Time-periodic Variation Research Articles

Lotka-Volterra prey-predator model with harvesting and environmental perturbations

The authors considered the two-species Lotka-Volterra prey-predator system where both species are subjected to combined harvesting effort. The system is studied by taking into account time-periodic variations in biological parameters and harvesting effort. The significant results are discussed.

Alterations in the cellular DNA and protein content determined by flow cytometry as indicators for chemically induced structural and numerical chromosome aberrations

Cellular DNA and protein content were determined simultaneously in freshly isolated fibroblast-like rat cells by flow cytometry. After exposure to doxorubicin, nitrofurantoin, propranolol and practolol at a low, tissue like oxygen concentration (5% O2), drug-induced alterations in cell cycle kinetics, in the distribution of DNA and in the protein content of G1-phase cells (nucleus/cytoplasm ratio) were analysed. Optimal exposure time (5 or 24 h) and recovery interval (24 or 48 h) were determined. Variation in the exposure time and recovery period can affect cell cycle kinetics both qualitatively and quantitatively, whereas the distribution of DNA and protein content are affected quantitatively only. A 24-h exposure combined with a 24-h recovery period proved to be the most efficient approach. Each of the tested chemicals induced a specific, dose-dependent pattern of altered cellular DNA and protein content. Comparison with results obtained in other genotoxicity tests, and with data reported earlier, showed that this two-parameter protocol can be used to recognize and to characterize chemicals as clastogens, or as compounds with a combined cytostatic/clastogenic activity, or as spindle-poison-like compounds.

Comparison of time-periodic variations in temperature and wind from meteorological rockets and satellites

Although the Meteorological Rocket Network operated by or in cooperation with the United States has decreased from fourteen to nine stations since 1979, there have been many observations accumulated in the years since CIRA 1972 was prepared with data up to 1969. The mean, annual and semiannual variations of temperature and wind are presented and compared with the Oxford SCR-PMR five-year data set, the CDC-SCR seven-year data, and CIRA 1972 with respect to both temperature and zonal winds, as far as presently available. The agreement among the data sets is generally very good.

Comparison of time-periodic variations in temperature and wind from meteorological rockets and satellites

Although the Meteorological Rocket Network operated by or in cooperation with the United States has decreased from fourteen to nine stations in the past five years, there have been many observations accumulated in the ten years since CIRA 1972 was prepared with data up to 1969. The mean, annual and semiannual variations of temperature and wind are presented and special attention is directed to the polar semiannual wave. The results are compared with the Oxford SCR-PMR five-year data set, the CDC-SCR seven-year data, and CIRA 1972 with respect to both temperature and zonal winds, as far as presently available. The agreement among the data sets is very good.

Comparison of teleradiometric and sensitometric techniques for visibility measurements

Visibility measurements of power plant and smelter plumes were conducted as part of the 1981 VISTTA field programs by means of sensitometry of color photographic slides and teleradiometry. The results obtained for 10 sets of measurements with these two techniques at three wavelengths (450, 550 and 630 nm) were compared and showed good agreement for plume/sky radiance ratios, with correlation coefficients on the order of 0.9 and average absolute differences of 2–4 per cent. The uncertainty in the plume/sky radiance ratio is about 4 per cent for sensitometry. Although this value is slightly higher than the uncertainty of 2 per cent previously reported for teleradiometry (Bergstrom et al., 1982), sensitometry offers the advantage of an instantaneous measurement. In contrast, a manual teleradiometer scan takes on the order of 15 min, and plume visual effects can show large variations in this short time period.

The stability of thermal oscillations in a reactive slab: reduction to Hill’s equation

The thermal stability of an exothermic chemically reacting slab with time-periodic surface temperature variation is examined. It is shown, on the basis of a good approximation due to Boddington, Gray and Walker, that the behaviour depends on the solutions of an ordinary differential equation of first order. The equation contains a modified amplitude, for small values of which it can be reduced to a particular form of Hill’s equation. Critical values of the Frank-Kamenetskii parameter, as a function of the amplitude ϵ and frequency ω of the surface temperature oscillations, are derived from the latter equation. For ω = 2π and 0 ≼ ϵ ≼ 2 the values are in good agreement with previously calculated ones.

Informant accuracy in social-network data V. An experimental attempt to predict actual communication from recall data

This paper seeks to discover whether the known inaccuracy of informant recall about their communication behavior can be accounted for by experimentally varying the time period over which recall takes place. The experiment took advantage of a new communications medium (computer conferencing) which enabled us to monitor automatically all the interactions involving a subset of the computer network. The experiment itself was administered entirely by the computer, which interviewed informants and recorded their responses. Variations in time period failed to account for much of the inaccuracy, which continues, as in previous experiments at an unacceptably high level. One positive finding did emerge: although the informants did not know with whom they communicated, the informants en masse seemed to know certain broad facts about the communication pattern. All other findings were negative. For example, it is impossible to predict the people an informant claimed to communicate with but did not; and it is impossible to predict who the five people are that an informant forgot to mention that she or he had communication with. Thus, despite their presumed good intentions, our findings here confirm what we have learned from six previous experiments: What people say about their communications bears no resemblance to their behavior. This suggests that other forms of data gathering, based on questions which require that informants recall their behavior, may well be suspect.

