A model of the late universe with viscous Zel’ldovich fluid and decaying vacuum     

K. Rajagopalan Nair  Titus K. Mathew 《Astrophysics and Space Science》2018,363(9):183

Many have speculated about the presence of a stiff fluid in very early stage of the universe. Such a stiff fluid was first introduced by Zel’dovich. Recently the late acceleration of the universe was studied by taking bulk viscous stiff fluid as the dominant cosmic component, but the age predicted by such a model is less than the observed value. We consider a flat universe with viscous stiff fluid and decaying vacuum energy as the cosmic components and found that the model predicts a reasonable background evolution of the universe with de Sitter epoch as end phase of expansion. More over, the model also predicts a reasonable value for the age of the present universe. We also performed a dynamical system analysis of the model and found that the end de Sitter phase predicted by the model is stable.  相似文献   

Magnetic Disconnections at the Boundary of a Small Interplanetary Magnetic Flux Rope Associated with a Reconnection Exhaust     

JieMin Wang  Qiang Liu  Yan Zhao 《Solar physics》2018,293(8):116

A local current sheet and a subsequent small interplanetary magnetic-flux rope were observed on 1 April 2003 by Wind and the Advanced Composition Explorer (ACE). A Petschek reconnection-like exhaust crossing of the local current sheet was identified using the Walén test. The Wind spacecraft re-entered the reconnection exhaust after the main exhaust encounter, and the reentry may be due to a spatial fold of the current-sheet surface itself. The absence of parallel strahls and the presence of antiparallel strahls on either side of the current sheet suggest that the magnetic-field lines before the exhaust and in the subsequent small flux rope are all open. The \(180^{\circ}\) pitch-angle strahls were clearly absent, and halo-suprathermal electron pitch-angle distributions were observed in the exhaust. This finding means that the open field lines of the magnetic-flux rope were reconnecting to the adjacent open field lines to produce U-shaped field lines disconnected from the Sun. These observations provide direct evidence that the magnetic fields of the interplanetary small magnetic-flux rope were disconnecting from the Sun through magnetic reconnection. This type of disconnected event potentially has important implications for the magnetic-flux budget of the heliosphere.  相似文献   

Gravitational instability in isotropic MHD plasma waves     

Alemayehu Mengesha Cherkos 《Journal of Astrophysics and Astronomy》2018,39(2):22

The effect of compressive viscosity, thermal conductivity and radiative heat-loss functions on the gravitational instability of infinitely extended homogeneous MHD plasma has been investigated. By taking in account these parameters we developed the six-order dispersion relation for magnetohydrodynamic (MHD) waves propagating in a homogeneous and isotropic plasma. The general dispersion relation has been developed from set of linearized basic equations and solved analytically to analyse the conditions of instability and instability of self-gravitating plasma embedded in a constant magnetic field. Our result shows that the presence of viscosity and thermal conductivity in a strong magnetic field substantially modifies the fundamental Jeans criterion of gravitational instability.  相似文献   

Nonlinear model to study filamentation instability of circularly polarized dispersive Alfvén waves in solar wind plasmas     

Swati Sharma  R. P. Sharma  M. K. Mishra 《Astrophysics and Space Science》2018,363(8):164

This paper examines the small-scale solar wind turbulence driven in view of the Alfvén waves subjected to ponderomotive nonlinearity. Filamentation instability is known to take place for the case of dispersive Alfvén wave (DAW) propagating parallel to the ambient magnetic field. The ponderomotive force associated with DAW is responsible for wave localization and these webs of filaments become more intense and irregular as one proceeds along the spatial domain. The ponderomotive force associated with pump changes with pump parameters giving rise to different evolution patterns. This paper studies in detail the nonlinear evolution of filamentation instability introduced by dispersive Alfven waves (DAWs) which becomes dispersive on account of the finite frequency of DAW i.e., pump frequency is comparable to the ion cyclotron frequency. We have explicitly obtained the perturbation dynamics and then examined the impact of pump magnitude on the driven magnetic turbulence using numerical simulation. The results show steepening at small scales with increasing pump amplitude. The compressibility associated with acoustic fluctuations may explain the variation in spectral scaling of solar wind turbulence as observed by Alexandrova et al. (Astrophys. J. 674:1157, 2008).  相似文献   

Fraile Detection of Solar $$-Modes by Fossat et al.     

