Large Time Domain Research Articles

Spatially Explicit and Stochastic Simulation of Forest-Landscape Fire Disturbance and Succession

Understanding disturbance and recovery of forest landscapes is a challenge because of complex interactions over a range of temporal and spatial scales. Landscape simulation models offer an approach to studying such systems at broad scales. Fire can be simulated spatially using mechanistic or stochastic approaches. We describe the fire module in a spatially explicit, stochastic model of forest landscape dynamics (LANDIS) that incorporates fire, windthrow, and harvest disturbance with species-level succession. A stochastic approach is suited to forest landscape models that are designed to simulate patterns over large spatial and time domains and are not used deterministically to predict individual events. We used the model to examine how disturbance regimes and species dynamics interact across a large (500 000 ha), heterogeneous landscape in northern Wisconsin, USA, with six land types having different species environments, and fire disturbance return intervals varying from 200 to 1000 yr. The model shows that there are feedbacks over time between species, disturbance, and environment, resulting in the re-emergence of patterns that characterized the landscape before extensive alteration. Landscape equilibrium of species composition and age-class structure develops at three scales from the initial, disturbed landscape. Over 100–150 yr, fine-grained successional processes cause gradual disaggregation of the initial pattern of relatively homogenous composition and age classes. Species such as eastern hemlock (Tsuga canadensis), largely removed from the landscape by past human activities, only slowly re-invade. Next, patterns on the various land types diverge, driven by different disturbance regimes and dominant species. Finally, aging of the landscape causes the probabilities of larger and more severe fires to increase, and a coarse-grained pattern develops from the disturbance patches. Influence of adjacent land types is shown as fires spread across land type boundaries, although modified in spread and severity. As found by others, altered landscapes are likely to retain their modified pattern for centuries, suggesting that nonequilibrium conditions between tree species and climate will persist under predicted rates of climate change. The results suggest that this modeling approach can be useful in examining species-level, broad-scale responses of heterogeneous landscapes to changes in landscape disturbance, such as modified management or land-use scenarios, or effects of global change.

Time-domain near-field analysis of short-pulse antennas .II. Reactive energy and the antenna Q

For pt. I see ibid., vol.47, no.2, p.271-79 (1999). The time-domain (TD) multipole expansion, developed in the first part of this two-part sequence, is extended here to analyze the power-flow and energy balance in the vicinity of a pulsed antenna. Using the spherical transmission line formulation, we derive expressions for the pulsed power-flow and energy and identify the radiative and the reactive constituents. For time-harmonic fields, the reactive concepts are well understood in terms of the stored energy, but this interpretation is not applicable for short-pulse fields where there is no stored energy. By considering the TD energy balance, we clarify the transition of the near-zone pulsed reactive energy to the radiation power and show that the pulsed reactive energy discharges back to the source once the pulse has been radiated. We thus introduce a TD Q factor that quantifies the radiation efficiency. In particular, we show that super resolution using short-pulse fields involves large TD reactive energies and Q and is, therefore, not feasible. The general TD concepts discussed are demonstrated through a numerical example of radiation from a circular disk carrying a pulsed current distribution.

Reduced-cost methods for large time domain integral equation scattering analyses

Analysis of high frequency scattering using pulsed illumination generates surface fields which are small over most of the scatterer most of the time. A reformulation of the usual integral equation time domain approach which exploits this is presented. It is shown that cost scaling can be reduced, with costs reduced by an order of magnitude for the examples presented, with negligible accuracy loss. © 1998 John Wiley & Sons, Ltd.

Charge transport in a-Si:H detectors: comparison of analytical and Monte Carlo simulations

To understand the signal formation in hydrogenated amorphous silicon (a-Si:H) p-i-n detectors, dispersive charge transport due to multiple trapping in a-Si:H tail states is studied both analytically and by Monte Carlo simulations. An analytical solution is found for the free electron and hole distributions n(x,t) and the transient current I(t) due to an initial electron-hole pair generated at an arbitrary depth in the detector for the case of exponential band tails and linear field profiles; integrating over all e-h pairs produced along the particle's trajectory yields the actual distributions and current; the induced charge Q(t) is obtained by numerically integrating the current. This generalizes previous models used to analyze time-of-flight experiments. The Monte Carlo simulation provides the same information but can be applied to arbitrary field profiles, field dependent mobilities and localized state distributions. A comparison of both calculations is made in a simple case to show that identical results are obtained over a large time domain. A comparison with measured signals confirms that the total induced charge depends on the applied bias voltage. The applicability of the same approach to other semiconductors is discussed. >

Abra Lead-Silver-Copper-Gold Deposit, Western Australia: A Geophysical Case History

