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81.
This paper proposes a method to structurally identify inherent dynamic characteristics based on long-term monitored acceleration data of nonlinear offshore platforms under sea-ice excitation. Not all the inherent characteristics can be excited due to the randomness of ice loading and its limited bandwidth. However, the long-term monitored data can reflect most of the conditions of sea-ice excitation. The change of natural characteristics of the platform under ice loads can be identified by analyzing of long-term monitored data. A nonlinear system of two-degree-of-freedom (2DOF) is simulated to identify dynamic characteristics and verify the effectiveness of the proposed method. The method is applied to analyze the acceleration data caused by ice-induction at a jacket platform in Liaodong Bay. The inherent characteristics of the structure can be identified and its variation under different ice-load intensities is summarized. 相似文献
82.
Recent results of the gamma-ray Cherenkov astronomy definitely prove the existence of fast variability in the very high energy
(V.H.E.) gamma-ray flux of some active galactic nuclei. The BL Lac PKS 2155-304 for instance showed variations down to a few
minutes time scale. From standard light travel time argument, these variations put extremely strong constraints on the size
of the TeV emitting zone, which has to be of the order of a few Schwarzschild radius, even for high values of the relativistic
Doppler factor of the emitting jets. Such discovery is a challenge for particle acceleration scenarios, which have to imagine
efficient acceleration processes at work in a very compact zone. Eventually, the immediate vicinity of the central black hole
appears as the most conservative choice for the location of the TeV emission region of active galactic nuclei. In this paper,
we propose a two-step mechanism for charged particle acceleration in the magnetosphere of a massive black hole surrounded
by an accretion disk. Particles first gain energy by a stochastic process during the accretion phase. It is shown that effective
proton acceleration up to energies 1017–1019 eV is possible in a low-luminosity magnetized accretion disk with 2D turbulent motion. The distribution function of energetic
protons over energies is a power law function with typical index ≃−1. Here electrons are not very efficiently accelerated
because of their drastic losses by synchrotron radiation. In a second time, part of the fast particles escape from the disk
and are then entrained by the magnetic structure above the disk, in the rotating black hole magnetosphere. They thus gain
additional energy by direct centrifugal mechanism, up to about 1020 eV for the protons and to 10–100 TeV for the electrons when they cross the light cylinder surface. Such energetic particles
can further radiate in the TeV spectral range observed by Cherenkov experiments as HESS, MAGIC and VERITAS. Energetic protons
can produce γ-radiation in the energy band 1 GeV–100 TeV and above mainly by nuclei collisions with the disk matter, clouds, or ambient
low energy photons. Energetic electrons can also reach the required spectral range by inverse Compton emission. However their
acceleration is less efficient due to heavy radiation losses, and only gained by centrifugal process during the second phase
of the whole mechanism we describe. Our present analysis would therefore favor hadronic scenarios for TeV emission of active
galactic nuclei. It is tempting to relate long term variability over years of TeV active galactic nuclei to the first stochastic
acceleration phase, which also provides the needed power law particle distributions, while short term variability over minutes
is more likely due to perturbations of the second fast direct acceleration phase. 相似文献
83.
Surajit Chattopadhyay Ujjal Debnath Goutami Chattopadhyay 《Astrophysics and Space Science》2008,314(1-3):41-44
In this letter, we have assumed that the Universe is filled in tachyonic field with potential, which gives the acceleration
of the Universe. For certain choice of potential, we have found the exact solutions of the field equations. We have shown
the decaying nature of potential. From recently developed statefinder parameters, we have investigated the role of tachyonic
field in different stages of the evolution of the Universe. 相似文献
84.
In this paper, we have considered a model for FRW space-time in the presence of coupled scalar field φ and potential V(φ) with causal viscous fluid and polytropic fluid. We have shown that irrespective of fluid the causality theory provides late
time acceleration of the universe. In all cases, the potential always decreases due to evolution of the universe. 相似文献
85.
86.
