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891.
Alberto Escapa 《Celestial Mechanics and Dynamical Astronomy》2011,110(2):99-142
We explore the evolution of the angular velocity of an elastic Earth model, within the Hamiltonian formalism. The evolution
of the rotation state of the Earth is caused by the tidal deformation exerted by the Moon and the Sun. It can be demonstrated
that the tidal perturbation to spin depends not only upon the instantaneous orientation of the Earth, but also upon its instantaneous
angular velocity. Parameterizing the orientation of the Earth figure axis with the three Euler angles, and introducing the
canonical momenta conjugated to these, one can then show that the tidal perturbation depends both upon the angles and the
momenta. This circumstance complicates the integration of the rotational motion. Specifically, when the integration is carried
out in terms of the canonical Andoyer variables (which are the rotational analogues to the orbital Delaunay variables), one
should keep in mind the following subtlety: under the said kind of perturbations, the functional dependence of the angular
velocity upon the Andoyer elements differs from the unperturbed dependence (Efroimsky in Proceedings of Journées 2004: Systèmes
de référence spatio-temporels. l’Observatoire de Paris, pp 74–81, 2005; Efroimsky and Escapa in Celest. Mech. Dyn. Astron. 98:251–283, 2007). This happens because, under angular velocity dependent perturbations, the requirement for the Andoyer elements to be canonical
comes into a contradiction with the requirement for these elements to be osculating, a situation that parallels a similar
antinomy in orbital dynamics. Under the said perturbations, the expression for the angular velocity acquires an additional
contribution, the so called convective term. Hence, the time variation induced on the angular velocity by the tidal deformation
contains two parts. The first one comes from the direct terms, caused by the action of the elastic perturbation on the torque-free
expressions of the angular velocity. The second one arises from the convective terms. We compute the variations of the angular
velocity through the approach developed in Getino and Ferrándiz (Celest. Mech. Dyn. Astron. 61:117–180, 1995), but considering the contribution of the convective terms. Specifically, we derive analytical formulas that determine the
elastic perturbations of the directional angles of the angular velocity with respect to a non-rotating reference system, and
also of its Cartesian components relative to the Tisserand reference system of the Earth. The perturbation of the directional
angles of the angular velocity turns out to be different from the evolution law found in Kubo (Celest. Mech. Dyn. Astron.
105:261–274, 2009), where it was stated that the evolution of the angular velocity vector mimics that of the figure axis. We investigate comprehensively
the source of this discrepancy, concluding that the difference between our results and those obtained in Ibid. stems from an oversimplification made by Kubo when computing the direct terms. Namely, in his computations Kubo disregarded
the motion of the tide raising bodies with respect to a non-rotating reference system when compared with the Earth rotational
motion. We demonstrate that, from a numerical perspective, the convective part provides the principal contribution to the
variation of the directional angles and of length of day. In the case of the x and y components in the Tisserand system, the convective contribution is of the same order of magnitude as the direct one. Finally,
we show that the approximation employed in Kubo (Ibid.) leads to significant numerical differences at the level of a hundred micro-arcsecond. 相似文献
892.
Kshitij Thorat 《Journal of Astrophysics and Astronomy》2011,32(4):495-496
We present the radio-optical imaging of ATLBS, a sensitive radio survey (Subrahmanyan et al. 2010). The primary aim of the ATLBS survey is to image low-power radio sources which form the bulk of the radio source population
to moderately high red-shifts (z ∼ 1.0). The accompanying multiband optical and near infra-red observations provide information about the hosts and environments
of the radio sources. We give here details of the imaging of the radio data and optical data for the ATLBS survey. 相似文献
893.