Thermal explosion theory for a slab with time-periodic surface temperature variation

The problem of the onset of thermal explosion in a slab subject to time-periodic surface temperature variation has been investigated where the slab is symmetrically heated by an exothermic zero-order chemical reaction. The main purpose of the investigation was to obtain the critical Frank-Kamenetskii parameter #c(e, w) as a function of amplitude e and frequency w/2n of the surface temperature oscillation. We have shown that with values of this parameter, such that 0 < e, w) steady temperature oscillations can be maintained within the slab, but that 8 > 8c{e, w) must lead to thermal explosion. For a period of oscillation of 24 h and suitable values for the thermal diffusivity and half-width of the slab, o) 27i, and the parameter range 0 < e ^ 4 covers ambient temperature fluctuations likely to be encountered by hazardous materials, such as in the hold of a ship in tropical seas. The problem has been examined in three different ways, (i) A comparison theorem for partial differential equations has been used to determine an analytical bound on 8,the result shows th at stable oscillations exist for all oj, provided 8^ 8C e~e (8C = 0.878), this represents a lower bound to the stability surface in (8, e, oj) space, (ii) Perturbation theory, for small amplitude, has been used to determine the critical parameter in the form oj) — 8C + e1 281(oj) + ... with £x(w) a function of frequency. Comparison with the exact numerical solution shows that this gives values which differ by less than about 0-2 % for oj^ 2k and 0 ^ e^ 1. (iii) The energy conservation equation has been solved numerically over a rectangular mesh representing the half-width of the slab and one period of steady oscillation. For given e, oj such solution could be found for 8 sufficiently small; it is shown th at the breakdown of the numerical process is associated with criticality, allowing the limiting parameter to be determined. This method has been used to obtain curves of £c(e, oj) versus e for oj = 2 4n, 8tt and the range 0 ^ ^ 4.

Infrasound Observations of Variability During Stratospheric Warmings

Abstract Observations of natural infrasound produce a continual record of the sound velocity, a function of wind and temperature, at the reflection level in the upper atmosphere. Under normal conditions in winter the reflection level, for sound generated by ocean waves to the east of Palisades, N. Y., is in the lower thermosphere. During the circulation changes associated with stratospheric warmings, winds near the stratopause may become east or north, allowing infrasound to be reflected from this level. We are then provided with a continuous record of sound velocity near the stratopause. The methods which are used to distinguish between stratosphere and thermospheric sound reflection are discussed, and circulation changes for each year are cataloged. During the warming event sound velocities in the stratosphere are shown to vary radically, with fluctuations of up to 60 m s−1 in a few hours time period. These short time period variations, observable only because of the continuous nature of infrasound reco...

THE VEGETATION HISTORY FROM 6000 b.p. TO PRESENT OF WILSONS PROMONTORY, VICTORIA, AUSTRALIA

SummaryPollen analysis of three sites on Wilsons Promontory provides a vegetation record from 6000 b.p. to the present day. No extensive changes in the vegetation have been found over this time period, but local and regional variations in the extent of communities may imply small climatic changes. Moister conditions than at present seem to have prevailed earlier than 4500 b.p., followed by a drier phase till less than 2000 b.p., when an increase in moisture gave conditions similar to those of today. Some evidence exists to support the theory of a relatively steady sea level close to the present level over the last 6000 years along the western Wilsons Promontory coastline.

Modulation spectroscopy using the beam-foil light source

When a beam of ions passes through a thin exciter foil, certain radiation emitted by the beam particles can exhibit time-periodic intensity variations. These variations can be induced by external E and H fields, or they can be the result of the field-free atomic structure itself. Intensity modulations observed so far in beam-foil spectroscopy can be divided into three classes: (1) Quantum mechanical interference of fine structure levels. This is a QM resonance arising from time-dependent populations of emitting states having different transition probabilities. The resonance is induced by external constant electric fields. (2) Initial coherent superpositions of radiating states. This results from the creation of M L alignment at the instant of excitation by the foil. The modulations are field free and are observed in polarized light. (3) Rotating electric dipole in a magnetic field. When alignment occurs, the intensity of the beam radiation after excitation satisfies the relation I( t) = = I 0[1+ A cos(2 γHt)] e − αt . The modulation will be a function of the magnetic field H and the gyromagnetic ratio γ. These effects can be used to study Lamb shifts, g-values, fine structure levels, and interaction processes.

In vitro response of lymphocytes to phytohemagglutinin (PHA) as studied with antiserum to PHA: I. Initiation period, daughter-cell proliferation, and restimulation

The time that individual lymphocytes must be exposed to PHA in order to be stimulated to synthesize DNA was determined by inactivating PHA with antiserum (anti-PHA). The variation of this required PHA exposure time was compared to the variation of the total initiation time period, that time from the beginning of PHA exposure to DNA synthesis. It was found that cells are PHA-dependent for a variable portion and then PHA-independent for a constant portion of that initiation period. These times are defined here as the in vitro part of the G0 phase (pre-G1) and the entire initial G1 phase, respectively, of the cells' proliferative cycles. The variation in the in vitro G0 phase thus accounts for the variation in the total initiation period. Cultures partially-inhibited with anti-PHA contained cells which divided in essentially PHA-free medium, and the daughter cells of these initial mitotic divisions were found to synthesize DNA and divide without exposure to PHA. When the DNA synthetic response to PHA waned a restimulation DNA synthetic response was produced with the addition of fresh PHA. This restimulation response had a short initiation period that was analysed with anti-PHA inactivation of PHA, and it, too, was found to have two components, PHA-dependent (G0) and PHA-independent (G1). Cells participating in restimulation were shown to be those that had responded earlier in the initial exposure to PHA rather than cells that had never responded at all in vitro. This supports the suggestion that the restimulation response is quick because the cells are ‘primed’ with their initial response. The restimulation response of cultures partially-inhibited with anti-PHA consisted of both previously stimulated cells and cells released from inhibition, with short and long initiation periods, respectively.