A review of progress in FDTD Maxwell's equations modeling of impulsive subionospheric propagation below 300 kHz

pre-printWave propagation at the bottom of the electromagnetic spectrum (below300 kHz) in the Earth-ionosphere waveguide system has been an interesting and important area of investigation for the last four decades. Such wave propagation is characterized by complex phenomena involving nonhomogeneous and anisotropic media, and can result in resonances of the entire Earth-ionosphere cavity. In the spirit of this Special Issue, the goal of this paper is to call attention to emerging finite-difference time-domain computational solutions of Maxwell's equations for wave propagation below 300 kHz which promise to complement and extend previous analyses by pioneers such as Profs. Wait and Felsen. The following topical areas are discussed: long-range two-dimensional propagation, lightning sources and radiation, global propagation, Schumann resonances, hypothesized pre-seismic lithosphere sources and radiation, detection of deep underground resource formations, and remote sensing of localized ionospheric anomalies. We conclude with a prospectus for future research, especially in incorporating the physics of the anisotropic, nonhomogeneous magnetized plasma in a global planetary ionosphere

A Review of Low Frequency Electromagnetic Wave Phenomena Related to Tropospheric-Ionospheric Coupling Mechanisms

Investigation of coupling mechanisms between the troposphere and the ionosphere requires a multidisciplinary approach involving several branches of atmospheric sciences, from meteorology, atmospheric chemistry, and fulminology to aeronomy, plasma physics, and space weather. In this work, we review low frequency electromagnetic wave propagation in the Earth-ionosphere cavity from a troposphere-ionosphere coupling perspective. We discuss electromagnetic wave generation, propagation, and resonance phenomena, considering atmospheric, ionospheric and magnetospheric sources, from lightning and transient luminous events at low altitude to Alfven waves and particle precipitation related to solar and magnetospheric processes. We review in situ ionospheric processes as well as surface and space weather phenomena that drive troposphere-ionosphere dynamics. Effects of aerosols, water vapor distribution, thermodynamic parameters, and cloud charge separation and electrification processes on atmospheric electricity and electromagnetic waves are reviewed. We also briefly revisit ionospheric irregularities such as spread-F and explosive spread-F, sporadic-E, traveling ionospheric disturbances, Trimpi effect, and hiss and plasma turbulence. Regarding the role of the lower boundary of the cavity, we review transient surface phenomena, including seismic activity, earthquakes, volcanic processes and dust electrification. The role of surface and atmospheric gravity waves in ionospheric dynamics is also briefly addressed. We summarize analytical and numerical tools and techniques to model low frequency electromagnetic wave propagation and solving inverse problems and summarize in a final section a few challenging subjects that are important for a better understanding of tropospheric-ionospheric coupling mechanisms

Current and future applications of 3-D global earth-ionosphere waveguide models based on the full-vector maxwells equations FDTD method

pre-printAdvances in computing technologies in recent decades have provided a means of generating and performing highly sophisticated computational simulations of electromagnetic phenomena. In particular, just after the turn of the 21st century, improvements to computing infrastructures provided for the first time the opportunity to conduct advanced, high-resolution three-dimensional full-vector Maxwell's equations investigations of electromagnetic propagation throughout the global Earth-ionosphere spherical volume. In particular, global models employing the finite-difference time-domain (FDTD) method are capable of including such details as the Earth's topography and bathymetry, as well as arbitrary horizontal / vertical geometrical and electrical inhomogeneities and anisotropies of the ionosphere, lithosphere, and oceans. Studies at this level of detail simply are not achievable using analytical methods. The goal of this Paper is to provide an historical overview and future prospectus of global FDTD computational research for both natural and man-made electromagnetic phenomena around the world. Current and future applications of global FDTD models relating to lightning sources and radiation, Schumann resonances, hypothesized earthquake precursors, remote sensing, and space weather are discussed

Efficient modeling of impulsive ELF antipodal propagation about the earth sphere using an optimized two-dimensional geodesic FDTD grid

pre-printThis letter reports the initial application of a geodesic finite-difference time-domain (FDTD) grid to model impulsive extremely low frequency electromagnetic wave propagation about the Earth sphere. The two-dimensional transverse-magnetic grid is comprised entirely of hexagonal cells, except for a small fixed number of pentagonal cells needed for grid completion. Grid-cell areas and locations are optimized to yield a smoothly varying area difference between adjacent cells, thereby maximizing numerical convergence. The new FDTD grid model is considerably superior to our previously reported latitude-longitude grid because it is simpler to construct, avoids geometrical singularities at the poles, executes about 14 times faster, provides much more isotropic wave propagation, and permits an easier interchange of data with state-of-the-art Earth-simulation codes used by the geophysics community. We verify our new model by conducting numerical studies of impulsive antipodal propagation and the Schumann resonance