A solution to the fixed-time minimum-fuel two-impulse rendezvous problem for the general non-coplanar elliptical orbits is
provided. The optimal transfer orbit is obtained using the constrained multiple-revolution Lambert solution. Constraints consist
of lower bound for perigee altitude and upper bound for apogee altitude. The optimal time-free two-impulse transfer problem
between two fixed endpoints implies finding the roots of an eighth order polynomial, which is done using a numerical iterative
technique. The set of feasible solutions is determined by using the constraints conditions to solve for the short-path and
long-path orbits semimajor axis ranges. Then, by comparing the optimal time-free solution with the feasible solutions, the
optimal semimajor axis for the two fixed-endpoints transfer is identified. Based on the proposed solution procedure for the
optimal two fixed-endpoints transfer, a contour of the minimum cost for different initial and final coasting parameters is
obtained. Finally, a numerical optimization algorithm (e.g., evolutionary algorithm) can be used to solve this global minimization
problem. A numerical example is provided to show how to apply the proposed technique. 相似文献
894.
We present the first set of polarimetric images made with the GMRT. We find that the instrumental polarization leakage at
the GMRT varies with frequency. It is possible to calibrate these terms to better than 1% accuracy, making it feasible to
study sources that are polarized at the few per cent level. We present 610 MHz polarization images of two extended FR-II radio
galaxies, viz. 3C 79 and 3C 265. We present high resolution polarization images of these two sources and also find that the
polarization fractions of the two sources as seen at the GMRT are consistent with those reported by Conway & Strom (1984). 相似文献
895.
Yoshio Kubo 《Celestial Mechanics and Dynamical Astronomy》2011,110(2):143-168
Under perturbations from outer bodies, the Earth experiences changes of its angular momentum axis, figure axis and rotational
axis. In the theory of the rigid Earth, in addition to the precession and nutation of the angular momentum axis given by the
Poisson terms, both the figure axis and the rotational axis suffer forced deviation from the angular momentum axis. This deviation
is expressed by the so-called Oppolzer terms describing separation of the averaged figure axis, called CIP (Celestial Intermediate
Pole) or CEP (Celestial Ephemeris Pole), and the mathematically defined rotational axis, from the angular momentum axis. The
CIP is the rotational axis in a frame subject to both precession and nutation, while the mathematical rotational axis is that
in the inertial (non-rotating) frame. We investigate, kinematically, the origin of the separation between these two axes—both
for the rigid Earth and an elastic Earth. In the case of an elastic Earth perturbed by the same outer bodies, there appear
further deviations of the figure and rotational axes from the angular momentum axis. These deviations, though similar to the
Oppolzer terms in the rigid Earth, are produced by quite a different physical mechanism. Analysing this mechanism, we derive
an expression for the Oppolzer-like terms in an elastic Earth. From this expression we demonstrate that, under a certain approximation
(in neglect of the motion of the perturbing outer bodies), the sum of the direct and convective perturbations of the spin
axis coincides with the direct perturbation of the figure axis. This equality, which is approximate, gets violated when the
motion of the outer bodies is taken into account. 相似文献
896.
Abimael Bengochea Manuel Falconi Ernesto Pérez-Chavela 《Astrophysics and Space Science》2011,333(2):399-408
We present some families of horseshoe periodic orbits in the general planar three-body problem for the case of two equal masses.
The considered system is a symmetric version of the one formed by Saturn, Janus and Epimetheus. We use a mass ratio equal
to 35×10−5, corresponding to 105 times the Saturn-Janus mass parameter of the restricted case; for this mass ratio the satellites have a significantly bigger
influence on the planet than in the classical Saturn, Janus and Epimetheus system. To obtain periodic orbits, we search those
horseshoe orbits passing through two reversible configurations. A particular kind of periodic orbits where the minor bodies
follow the same path is discussed. 相似文献
897.
This paper describes a new 2D model for the photospheric evolution of the magnetic carpet. It is the first in a series of
papers working towards constructing a realistic 3D non-potential model for the interaction of small-scale solar magnetic fields.
