Michael B. Silevitch

Three-dimensional subsurface analysis of electromagnetic scattering from penetrable/PEC objects buried under rough surfaces: use of the steepest descent fast multipole method

The electromagnetic scattering from a three-dimensional (3D) shallow object buried under a two-dimensional (2D) random rough dielectric surface is analyzed. The buried object can be a perfect electric conductor (PEC) or can be a penetrable dielectric with size and burial depth comparable to the free-space wavelength. The random rough ground surface is characterized with Gaussian statistics for surface height and for surface autocorrelation function. The Poggio, Miller, Chang, Harrington, and Wu (PMCHW) integral equations are implemented and extended. The integral equation-based steepest descent fast multipole method (SDFMM), that was originally developed at UIUC, has been used and the computer code based on this algorithm has been successfully modified to handle the current application. The significant potential of the SDFMM code is that it calculates the unknown moment method surface electric and magnetic currents on the scatterer in a dramatically fast, efficient, and accurate manner. Interactions between the rough surface interface and the buried object are fully taken into account with this new formulation. Ten incident Gaussian beams with the same elevation angle and different azimuth angles are generated for excitation as one possible way of having multiple views of a given target. The scattered electric fields due to these ten incident beams are calculated in the near zone and their complex vector average over the multiple views is computed. The target signature is obtained by subtracting the electric fields scattered from the rough ground only from those scattered from the ground with the hurled anti-personnel mine.

Prebreakup arcs: A comparison between theory and experiment

We have developed a model describing the structure of a prebreakup arc based on an ionospheric Cowling channel and its extension into the magnetosphere. A coupled two-circuit representation of the substorm current wedge is used which is locally superimposed on both westward and eastward electrojets. We find that brighter, more unstable prebreakup arcs are formed in the premidnight (southwest of the Harang Discontinuity) than in the postmidnight (northeast of the Harang Discontinuity) sector. This contributes to the observed prevalence of auroral activity in the premidnight sector. Also, our model predicts that the north-south dimensions of the current wedge in the ionosphere should vary from a few kilometers at an invariant latitude (Λ) of 62° to hundreds of kilometers above Λ = 68°. Comparison of the model results with the extensive observations of Marklund et al. (1983) for a specific arc observed just after onset shows good agreement, particularly for the magnitude of the polarization electric field and the arc size. We conclude that this agreement is further evidence that the substorm breakup arises from magnetosphere-ionosphere coupling in the near magnetosphere and that the steady state model developed here is descriptive of the breakup arc before inductive effects become dominant.

Monte Carlo simulations of electromagnetic wave scattering from a random rough surface with three-dimensional penetrable buried object: mine detection application using the steepest-descent fast multipole method.

We present a statistical study of the electric field scattered from a three-dimensional penetrable object buried under a two-dimensional random rough surface. Monte Carlo simulations using the steepest-descent fast multipole method (SDFMM) are conducted to calculate the average and the standard deviation of the near-zone scattered fields. The SDFMM, originally developed at the University of Illinois at Urbana-Champaign, has been modified to calculate the unknown surface currents both on the rough ground and on the buried object that are due to excitation by a tapered Gaussian beam. The rough ground medium used is an experimentally measured typical dry Bosnian soil with 3.8% moisture, while the buried object represents a plastic land mine modeled as an oblate spheroid with dimensions and burial depth smaller than the free-space wavelength. Both vertical and horizontal polarizations for the incident waves are studied. The numerical results show that the TNT mine signature is almost 5% of the total field scattered from the ground. Moreover, relatively recognizable object signatures are observed even when the object is buried under the tail of the incident beam. Interestingly, even for the small surface roughness parameters considered, the standard deviation of the object signature is almost 30% of the signal itself, indicating significant clutter distortion that is due to the roughness of the ground.

Particle dynamics in a spatially varying electric field

For an MHD description of a plasma a distinct separation between the macroscopic and microscopic spatial and temporal scales is assumed. In this paper we solve the particle dynamics with finite first and second spatial derivatives in the electric field. We find that (1) MHD (ideal and nonideal) becomes invalid for a sufficiently strong constant electric field gradient perpendicular to the magnetic field; (2) a sufficiently large second derivative in the electric field can cause heavy ions to become chaotically untrapped; (3) for an electric field with a constant gradient the ion drift velocity is equal to (E×B)/|B|2 as long as the orbit-averaged value of E is used. There are no finite currents associated with the ion drift for such an electric field; (4) perturbation technique gives a poor approximation to the ion drift velocity even for values of the second derivative that may well occur in the magnetosphere. Results 1 and 2 provide necessary criteria for the applicability of magnetospheric MHD models of spatially varying electric fields. They also predict an asymmetry in the heavy ion fluxes, a feature that could be useful in inferring magnetospheric electric field structure. We illustrate these results by application to the Harang discontinuity. It is found that if the interplanetary magnetic field swings northward under substorm growth conditions the orbits of the equatorial O+ may dramatically change due to result 2. This effect may contribute to the substorm onset process.

