Viscous fluid universe filled with stiff fluid in general relativity

We have investigated two stiff-fluid models in which the material distribution is that of viscous fluid. In the first model, the coefficient of shear viscosity is assumed to be constant while in the second model the coefficient of shear viscosity is proportional to the rate of expansion in the model. The paper also discusses some physical and geometrical aspects of the model. The behaviour of the model in absence of viscosity is also discussed.     

Agegraphic Chameleon-Tachyon dark energy model: stability and observational tests

In this paper interacting chameleon-tachyon model with agegraphic dark energy is revisited. The model in two cases of matter and radiation dominated universe is best fitted with the observational data for distance modulus. Stability of the model is investigated. The model then tested against observational data for Hubble parameter. With respect to the best fitted model parameters, our results show that while both scenarios are in good match with the observational data in low redshifts, the model in radiation dominated case better fits the data in high redshifts.     

A new calibration model for pointing a radio telescope that considers nonlinear errors in the azimuth axis

A new calibration model of a radio telescope that includes pointing error is presented, which considers nonlinear errors in the azimuth axis. For a large radio telescope, in particular for a telescope with a turntable, it is difficult to correct pointing errors using a traditional linear calibration model, because errors produced by the wheel-on-rail or center bearing structures are generally nonlinear. Fourier expansion is made for the oblique error and parameters describing the inclination direction along the azimuth axis based on the linear calibration model, and a new calibration model for pointing is derived. The new pointing model is applied to the 40 m radio telescope administered by Yunnan Observatories, which is a telescope that uses a turntable. The results show that this model can significantly reduce the residual systematic errors due to nonlinearity in the azimuth axis compared with the linear model.     

灰色模型用于卫星钟差长期预报的性能研究

利用灰色模型对GPSRb钟和Cs钟进行了长期预报,并与常用的二阶多项式模型预报进行比较,结果表明:灰色模型对GPSCs钟进行长期210d(天)预报时精度高达ns量级,对GPSRb钟的预报精度在10ns量级,明显高于二阶多项式模型的预报精度,满足实际应用中的精度要求。     

Performance results of HESP physical model

As a continuation to the published work on model based calibration technique with HESP(Hanle Echelle Spectrograph) as a case study, in this paper we present the performance results of the technique. We also describe how the open parameters were chosen in the model for optimization, the glass data accuracy and handling the discrepancies. It is observed through simulations that the discrepancies in glass data can be identified but not quantifiable. So having an accurate glass data is important which is possible to obtain from the glass manufacturers. The model’s performance in various aspects is presented using the ThAr calibration frames from HESP during its pre-shipment tests. Accuracy of model predictions and its wave length calibration comparison with conventional empirical fitting, the behaviour of open parameters in optimization, model’s ability to track instrumental drifts in the spectrum and the double fibres performance were discussed. It is observed that the optimized model is able to predict to a high accuracy the drifts in the spectrum from environmental fluctuations. It is also observed that the pattern in the spectral drifts across the 2D spectrum which vary from image to image is predictable with the optimized model. We will also discuss the possible science cases where the model can contribute.     

A model for the penumbra of sunspots

A penumbra model in hydrostatic equilibrium is presented. The model accounts for the continuum observations as well as the observations of Fraunhofer lines in the penumbra. The uncertainty in the model in deeper layers is discussed. It is shown that the penumbra is probably not in strict radiative equilibrium.     

Bianchi Type-V Bulk Viscous Fluid String Dust Cosmological Model In General Relativity

Bianchi Type-V bulk viscous fluid string dust cosmological model in General Relativity is investigated. It has been shown that if coefficient of bulk viscosity (ζ) is inversely proportional to the expansion (θ) in the model then string cosmological model for Bianchi Type-V space-time is possible. In absence of bulk viscosity (ζ), i.e. when ζ → 0, then there is no string cosmological model for Bianchi Type-V space-time. The physical and geometrical aspects of the model are also discussed.     

Relativistic fireworks

We have studied a model of relativistic fireworks. In this model it is assumed that a series of explosions occur. In each explosion the fragments fly apart in arbitrary directions with a given velocity which is a parameter in the model.We have succeeded in obtaining an exact expression for the distribution of fragments in velocity space aftern explosions.We present an exact solution also in the limiting case of small velocity steps where the process turns into a diffusion in velocity space.The development in configuration space has been obtained through Monte-Carlo numerical simulations.The model has been applied to metagalactic cosmology. Although single explosions cannot reach the highest redshifts observed in the Hubble expansion the fireworks model offers a possibility to reach thesez-values in a few explosions.The model gives a density inhomogeneity of 20% over a tenth of the Hubble distance as seen from a typical position. Observations show a considerably greater irregular variation.The model gives a local velocity dispersion which is too great to comply with observations. A development of the model is suggested.     