In the model, the basic evolution of the magnetic elements is governed by a supergranular flow profile. In addition, magnetic
elements may evolve through the processes of emergence, cancellation, coalescence and fragmentation. Model parameters for
the emergence of bipoles are based upon the results of observational studies. Using this model, several simulations are considered,
where the range of flux with which bipoles may emerge is varied. In all cases the model quickly reaches a steady state where
the rates of emergence and cancellation balance. Analysis of the resulting magnetic field shows that we reproduce observed
quantities such as the flux distribution, mean field, cancellation rates, photospheric recycle time and a magnetic network.
As expected, the simulation matches observations more closely when a larger, and consequently more realistic, range of emerging
flux values is allowed (4×1016 – 1019 Mx). The model best reproduces the current observed properties of the magnetic carpet when we take the minimum absolute flux
for emerging bipoles to be 4×1016 Mx. In future, this 2D model will be used as an evolving photospheric boundary condition for 3D non-potential modeling. 相似文献
898.
Heinrich Hora George H. Miley Xiaoling Yang Paraskevas Lalousis 《Astrophysics and Space Science》2011,336(1):225-228
An extreme anomaly of laser-plasma interaction with petawatt-picosecond (PW-ps) pulses of very high contrast ratio for suppression
of relativistic self-focusing permitted a come-back of the Bobin-Chu side-on ignition of uncompressed deuterium-tritium (DT)
fusion fuel. The plasma blocks for the side-on ignition have to be produced by the well confirmed nonlinear force acceleration
which is about 100,000 times higher than thermo-kinetic fluid-dynamic acceleration for comparison with astrophysical cases.
It is essential that the dielectric plasma properties within the nonlinear force are used. Using the measured ion beam densities
above 1011 A s/cm2 the ignition mechanism needed numerical and theoretical studies of extremely strong shock phenomena. When extending these
results to the side-on ignition of uncompressed hydrogen-boron11 (HB11), surprisingly, the ignition by this shock mechanism
was only about 10 times more difficult than for DT in contrast to ignition by spherical laser driven compression using thermo-kinetic
conditions in which case HB11 ignition is 100,000 times more difficult than DT. 相似文献
899.
Colin R. McInnes 《Celestial Mechanics and Dynamical Astronomy》2011,110(3):199-215
This paper investigates new families of displaced, highly non-Keplerian orbits in the two-body problem and artificial equilibria
in the circular restricted three-body problem. The families of orbits presented extend prior work by using periodic impulses
to generate displaced orbits rather than continuous thrust. The new displaced orbits comprise a sequence of individual Keplerian
arcs whose intersection is continuous in position, with discontinuities in velocity removed using impulses. For frequent impulses
the new families of orbits approximate continuous thrust non-Keplerian orbits found in previous studies. To generate approximations
to artificial equilibria in the circular restricted three-body problem, periodic impulses are used to generate a sequence
of connected three-body arcs which begin and terminate at a fixed position in the rotating frame of reference. Again, these
families of orbits reduce to the families of artificial equilibria found using continuous thrust. 相似文献
900.
The Wenchuan Earthquake that occurred in May of 2008 caused damages to large areas of Sichuan, Gansu, and Shaanxi provinces
in China. Reports from local governments and related management agencies show that the giant panda nature reserves in the
earthquake-hit areas were heavily damaged. Our estimates in this paper of the impacts of the earthquake on the giant panda
in the earthquake-hit areas were made based on the interpretation of remote sensing images and information collected by field
survey. A rapid assessment method was designed to estimate the damages of the earthquake on giant panda habitats. By using
visual interpretation methods, we decoded the remote sensing images of the disaster area in the 49 giant panda nature reserves.
Research results showed that the Wenchuan Earthquake and the succeeding secondary geological disasters caused great damages
to the giant panda nature reserves and disturbed the normal life of the giant pandas there (e.g., landscape fragmentation
increased significantly). Undoubtedly, the life of the giant pandas there was affected. However, although the earthquake caused
certain impacts on the giant pandas, it did not really threat their survival. Even so, we still strongly advocate for protection
of the giant pandas, and have prioritized a couple of measures to be taken to restore the giant panda nature reserves in the
earthquake-hit areas. 相似文献