A new model for auroral breakup during substorms

A model for substorm breakup is developed, based on (1) the relaxation of stretched (closed) dipolar field lines, and (2) the formation of an incipient current wedge within a single arc structure. It is argued that the establishment of a coupled current structure within a single arc leads to a quasistable system, i.e. the prebreakup regime. Perturbation of the prebreakup structure leads to an instability criterion. It is found, consistent with observations, that the narrower auroral arcs at lower L shells undergo the most explosive poleward expansion. According to this model, the precise location at which breakup occurs depends on the O/sup +/ density in the plasma sheet, the level of magnetic activity (K/sub p/), and the intensity of the substorm westward electrojet in the ionosphere. An enhancement of any of these features will cause breakup to occur at lower L shells. Comparison of the proposed model with the Heppner-Maynard polar-cap potential model indicates that breakup is restricted to the west of the Harang discontinuity, consistent with observations from the Viking satellite. >

Acceleration and stochastic heating of ions drifting through an auroral arc

We find that ions E x B drifting through an auroral arc can undergo transverse acceleration and stochastic heating. This result is very analogous to recent work regarding similar phenomena in the m ...

Advantages of electric circuit models for treating the substorm breakup problem

It is shown, by using a circuit model for the magnetospheric current system, that the substorm breakup can be triggered either by some instability anywhere in the circuit or by a decrease in the generator emf, i.e., a northward turning of the interplanetary magnetic field.

A Unified Discipline of Subsurface Sensing and Imaging Systems

Subsurface sensing and imaging seeks to locate and identify objects or conditions underneath an obscuring media by monitoring a probe or wave outside the surface. Many of the mathematical and physical models used in this process are common to underground and underwater environmental exploration, medical imaging, and three-dimensional microscopies, allowing a common framework of physic-based signal processing (PBSP) to be applied. The basis for a unified discipline of subsurface sensing and imaging can be identified from a few general subsurface information extraction strategies. These strategies and their related families of PBSP algorithms can be used to guide a systems-oriented approach to subsurface solutions.

Case Study–Puerto Rico Test Site for Exploring Contamination Threats

Puerto Rico has one of the highest densities of Superfund and National Priority List sites per square mile. The specific pollutants in those sites have been well documented and monitored over time. Puerto Rico has its share of public health problems as it has the highest rates of preterm birth compared to other jurisdiction in the US. Other important attributes, such as the stability and diversity of the population, and the presence of a karst environment with a major aquifer that provides most of the water supply to the study area, make Puerto Rico a good case study to evaluate the impact of exposure to contamination on public health problems. In this paper, we present an interdisciplinary approach to evaluate the relation between preterm birth, a common and well defined health outcome, and exposure to contamination from well-defined sources, pathways and receptors. The rate of preterm birth in Puerto Rico in 2008, the most recent year with available data, was 19.6% or nearly 1 out of every five births. Puerto Rico has gradually experienced nearly a doubling in the preterm rate since 1990, when it was at just 11.4%. The usual causes of preterm births, such as lack of prenatal care in the first trimester, maternal education, maternal smoking in pregnancy, and increased use of assisted reproductive technology (ART), do not explain the increase in the preterm rate observed in Puerto Rico. Through integrated analytical, mechanistic, epidemiology, fate-transport, and remediation studies, along with a centralized, indexed data repository, the program will deliver new knowledge and technology in the area of contaminants as a potential cause of preterm birth.

O+ phase bunching as a source for stable auroral arcs

We propose a model to explain how ion dynamics create an Alfven wave generator in the equatorial region that can be applied to the stable arc problem. For example, in the earthward drifting magnetotail plasma, phase bunching of O+ ions (and to a much lesser extent of the H+ ions) can be caused by a weak (∼1×10−9 Vm−2) electric field gradient [Rothwell et al., 1994]. This leads to density striations in the GSM frame. O+ density striations in the earthward drifting plasma frame are seen as a tailward propagating source of Alfven waves where the hydrogen ions provide the polarization current of the wave. A transformation to the GSM frame will yield a static, oblique wave structure similar to that previously treated. The waves propagate from the equatorial region to both ionospheres where they are reflected. The ionospheric boundary condition when combined with a magnetospheric boundary condition allows a solution of the wave amplitudes in terms of the striation structure. The frequency of the Alfven wave and the associated wavelengths are also determined by the striation driver. We find that the magnitude of the parallel current density at the ionosphere has a spatial resonance when the distance between the ionosphere and the equatorial plane is equal to a quarter wavelength along Bo. In that case, the magnitude of the parallel current density at the ionosphere is of the order of 10 μA m−2 and peaks for striation wavelengths (as mapped to the ionosphere) of 10–40 km, which is comparable to the transverse scale of auroral arcs. The associated Poynting flux incident on the ionosphere is found to be ∼ 2 mWm−2 and represents a net transfer of energy from the magnetosphere to the ionosphere as recently observed by experimenters studying substorm onsets. We find that in the steady state the power extracted from the bulk flow to power the arc is balanced by energy provided by the solar wind through the cross-tail electric field.