《New Astronomy》2015
We present PICsar – a new Particle in Cell code geared towards efficiently simulating the magnetosphere of the aligned rotator. PICsar is a special relativistic, electromagnetic, charge conservative code that can be used to simulate arbitrary electromagnetics problems in axisymmetry. It features stretchable body-fitted coordinates that follow the surface of a sphere, simplifying the application of boundary conditions in the case of the aligned rotator; a radiation absorbing outer boundary, which allows a steady state to be set up dynamically and maintained indefinitely from transient initial conditions; and algorithms for injection of charged particles into the simulation domain. The code is parallelized using MPI and scales well to a large number of processors. We discuss the numerical methods used in PICsar and present tests of the code. In particular, we show that PICsar can accurately and efficiently simulate the magnetosphere of the aligned monopole rotator in the force-free limit. We present simulations of the aligned dipole rotator in a forthcoming paper. 相似文献
87.
Dmitry Khangulyan Slavomir Hnatic Felix Aharonian 《Astrophysics and Space Science》2007,309(1-4):261-265
The inverse Compton (IC) scattering of ultrarelativistic electrons accelerated at the pulsar wind termination shock is generally
believed to be responsible for TeV gamma-ray signal recently reported from the binary system PSR B1259-63/SS2883. In such
a system the acceleration takes place in the presence of a dense radiation field provided by a companion Be2-type star. Thus
it is natural to expect an orbital phase dependence of the acceleration efficiency in the system. The HESS collaboration reported
the tendency of reduction of TeV γ-rays around the periastron. In this paper we study a possible explanation of this effect by the “early” (sub-TeV) cutoffs
in the energy spectrum of accelerated electrons due to the enhanced rate of Compton losses close to the periastron. 相似文献
88.
E. Martínez-Gómez H. J. Durand-Manterola H. Pérez de Tejada 《Astrophysics and Space Science》2006,306(4):259-267
Voyager's plasma probe observations suggest that there are at least three fundamentally different plasma regimes in Saturn:
the hot outer magnetosphere, the extended plasma sheet, and the inner plasma torus. At the outer regions of the inner torus
some ions have been accelerated to reach energies of the order of 43 keV. We develop a model that calculates the acceleration
of charged particles in the Saturn's magnetosphere. We propose that the stochastic electric field associated to the observed
magnetic field fluctuations is responsible of such acceleration. A random electric field is derived from the fluctuating magnetic
field – via a Monte Carlo simulation – which then is applied to the momentum equation of charged particles seeded in the magnetosphere.
Taking different initial conditions, like the source of charged particles and the distribution function of their velocities,
we find that particles injected with very low energies ranging from 0.129 eV to 5.659 keV can be strongly accelerated to reach
much higher energies ranging from 22.220 eV to 9.711 keV as a result of 125,000 hitting events (the latter are used in the
numerical code to produce the particle acceleration over a predetermined distance). 相似文献
89.
根据采用动力学方程对亚暴期间磁尾磁场向偶极形弛豫过程中离子分布函数的模拟结果 ,研究了磁尾来自电离层的O+,H+和He+离子的速度及能量随时间的变化 .主要结果为 :(1 )离子的加速及能量变化主要发生在磁场偶极化过程的中期 ,对应的地心距离位于- 1 2RE到 - 8RE 之间 ;(2 )垂直于磁场方向上离子加速及能量变化较快 ,平行方向上较慢 ;(3)轻离子较重离子加速及能量变化快 ,磁场偶极化终结 ,3种离子的能量均可增加 2 0 0倍左右 ;(4)初始能量较高时 ,离子加速及能量变化较快 ,离子最终获得的能量较大 .理论计算的磁尾离子能量在磁场偶极化过程终了可达 1 0 2 keV的量级 ,这与观测结果一致 . 相似文献
90.
Abdelmajid Benali 《Comptes Rendus Geoscience》2013,345(1):36-46
When running a groundwater flow model, a recurrent and seemingly subsidiary question arises at the starting step of computations: what value of acceleration parameter do we need to optimize the numerical solver? A method is proposed to provide a practical estimate of the optimal acceleration parameter via a geostatistical analysis of the spatial variability of the logarithm of the transmissivity field Y. The background of the approach is illustrated on the successive over-relaxation method (SOR) used, either as a stand-alone solver, or as a symmetric preconditioner (SSOR) to the gradient conjugate method, or as a smoother in multigrid methods. It shows that this optimum acceleration factor is a function of the standard deviation and the correlation length of Y. This provides an easy-to-use heuristic procedure to estimate the acceleration factors, which could even be incorporated in the software package. A case study illustrates the steps needed to perform this estimation. 相似文献