Spectra of quasiperiodic oscillations of galactic X-ray sources–dynamical regimes from a simple model

We suggest that the dynamical regime(s) underlying quasi-periodic oscillations observed in the spectra of bright galactic-bulge X-ray sources are nonlinear with a mixed phase space. The important feature of such regimes is that they are generic among nonlinear Hamiltonian and nearly Hamiltonian systems of more than two degrees of freedom. We give a simple example of such chaotic (deterministic) systems whose spectra share a number of features with those observed for quasiperiodic oscillations of such sources.     

Dynamics of Axisymmetric Truncated Dynamo Models

An important question regarding the study of mean field dynamo models is how to make precise the nature of their underlying dynamics. This is difficult both because relatively little is known about the dynamical behaviour of infinite dimensional systems and also due to the numerical cost of studying the related partial differential equations. As a first step towards their understanding, it is useful to consider the corresponding truncated models. Here we summarise some recent results of the study of a class of truncated axisymmetric mean field dynamo models. We find conclusive evidence in these models for various types of intermittency as well as multiple attractors and final state sensitivity. We also find that the understanding of the underlying dynamics of such dynamo models requires the study of a new class of dynamical systems, referred to as the non-normal systems. Current work demonstrates that these types of systems are capable of a novel type of intermittency and also of relevance for the understanding of the full axisymmetric PDE dynamo models.     

Determination of organophosphorus insecticide residues in the rice paddies

In most rice paddies in Mazandaran Province, diazinon is applied to control Chilo suppressalis. Due to the extensive application of insecticides in the rice paddies of the Caspian coasts of Iran, this investigation was carried out on the rice fields in order to obtain the necessary data and information on the concentration of insecticide residues. A total of 125 samples were taken from five areas of Amol township in 2007. Then, according to the analysis of variance procedures, the water samples were statistically analyzed after the spraying of diazinon. Results indicate that the insecticide was used frequently to control stem boring caterpillar of rice. The residuals of this toxic chemical were observed in the majority of stations from the day after the spraying until one to two months later. The greatest level of diazinon was observed in station 1 in amount of 1.14 ppm. This study revealed that the physical and chemical properties of the studied diazinon such as fumigation characteristic as well as the ecological conditions and soil type influence the reduction and eventual removal of the insecticides during the cultivation and harvest periods.     

Redistribution of energy by vertical oscillations in the solar atmosphere

The effect of vertical oscillations with periods between 90 s and 300 s on a solar atmosphere governed by heat conduction and radiation loss is examined. The effect is found to be primarily a redistribution, rather than a net addition or subtraction, of energy within the low corona, mainly by long period (180 to 300 s) oscillations. The redistribution of energy is found to affect the time-averaged temperature and density profiles of such an atmosphere, particularly in the low corona. The amount of energy redistributed is found to increase with increasing period.     

Intermittent Behaviour in Axisymmetric Mean-Field Dynamo Models in Spherical Shells

Axisymmetric mean-field dynamo models in spherical shells are shown to be capable of producing temporally intermittent behaviour. This is of potential importance since (i) it is, as far as we are aware, the first time such behaviour has been produced internally by a mean-field dynamo model in a spherical shell, without requiring any additional assumptions or truncations, and (ii) it may be characteristic of the type of behaviour observed in the long-term record of solar activity, such as Maunder minima. We also show that these types of behaviour persist when the functional form of the alpha quenching is altered and also occur over intervals of the shell thickness and the dynamo number.     

Robustness of truncated α Ω dynamos with a dynamic α

In a recent work (Covas et al., 1996), the behaviour and the robustness of truncated dynamos with a dynamic were studied with respect to a number of changes in the driving term of the dynamic equation, which was considered previously by Schmalz and Stix (1991) to be of the form AB. Here we review and extend our previous work and consider the effect of adding a quadratic quenching term of the form |B|². We find that, as before, such a change can have significant effects on the dynamics of the related truncated systems. We also find intervals of (negative) dynamo numbers, in the system considered by Schmalz and Stix (1991), for which there is sensitivity with respect to small changes in the dynamo number and the initial conditions, similar to what was found in our previous work. This latter behaviour may be of importance in producing the intermittent type of behaviour observed in the Sun.     

Convective dynamos in spherical wedge geometry

Self‐consistent convective dynamo simulations in wedge‐shaped spherical shells are presented. Differential rotation is generated by the interaction of convection with rotation. Equatorward acceleration and dynamo action are obtained only for sufficiently rapid rotation. The angular velocity tends to be constant along cylinders. Oscillatory large‐scale fields are found to migrate in the poleward direction. Comparison with earlier simulations in full spherical shells and Cartesian domains is made (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Statistical response of randomly excited nonlinear radial oscillations in polytropes

The effect of internal noise on the response characteristics of nonlinear radial pulsation in polytropic model has been investigated using the equivalent linearization technique. The model of the noise is taken to be δ-correlated with zero mean. Both non-stationary and stationary response characteristics have been computed for various nonlinearity parameters in different polytropes. This work is supported by UGC, New Delhi.     

Mean Field Dynamos with Algebraic and Dynamic α-Quenchings

Calculations for mean field dynamo models (in both full spheres and spherical shells), with both algebraic and dynamic -quenchings, show qualitative as well as quantitative differences and similarities in the dynamical behaviour of these models. We summarise and enhance recent results with extra examples. Overall, the effect of using a dynamic appears to be complicated and is affected by the region of parameter space examined.     

Equatorial magnetic helicity flux in simulations with different gauges

We use direct numerical simulations of forced MHD turbulence with a forcing function that produces two different signs of kinetic helicity in the upper and lower parts of the domain. We show that the mean flux of magnetic helicity from the small‐scale field between the two parts of the domain can be described by a Fickian diffusion law with a diffusion coefficient that is approximately independent of the magnetic Reynolds number and about one third of the estimated turbulent magnetic diffusivity. The data suggest that the turbulent diffusive magnetic helicity flux can only be expected to alleviate catastrophic quenching at Reynolds numbers of more than several thousands. We further calculate the magnetic helicity density and its flux in the domain for three different gauges. We consider the Weyl gauge, in which the electrostatic potential vanishes, the pseudo‐Lorenz gauge, where the speed of light is replaced by the sound speed, and the ‘resistive gauge’ in which the Laplacian of the magnetic vector potential acts as a resistive term. We find that, in the statistically steady state, the time‐averaged magnetic helicity density and the magnetic helicity flux are the same in all three gauges (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)