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201.
Reba M. Bandyopadhyay K.S. Wood P. Hertz M.N. Lovellette P.S. Ray M.T. Wolff B. Giebels E.D. Bloom 《Astrophysics and Space Science》2001,276(1):23-24
We present first results from a monitoring campaign of GRS 1915+105 undertaken with the USA X-ray timing experiment on the ARGOS satellite. A variety of behaviour has been observed, ranging from low, steady X-ray emission to rapid quasi-periodic flaring on timescales of approximately 10–120 seconds. 相似文献
202.
A mechanism of damped oscillations of a coronal loop is investigated. The loop is treated as a thin toroidal flux rope with
two stationary photospheric footpoints, carrying both toroidal and poloidal currents. The forces and the flux-rope dynamics
are described within the framework of ideal magnetohydrodynamics (MHD). The main features of the theory are the following:
i) Oscillatory motions are determined by the Lorentz force that acts on curved current-carrying plasma structures and ii) damping is caused by drag that provides the momentum coupling between the flux rope and the ambient coronal plasma. The
oscillation is restricted to the vertical plane of the flux rope. The initial equilibrium flux rope is set into oscillation
by a pulse of upflow of the ambient plasma. The theory is applied to two events of oscillating loops observed by the Transition Region and Coronal Explorer (TRACE). It is shown that the Lorentz force and drag with a reasonable value of the coupling coefficient (c
d
) and without anomalous dissipation are able to accurately account for the observed damped oscillations. The analysis shows
that the variations in the observed intensity can be explained by the minor radial expansion and contraction. For the two
events, the values of the drag coefficient consistent with the observed damping times are in the range c
d
≈2 – 5, with specific values being dependent on parameters such as the loop density, ambient magnetic field, and the loop
geometry. This range is consistent with a previous MHD simulation study and with values used to reproduce the observed trajectories
of coronal mass ejections (CMEs). 相似文献
203.
Valerio Iafolla David M. Lucchesi Sergio Nozzoli Francesco Santoli 《Celestial Mechanics and Dynamical Astronomy》2007,97(3):165-187
We have estimated a preliminary error budget for the Italian Spring Accelerometer (ISA) that will be allocated onboard the
Mercury Planetary Orbiter (MPO) of the European Space Agency (ESA) space mission to Mercury named BepiColombo. The role of
the accelerometer is to remove from the list of unknowns the non-gravitational accelerations that perturb the gravitational
trajectory followed by the MPO in the strong radiation environment that characterises the orbit of Mercury around the Sun.
Such a role is of fundamental importance in the context of the very ambitious goals of the Radio Science Experiments (RSE)
of the BepiColombo mission. We have subdivided the errors on the accelerometer measurements into two main families: (i) the
pseudo-sinusoidal errors and (ii) the random errors. The former are characterised by a periodic behaviour with the frequency
of the satellite mean anomaly and its higher order harmonic components, i.e., they are deterministic errors. The latter are
characterised by an unknown frequency distribution and we assumed for them a noise-like spectrum, i.e., they are stochastic
errors. Among the pseudo-sinusoidal errors, the main contribution is due to the effects of the gravity gradients and the inertial forces, while among the random-like errors the main disturbing effect is due to the MPO centre-of-mass displacements produced by the onboard High Gain Antenna (HGA) movements and by the fuel consumption and sloshing. Very subtle to be considered
are also the random errors produced by the MPO attitude corrections necessary to guarantee the nadir pointing of the spacecraft.
We have therefore formulated the ISA error budget and the requirements for the satellite in order to guarantee an orbit reconstruction
for the MPO spacecraft with an along-track accuracy of about 1 m over the orbital period of the satellite around Mercury in
such a way to satisfy the RSE requirements. 相似文献
204.
Hypothesis of possible superconductivity of the iced matter of the rings of Saturn (based on the data of Voyager and Pioneer
space missions) allow us to explain many phenomena which have not been adequately understood earlier. Introducing into planetary
physics force of magnetic levitation of the superconducting iced particle of the rings, which interact with magnetosphere
of the planet, becomes to be possible to explain origin, evolution, and dynamics of the rings; to show how the consequent
precipitation of the rings’ matter upon the planet was concluded; how the rings began their rotation; how they were compressed
by the magnetic field into the thin disc, and how this disc was fractured into hundreds of thousands of separated rings; why
in the ring B do exist “spokes”; why magnetic field lines have distortion near by ring F; why there is a variable azimuth brightness of the ring A; why the rings reflected radio waves so efficiently; why exists strong electromagnetic radiation of the rings in the 20.4
kHz–40.2 MHz range and Saturnian kilometric radiation; why there is anomalous reflection of circularly polarized microwaves;
why there are spectral anomalies of the thermal radiation of the rings; why the matter of the various rings does not mix but
preserves its small-scale color differences; why there is an atmosphere of unknown origin nearby the rings of Saturn; why
there are waves of density and bending waves within Saturn’s rings; why planetary rings in the solar system appear only after
the Belt of Asteroids (and may be the Belt of Asteroids itself is a ring for the Sun); why our planet Earth has no rings of
its own. 相似文献
205.