Hannah Schunker  Jesper Schou  Patrick Gaulme  Laurent Gizon 《Solar physics》2018,293(6):95

The internal gravity modes of the Sun are notoriously difficult to detect, and the claimed detection of gravity modes presented by Fossat et al. (Astron. Astrophys.604, A40, 2017) is thus very exciting. Given the importance of these modes for understanding solar structure and dynamics, the results must be robust. While Fossat et al. described their method and parameter choices in detail, the sensitivity of their results to several parameters was not presented. Therefore, we test the sensitivity of the results to a selection of the parameters. The most concerning result is that the detection vanishes when we adjust the start time of the 16.5-year velocity time-series by a few hours. We conclude that this reported detection of gravity modes is extremely fragile and should be treated with utmost caution.  相似文献   

Dynamical model of binary asteroid systems using binary octahedrons     

Yu Jiang  Hexi Baoyin  Mo Yang 《Journal of Astrophysics and Astronomy》2018,39(5):54

We used binary octahedrons to investigate the dynamical behaviors of binary asteroid systems. The mutual potential of the binary polyhedron method is derived from the fourth order to the sixth order. The irregular shapes, relative orbits, attitude angles, as well as the angular velocities of the binary asteroid system are included in the model. We investigated the relative trajectory of the secondary relative to the primary, the total angular momentum and total energy of the system, the three-axis attitude angular velocity of the binary system, as well as the angular momentum of the two components. The relative errors of the total angular momentum and the total energy indicate that the calculation has a high precision. It is found that the influence of the orbital and attitude motion of the primary from the gravitational force of the secondary is obvious. This study is useful in understanding the complicated dynamical behaviors of the binary asteroid systems discovered in our Solar system.  相似文献   

Antenna design and implementation for the future space Ultra-Long wavelength radio telescope     

Linjie Chen  Amin Aminaei  Leonid I. Gurvits  Marc Klein Wolt  Hamid Reza Pourshaghaghi  Yihua Yan  Heino Falcke 《Experimental Astronomy》2018,45(2):231-253

In radio astronomy, the Ultra-Long Wavelengths (ULW) regime of longer than 10 m (frequencies below 30 MHz), remains the last virtually unexplored window of the celestial electromagnetic spectrum. The strength of the science case for extending radio astronomy into the ULW window is growing. However, the opaqueness of the Earth’s ionosphere makes ULW observations by ground-based facilities practically impossible. Furthermore, the ULW spectrum is full of anthropogenic radio frequency interference (RFI). The only radical solution for both problems is in placing an ULW astronomy facility in space. We present a concept of a key element of a space-borne ULW array facility, an antenna that addresses radio astronomical specifications. A tripole–type antenna and amplifier are analysed as a solution for ULW implementation. A receiver system with a low power dissipation is discussed as well. The active antenna is optimized to operate at the noise level defined by the celestial emission in the frequency band 1 ? 30 MHz. Field experiments with a prototype tripole antenna enabled estimates of the system noise temperature. They indicated that the proposed concept meets the requirements of a space-borne ULW array facility.  相似文献   

Nonlinear analytic solution for perturbed relative motion using differential equinoctial elements     

Zhen Yang  Yazhong Luo  Jin Zhang  Hengnian Li 《Celestial Mechanics and Dynamical Astronomy》2018,130(9):56

This paper develops a nonlinear analytic solution for satellite relative motion in J₂-perturbed elliptic orbits by using the geometric method that can avoid directly solving the complex differential equations. The differential equinoctial elements (DEEs) are used to remove any singularities for zero-eccentricity or zero-inclination orbits. Based on the relationship between the relative states and the DEEs, state transition tensors (STTs) for transforming the osculating DEEs and propagating the mean DEEs have been derived. The formulation of these STTs has been split into a set of vector and matrix operations, which avoids directly expanding the complex second-order terms, and thus, the obtained STTs could be easy-to-understand and easy-to-code. Numerical results show that the proposed nonlinear solution is valid for zero-eccentricity and zero-inclination reference orbit and is more accurate than the previous linear or nonlinear methods for the long-term prediction of satellite relative motion.  相似文献   

Optimization Design of Minimum Total Resistance Hull Form Based on CFD Method     

Bao-ji Zhang  Sheng-long Zhang  Hui Zhang 《中国海洋工程》2018,32(3):323-330

In order to reduce the resistance and improve the hydrodynamic performance of a ship, two hull form design methods are proposed based on the potential flow theory and viscous flow theory. The flow fields are meshed using body-fitted mesh and structured grids. The parameters of the hull modification function are the design variables. A three-dimensional modeling method is used to alter the geometry. The Non-Linear Programming (NLP) method is utilized to optimize a David Taylor Model Basin (DTMB) model 5415 ship under the constraints, including the displacement constraint. The optimization results show an effective reduction of the resistance. The two hull form design methods developed in this study can provide technical support and theoretical basis for designing green ships.  相似文献   

Observations and physics of prompt emission of gamma ray bursts     

Shabnam Iyyani 《Journal of Astrophysics and Astronomy》2018,39(6):75

Gamma ray bursts (GRBs) are the brightest explosions known to occur in the Universe. For the last several decades, they have been extensively observed and studied using both space as well as ground based observatories. In this review, the observational breakthroughs made till date, the techniques of observation and analyses of obtained data, temporal and spectral properties of the observed prompt emission of GRBs including polarisation, as well as the various theoretical models adopted to explain them are discussed.  相似文献