A microphysically-based approach to modeling emissivity and albedo of the martian seasonal caps

A new model of albedo and emissivity of the martian seasonal caps represented as porous CO₂ slabs containing spherical voids and dust particles is described. In the model, a radiative transfer model is coupled with a microphysical model in order to link changes in albedo and emissivity to changes in porosity caused by ice metamorphism. The coupled model is capable of reproducing temporal changes in the spectra of the caps taken by the Thermal Emission Spectrometer onboard the Mars Global Surveyor and it can be used as the forward model in the retrievals of the caps' physical properties (porosity, dust abundance, void and dust grain size) from the spectra. Preliminary results from such inversion studies are presented.     

三种对流层延迟改正模型精度评估

利用36个全球分布的IGS站2003全年GPS实测的对流层天顶延迟数据和气象数据,对目前国内外常用的两个对流层延迟改正模型;Hopfield模型、Saastamoinen模型和最近几年发展起来的EGNOS模型的改正精度和适用范围进行评估,指出Hopfield模型在应用中存在的缺陷,EGNOS模型可用作GNSS实时定位和导航的对流层天顶延迟的改正模型.     

Toward some new directions in Pc5 pulsation theory—II. Extension of the analysis to two hemispheres and determination of the model's polarization characteristics

The model of morning sector Pc5 pulsation activity developed in the companion paper is extended to account for both north and south ionospheres. The result is consistent with recent work by Allan and Knox (1979a and b). Using the spatial oscillation model proposed by McDiarmid (1979), the polarization properties of the model are determined. It is shown that when the phase of the ground induced response is taken into account, the model predicts the results observed by Lam and Rostoker (1979). Allan and Knox (1979) have also discussed the case where the wave admittance at the ionosphere equals the height integrated Pedersen conductivity (matching) and concluded that no standing wave solution exists. Here it is shown that for pulsation systems whose essential physics is described by the proposed model, the matching condition is more complicated and is, in fact, unlikely to be satisfied. Finally, it is concluded that the results obtained with our model are sufficient to warrant the development of an equivalent hydromagnetic wave model.     

A gravitationally non-degenerate cosmological model with expanding and shearing viscous fluid in general relativity

The object of this paper is to investigate the behaviour of viscosity in a cosmological model, in which the coefficient of shear viscosity is assumed to be proportional to rate of expansion in the model. The behaviour of the model in the absence of shear viscosity is also discussed.     

A model of the solar convection zone

A model of the convection zone is presented which matches an empirical model atmosphere (HSRA) and an interior model. A mixing length formalism containing four adjustable parameters is used. Thermodynamical considerations provide limits on two of these parameters. The average temperature-pressure relation depends on two or three combinations of the four parameters. Observational information on the structure of the outermost layers of the convection zone, and the value of the solar radius limit the range of possible parameter combinations. It is shown that in spite of the remaining freedom of choice of the parameters, the mean temperature-pressure relation is fixed well by these data.The reality of a small density inversion in the HSRA model is investigated. The discrepancy between the present model and a solar model by Mullan (1971) is discussed briefly.     

Adiabatic vibrational frequencies of local and nonlocal convection models of the sun

The eigen-vibrational frequencies of Xiong Da-run's nonlocal and local convection models of solar envelope are calculated and compared. The differences between the observational and theoretical vibrational frequencies are less than 1%. They can be divided into two isolated groups. For modes with l ≥ 60, all the differences between observed and theoretical eigen-vibrational frequencies are distributed in a narrow and inclined belt in the (Δv ≈ v)-diagram. This shows that the theoretical model of solar convective region can approximately reflect the intrinsic structure of the sun in the region of r = (0.70–0.95)R. The discrepancies between the theoretical and observational frequencies come from the outer layers. For modes with l < 60, the theoretical vibrational frequency is smaller than the observational one. This implies that the temperature of the upper part of the convectively unstable region is rather low. The frequency difference is more dispersed in the local convection model than in the nonlocal convection model. For the intermediate- and low-frequency ranges (v < 3000), the difference between the two models is small, while for the high-frequency range (v ≥ 3000) the frequency in the local model is higher than in the nonlocal model. This means that the temperature of the radiation region beneath the convective region is higher in the local convection model than in the nonlocal convection model. The nonlocal model is nearer to the observation than the local model.     