José Antonio Belinchón 《Astrophysics and Space Science》2009,323(3):307-315
We study a massive cosmic strings with BII symmetries cosmological models in two contexts. The first of them is the standard
one with a barotropic equation of state. In the second one we explore the possibility of taking into account variable “constants”
(G and Λ). Both models are studied under the self-similar hypothesis. We put special emphasis in calculating the numerical values
for the equations of state. We find that for ω∈(0,1], G, is a growing time function while Λ, behaves as positive decreasing time function. If ω=0, both “constants”, G and Λ, behave as true constants. 相似文献
206.
Motivated by the recent interest in phantom fields as candidates for the dark energy component, we investigate the consequences
of the phantom field when is minimally coupled to gravity. In particular, the necessary (but insufficient) conditions for
the acceleration and superacceleration of the universe are obtained when the non-minimal coupling term is taken into account.
Furthermore, the necessary condition for the cosmic acceleration is derived when the phantom field is non-minimally coupled
to gravity and baryonic matter is included. 相似文献
207.
Lazarus E. Kopilovich 《Experimental Astronomy》2010,28(1):1-9
Estimates for the maximum number of elements of non-redundant configurations on integer square and hexagonal grids of given
sizes are derived (a “non-redundant” configuration implies that the vector differences between its elements are all distinct).
When projecting a large 2-D interferometer or a telescope, such an estimate can be used as a guide for evaluating the maximum
possible number of antennas in a non-redundant configuration that can be arranged within a given area. The suggested estimates
are empirical and based on the available data. They are obtained by reducing the problem to the linear case and by generalizing
the method applied therein. Examples of applying the method are presented. 相似文献
208.
Hiroko Watanabe Alexandra Tritschler Reizaburo Kitai Kiyoshi Ichimoto 《Solar physics》2010,266(1):5-16
We performed two-dimensional spectroscopic observations of the preceding sunspot of NOAA 10905 located off disk center (S8
E36, μ≈0.81) by using the Interferometric BI-dimensional Spectrometer (IBIS) operated at the Dunn Solar Telescope (DST) of the National
Solar Observatory, New Mexico. The magnetically insensitive Fe I line at 709.04 nm was scanned in wavelength repetitively
at an interval of 37 s to calculate sequences of maps of the line-wing and line-core intensity, and the line-of-sight Doppler
velocity at different line depths (3% to 80%). Visual inspection of movies based on speckle reconstructions computed from
simultaneous broadband data and the local continuum intensity at 709.04 nm revealed an umbral dot (UD) intruding rapidly from
the umbral boundary to the center of the umbra. The apparent motion of this object was particularly fast (1.3 km s−1) when compared to typical UDs. The lifetime and size of the UD was 8.7 min and 240 km, respectively. The rapid UD was visible
even in the line-core intensity map of Fe I 709.04 nm and was accompanied by a persistent blueshift of about 0.06 km s−1. 相似文献
209.
Jayanta Roy Yashwant Gupta Ue-Li Pen Jeffrey B. Peterson Sanjay Kudale Jitendra Kodilkar 《Experimental Astronomy》2010,28(1):25-60
The new era of software signal processing has a large impact on radio astronomy instrumentation. Our design and implementation
of a 32 antennae, 33 MHz, dual polarization, fully real-time software backend for the GMRT, using only off-the-shelf components,
is an example of this. We have built a correlator and a beamformer, using PCI-based ADC cards and a Linux cluster of 48 nodes
with dual gigabit inter-node connectivity for real-time data transfer requirements. The highly optimized compute pipeline
uses cache efficient, multi-threaded parallel code, with the aid of vectorized processing. This backend allows flexibility
in final time and frequency resolutions, and the ability to implement algorithms for radio frequency interference rejection.
Our approach has allowed relatively rapid development of a fairly sophisticated and flexible backend receiver system for the
GMRT, which will greatly enhance the productivity of the telescope. In this paper we describe some of the first lights using
this software processing pipeline. We believe this is the first instance of such a real-time observatory backend for an intermediate
sized array like the GMRT. 相似文献
210.
Markus J. Aschwanden Jong Kwan Lee G. Allen Gary Michael Smith Bernd Inhester 《Solar physics》2008,248(2):359-377
The three-dimensional (3D) modeling of coronal loops and filaments requires algorithms that automatically trace curvilinear
features in solar EUV or soft X-ray images. We compare five existing algorithms that have been developed and customized to
trace curvilinear features in solar images: i) the oriented-connectivity method (OCM), which is an extension of the Strous pixel-labeling algorithm (developed by Lee,
Newman, and Gary); ii) the dynamic aperture-based loop-segmentation method (developed by Lee, Newman, and Gary); iii) unbiased detection of curvilinear structures (developed by Steger, Raghupathy, and Smith); iv) the oriented-direction method (developed by Aschwanden); and v) ridge detection by automated scaling (developed by Inhester). We test the five existing numerical codes with a TRACE image
that shows a bipolar active region and contains over 100 discernable loops. We evaluate the performance of the five codes
by comparing the cumulative distribution of loop lengths, the median and maximum loop length, the completeness or detection
efficiency, the accuracy, and flux sensitivity. These algorithms are useful for the reconstruction of the 3D geometry of coronal
loops from stereoscopic observations with the STEREO spacecraft, or for quantitative comparisons of observed EUV loop geometries
with (nonlinear force-free) magnetic field extrapolation models. 相似文献