Global FDTD maxwell's equations modeling of elctromagnetic propagation from currents in the lithosphere

pre-printElectromagnetic wave propagation from electric currents within the Earth's crust is investigated using a three-dimensional finite-difference time-domain (FDTD) full-vector Maxwell's equations model of the global Earth-ionosphere cavity. The FDTD model employed extends from -100 km below sea level to an altitude of -100 km, and can account for arbitrary horizontal as well as vertical geometrical and electrical inhomogeneities and anisotropies of the ionosphere, lithosphere, and oceans. Using this model, the surface horizontal magnetic field is calculated for different depths and orientations of an electric current occurring below the epicenter of the 1989 Loma Prieta earthquake. Results show that the alignment and depth of the electric current within the Earth's crust yields significant differences for the calculated surface magnetic field time-waveforms and spectra. Further, it is found that EM wave phenomena measured at the Earth's surface due to electric currents buried in the Earth's crust will only have significant spectra below - 1 Hz

A 3D TLM code for the study of the ELF electromagnetic wave propagation in the Earth's atmosphere

The interest in the study of electromagnetic propagation through planetary atmospheres is briefly discussed. Special attention is devoted to extremely-low-frequency fields in the Earth's atmosphere for its global nature and possible applications to climate monitoring studies among others. In the Earth's case, the system can be considered as a spherical electromagnetic shell resonator in which two concentric and large conducting spheres with a radius around 6300 km are separated by a very small distance of around 100 km, the atmosphere height. A numerical solution using the Transmission Line Method is proposed. The classical spherical-coordinate description is easy to use, however, the important difference in the dimensions along the three coordinate directions causes high numerical dispersion in the results. A Cartesian scheme with equal node size for all directions is used to reduce this undesired dispersion. A pre-processing stage is the key point introduced to lessen the resulting high demand of memory and time calculation and make the solution feasible. A parallelized Fortran code together with pre- and post-processing Python programs to ease the user interface are provided with this work. Details on the Fortran code and the Python modules are included both in the paper and the source codes to allow the use and modifications by other researchers interested in electromagnetic propagation through planetary atmospheres. The program allows calculation of the time evolution of the electromagnetic field at any point in the atmosphere. It includes the possibility of considering multiple time-dependent sources and different homogeneous and inhomogeneous conductivity profiles to model different situations. Profiles to study day-night asymmetries or locally perturbed profiles which have been attributed to earthquakes in the literature are implemented, for instance.MCIN/AEI 10.13039/501100011033 (grant PID 2020-112805 GA-I00)Grant PID 2020-112805 GA-I00 funded by MCIN/AEI/10.13039/ 50110001103

Three-dimensional FDTD modeling of impulsive ELF propagation about the earth-sphere

pre-printThis paper reports the application of an efficient finite-difference time-domain (FDTD) algorithm to model impulsive extremely low frequency (ELF) propagation within the entire Earth-ionosphere cavity. Periodic boundary conditions are used in conjunction with a three-dimensional latitude-longitude FDTD space lattice which wraps around the complete Earth-sphere. Adaptive combination of adjacent grid cells in the east-west direction minimizes cell eccentricity upon approaching the poles and hence maintains Courant stability for relatively large time steps. This technique permits a direct, three-dimensional time-domain calculation of impulsive, round-the-world ELF propagation accounting for arbitrary horizontal as well as vertical geometrical and electrical inhomogeneities/anisotropies of the excitation, ionosphere, lithosphere, and oceans. The numerical model is verified by comparing its results for ELF propagation attenuation with corresponding data reported in the literature

Doctor of Philosophy

dissertationThe finite-difference time-domain (FDTD) method is a robust numerical modeling approach that has been widely utilized over the past couple decades to solve for electro- magnetic (EM) wave propagation in the Earth-ionosphere waveguide. There are two main approaches to modeling EM wave propagation in the ionosphere: (1) treating the ionosphere as an isotropic medium; or (2) treating the ionosphere as an anisotropic medium (i.e., magnetized ionospheric plasma). The first approach simply utilizes an electrical conductivity profile to represent the ionosphere and ignores the influence of the geomagnetic field. The second approach accounts for the Earth\u27s magnetic field as well as the density and collision frequencies of the electrons. All of the existing FDTD-based Earth-ionosphere models to date account for only the average composition values of the ionosphere and then solve for only the expected average EM fields without considering uncertainties. Not accounting for the variability of the ionosphere content limits the utility and capability of EM modeling for applications such as communications, surveillance, navigation, and geophysical applications. The primary objective of this dissertation is to improve the versatility and computational efficiency of FDTD models by treating the ionosphere as a random medium. Specifically, stochastic methods are applied to FDTD models in order to better assess how ionosphere variability affects the characteristics of EM wave propagation in the Earth-ionosphere waveguide. Two different stochastic algorithms are implemented into FDTD models: the Galerkin-based polynomial chaos expansion, namely PCE-FDTD, and the delta method, namely S-FDTD. The former is applied to both isotropic and anisotropic ionosphere models. While its accuracy and efficiency show potential advantages compared with the conventional Monte Carlo method, its efficiency is declined when applying to anisotropic model due to the complexity nature of the anisotropic magnetized plasma algorithm. Therefore, the latter is applied to anisotropic model in order to search a more effective model in term of computational cost