The Abra base-metal deposit is a large, deeply buried, low-grade mineralised body located in the Jillawarra mineralised belt in the Bangemall Basin, Western Australia. It has no surface geological or geochemical expression. The deposit was discovered in 1981 by drill testing a 270 m deep target, based on the detailed modelling of a 400 nT bullseye magnetic anomaly which had a coincident weak residual gravity anomaly. Follow-up drilling broadly confirmed the original magnetic interpretation, and outlined an estimated 200 Mt of low-grade iron-barium-lead-silver-copper-gold mineralisation. Both moving-loop and large fixed-loop time-domain electromagnetic surveys recorded a broad anomaly over the Abra mineralised system. The anomalous time-domain electromagnetic transients have an exponential decay with a time constant of about 1.3 ms, indicative of low conductance. Downhole time-domain electromagnetic and mise-a-la-masse surveying confirms that the whole of the mineralised system, including both the stratiform zone and the underlying stringer zone, is weakly and uniformly conductive. No specific zones of locally greater conductance, which might be indicative of higher-grade mineralisation, were detected in the surface time-domain electromagnetic work. In the downhole time-domain electromagnetic surveying, the broad response to the bulk mineralised body is complicated by the use of relatively small transmitter loops which have 'selectively' energised portions of the large conductive body, resulting in both 'in-hole' and 'off-hole' responses being recorded, depending upon the relative transmitter loop-conductor-drillhole geometry. Several localised secondary conductor responses (both in-hole and off-hole) were also detected but, to date, this work has not successfully demonstrated the presence of discrete high-grade zones of significant dimensions within the overall system.

Diffusion of the 241Am → 237Np Decay Chain Limited by Their Elemental Solubilities in the Artificial Barriers of High-Level Radioactive Waste Repositories

A mathematical analysis of the diffusion of the [sup 241]Am [r arrow] [sup 237]Np decay chain in the artificial barrier of a high-level radioactive waste repository is presented. First, analytical solutions obtained are for the space- and time-dependent concentration of [sup 241]Am in the artificial barrier and the time-dependent amount of americium precipitated at the surface of the waste glass, based on the assumption of the congruency of the radionuclides with solubility-limited dissolution of the glass matrix. The effects of solubility sharing with coexisting [sup 243]Am are considered. Transport and precipitation of [sup 237]Np in the artificial barrier are analyzed by dividing the time domain into a small time domain, where the [sup 241]Am concentration is so large that [sup 237]Np precipitation is dominant, and a large time domain, where the [sup 241]Am becomes negligible and the precipitation region shrinks by diffusion from the precipitation front. The equation for the movement of the precipitation front is obtained. As the overpack lifetime increases, the effect of neptunium precipitation becomes less significant. With a lifetime longer than [approximately]6000 yr, an earlier model, where neptunium is treated as a mother nuclide and the precipitation occurs only at the glass surface, can be used.

Dynamic surface properties of aqueous sodium dioctyl sulfosuccinate solutions

To investigate the very slow surface equilibration of submicellar solutions of straight-chain soidum dioctyl sulfosuccinate (Na-DOSS), different techniques covering a large time domain are used. An adsorption model for Na-DOSS is introduced and the compatibility of the different experiments with this model is explained. Therefore, for longitudinal wave experiments as well as for stress—relaxation experiments, theoretical equations are calculated and interpreted in light of this model.

Edwards-Anderson type relaxation in the frustraded quasi-1d spin glass FeMgBO 4

The spin relaxation process, investigated by inelastic neutron scattering in a large time domain 6 x 10 -13 s ⩽ t ⩽ 10 -9 s is interpreted in terms of an Edwards-Anderson order parameter q. The freezing temperature does not correspond to the appearance of q but to the freezing of the low frequency spin fluctuations.

A pulsed particle beam source for nano- to microsecond luminescence decay studies

A simple device for pulsing accelerated particle beams especially suited for luminescence decay measurements over a large time domain (ns to several μs) is presented: the continuous particle beam is permanently deflected by a transverse magnetic field and periodically realigned by nanosecond electrical field pulses. Particle pulses with a duration of 1 ns and an adjustable repetition rate up to 100 kHz have been obtained. The main advantages of this “external” type pulsing method are (i) the suitability for various kinds of particle beams, (ii) the delivery of a well-time-defined synchronous electrical pulse, and (iii) the independence of the particle energy.

The spin-lattice relaxation times of water associated with early post mortem changes in skeletal muscle

We studied the spin-echo signal of muscle in a large time domain found that the motion of the nuclear magnetic moment of tissue water cannot be characterized by a single spin-lattice relaxation time ( T 1). The relaxation time T 1B, which is the T 1 characterized by those protons with a slower relaxation rate, is influenced by the early post mortem changes in skeletal muscle. T 1B increased with time after the tissue was taken from the animal and reached a maximum at 3 h. However, the weighted average of T 1 of all water protons ( T 1A) did not change throughout the time course of the experiments.