An analytic model of the Earth's magnetosphere

An analytic magnetic field model for the Earth's magnetosphere is constructed from a dipole field and a tail field. This model can be taken as a generalization of the Dungey's model, after one adds to it a horizontal component. The magnetic topology in the noon-midnight meridian plane of this model is fully determined and it is compared with the topology of other models. In this study it is found that, for a specific value of the parameterk, which is associated to any form of the model, the noon's side neutral points obey a bifurcation scheme.     

The Data Depth-weight-kernel Estimation of the Model Error of Satellite Orbit Determination

It is an objective fact that there exists error in the satellite dynamic model and it will be transferred to satellite orbit determination algorithm, forming a part of the connotative model error. Mixed with the systematic error and random error of the measurements, they form the unitive model error and badly restrict the precision of the orbit determination. We deduce in detail the equations of orbit improvement for a system with dynamic model error, construct the parametric model for the explicit part of the model and nonparametric model for the error that can not be explicitly described. We also construct the partially linear orbit determination model, estimate and fit the model error using a two-stage estimation and a kernel function estimation, and finally make the corresponding compensation in the orbit determination. Beginning from the data depth theory, a data depth weight kernel estimator for model error is proposed for the sake of promoting the steadiness of model error estimation. Simulation experiments of SBSS are performed. The results show clearly that the model error is one of the most important effects that will influence the precision of the orbit determination. The kernel function method can effectively estimate the model error, with the window width as a major restrict parameter. A data depth-weight-kernel estimation, however, can improve largely the robustness of the kernel function and therefore improve the precision of orbit determination.     

Generalized Chaplygin gas model: cosmological consequences and statefinder diagnosis

The generalized Chaplygin gas (GCG) model in spatially flat universe is investigated. The cosmological consequences led by GCG model including the evolution of EoS parameter, deceleration parameter and dimensionless Hubble parameter are calculated. We show that the GCG model behaves as a general quintessence model. The GCG model can also represent the pressureless CDM model at the early time and cosmological constant model at the late time. The dependency of transition from decelerated expansion to accelerated expansion on the parameters of model is investigated. The statefinder parameters r and s in this model are derived and the evolutionary trajectories in s–r plane are plotted. Finally, based on current observational data, we plot the evolutionary trajectories in s–r and q–r planes for best fit values of the parameters of GCG model. It has been shown that although, there are similarities between GCG model and other forms of Chaplygin gas in statefinder plane, but the distance of this model from the ΛCDM fixed point in s–r diagram is shorter compare with standard Chaplygin gas model.     

An explanation for the soft X-ray excess in active galactic nuclei

We present a large sample of type 1 active galactic nuclei (AGN) spectra taken with XMM–Newton , and fit them with both the conventional model (a power law and blackbody) and the relativistically blurred photoionized disc reflection model of Ross & Fabian. We find that the disc reflection model is a better fit. The disc reflection model successfully reproduces the continuum shape, including the soft excess, of all the sources. The model also reproduces many features that would conventionally be interpreted as absorption edges. We are able to use the model to infer the properties of the sources, specifically that the majority of black holes in the sample are strongly rotating, and that there is a deficit in sources with an inclination >70°. We conclude that the disc reflection model is an important tool in the study of AGN X-ray spectra.     

A Stochastic Prediction Model for the Sunspot Cycles

A stochastic prediction model for the sunspot cycle is proposed. The prediction model is based on a modified binary mixture of Laplace distribution functions and a moving-average model over the estimated model parameters. A six-parameter modified binary mixture of Laplace distribution functions is used for the modeling of the shape of a generic sunspot cycle. The model parameters are estimated for 23 sunspot cycles independently, and the primary prediction-model parameters are derived from these estimated model parameters using a moving-average stochastic model. A correction factor (hump factor) is introduced to make an initial prediction. The hump factor is computed for a given sunspot cycle as the ratio of the model estimated after the completion of a sunspot cycle (post-facto model) and the prediction of the moving-average model. The hump factors can be applied one at a time over the moving-average prediction model to get a final prediction of a sunspot cycle. The present model is used to predict the characteristics of Sunspot Cycle 24. The methodology is validated using the previous Sunspot Cycles 21, 22, and 23, which shows the adequacy and the applicability of the prediction model. The statistics of the variations of sunspot numbers at high solar activity are used to provide the lower and upper bound for the predictions using the present model.