of radio in NBS has been thoroughly covered by Snyder and Bragaw [5]. CRPL was moved to Boulder in 1954, and Wait joined the organization in 1955. The mathematics of electromagnetic wave propagation in stratified (layered) media is very complicated, and progress in propagation theory in the early 1900s was fairly slow. Wait’s book [1] included the most useful theory (much of which he developed) and practical applications that were available in 1962. A hallmark

Seismic Wave Propagation in Stratified Media

Abstract: Publisher Summary This chapter focuses on elastic wave propagation in stratified media. The development of the theory of elastic wave propagation in stratified media has been strongly influenced by the problems of seismic wave propagation and the nature of the seismograms recorded from earthquakes. For purely analytic developments of elastic wave propagation, the level of manageable algebraic complexity is reached in a model with one or two uniform layers overlying a uniform half space. This chapter shows how the excitation of elastic waves, within a horizontally stratified structure, can be conveniently developed in terms of reflection and transmission matrices. This procedure has allowed the construction of the full response of the medium or approximations with desired properties so that theoretical seismograms may be calculated for realistic distributions of elastic parameters. Although this development has been for isotropic media, nearly all the results apply directly to the case of full anisotropy if 3 × 3 reflection and transmission matrices allowing coupling between all wave types are employed. This development of the wavefield for both source and receiver within the stratification may be used for other classes of wave propagation.

Some features of field line resonances in the magnetosphere

Abstract: Field line resonances in the magnetosphere have received much attention. By using an extremely simplified model we examine the circumstances under which finite disturbance amplitude solutions of the coupled wave equation can be obtained in the vicinity of the resonant field line. General features of solutions are noted and the relevance of recent experimental work to the problem is pointed out. The observed latitude dependence of polarisation provides strong evidence of the role of resonating field lines as

Physics of strongly magnetized neutron stars

Abstract: There has recently been growing evidence for the existence of neutron stars possessing magnetic fields with strengths that exceed the quantum critical field strength of 4.4 × 1013 G, at which the cyclotron energy equals the electron rest mass. Such evidence has been provided by new discoveries of radio pulsars having very high spin-down rates and by observations of bursting gamma-ray sources termed magnetars. This paper will discuss the exotic physics of this high-field regime, where a new array of processes becomes possible and even dominant and where familiar processes acquire unusual properties. We review the physical processes that are important in neutron star interiors and magnetospheres, including the behaviour of free particles, atoms, molecules, plasma and condensed matter in strong magnetic fields, photon propagation in magnetized plasmas, free-particle radiative processes, the physics of neutron star interiors and field evolution and decay mechanisms. Application of such processes in astrophysical source models, including rotation-powered pulsars, soft gamma-ray repeaters, anomalous x-ray pulsars and accreting x-ray pulsars will also be discussed. Throughout this review, we will highlight the observational signatures of high magnetic field processes, as well as the theoretical issues that remain to be understood.

Electron cyclotron emission and absorption in fusion plasmas

Abstract: The state of the art in the field of electron cyclotron emission and absorption of fusion plasmas confined by a magnetic field is reviewed. The general theory is described concisely, explicit results being given mainly for plasmas in local thermodynamic equilibrium. Moreover, the practical implications of these effects with respect to cyclotron radiation losses, the use of cyclotron emission as a diagnostic tool, and electron cyclotron heating and current drive are discussed with emphasis on tokamak plasmas.