310 research outputs found
Construction of a model of the Venus surface and its use in processing radar observations
An algorithm is described for constructing the model of the Venus surface as an expansion in spherical functions. The relief expansion coefficients were obtained up to the coefficient S sub 99. The surface picture representation is given according to this expansion. The surface model constructed was used for processing radar observations. The use of the surface model allows improved agreement between the design and measured values of radar ranges
Asymmetry Function of Interstellar Scintillations of Pulsars
A new method for separating intensity variations of a source's radio emission
having various physical natures is proposed. The method is based on a joint
analysis of the structure function of the intensity variations and the
asymmetry function, which is a generalization of the asymmetry coefficient and
characterizes the asymmetry of the distribution function of the intensity
fluctuations on various scales for the inhomogeneities in the diffractive
scintillation pattern. Relationships for the asymmetry function in the cases of
a logarithmic normal distribution of the intensity fluctuations and a normal
distribution of the field fluctuations are derived. Theoretical relationships
and observational data on interstellar scintillations of pulsars (refractive,
diffractive, and weak scintillations) are compared. Pulsar scintillations match
the behavior expected for a normal distribution of the field fluctuations
(diffractive scintillation) or logarithmic normal distribution of the intensity
fluctuations (refractive and weak scintillation). Analysis of the asymmetry
function is a good test for distinguishing scintillations against the
background of variations that have different origins
Motion Parameters Determination of the SC and Phobos in the Project Phobos-Grunt
The SC "Phobos-Grunt" flight is planned to 2009 in Russia with the purpose to deliver to the Earth the soil samples of the Mars satellite Phobos. The mission will pass under the following scheme [1-4]: the SC flight from the Earth to the Mars, the SC transit on the Mars satellite orbit, the motion round the Mars on the observation orbit and on the quasi-synchronous one [5], landing on Phobos, taking of a ground and start in the direction to the Earth. The implementation of complicated dynamical operations in the Phobos vicinity is foreseen by the project. The SC will be in a disturbance sphere of gravitational fields from the Sun, the Mars and the Phobos. The SC orbit determination is carried out on a totality of trajectory measurements executed from ground tracking stations and measurements of autonomous systems onboard space vehicle relatively the Phobos. As ground measurements the radio engineering measurements of range and range rate are used. There are possible as onboard optical observations of the Phobos by a television system and ranges from the SC up to the Phobos surface by laser locator. As soon as the Phobos orbit accuracy is insufficient for a solution of a problem of landing its orbit determination will be carried out together with determination of the SC orbit. Therefore the algorithms for joint improving of initial conditions of the SC and the Phobos are necessary to determine parameters of the SC relative the Phobos motion within a single dynamical motion model. After putting on the martial satellite orbit, on the Phobos observation orbit, on the quasi-synchronous orbit in the Phobos vicinity the equipment guidance and the following process of the SC orbit determination relatively Phobos requires a priori knowledge of the Phobos orbit parameters with sufficiently high precision. These parameters should be obtained beforehand using both all modern observations and historical ones
Process based model sheds light on climate sensitivity of Mediterranean tree-ring width
We use the process-based VS (Vaganov-Shashkin) model to investigate whether a regional <i>Pinus halepensis</i> tree-ring chronology from Tunisia can be simulated as a function of climate alone by employing a biological model linking day length and daily temperature and precipitation (AD 1959–2004) from a climate station to ring-width variations. We check performance of the model on independent data by a validation exercise in which the model's parameters are tuned using data for 1982–2004 and the model is applied to generate tree-ring indices for 1959–1981. The validation exercise yields a highly significant positive correlation between the residual chronology and estimated growth curve (<i>r</i>=0.76 <i>p</i><0.0001, <i>n</i>=23). The model shows that the average duration of the growing season is 191 days, with considerable variation from year to year. On average, soil moisture limits tree-ring growth for 128 days and temperature for 63 days. Model results depend on chosen values of parameters, in particular a parameter specifying a balance ratio between soil moisture and precipitation. Future work in the Mediterranean region should include multi-year natural experiments to verify patterns of cambial-growth variation suggested by the VS model
Observation of a narrow baryon resonance with positive strangeness formed in Xe collisions
The charge-exchange reaction K^+ Xe --> K^0 p Xe' is investigated using the
data of the DIANA experiment. The distribution of the pK^0 effective mass shows
a prominent enhancement near 1538 MeV formed by \sim 80 events above the
background, whose width is consistent with being entirely due to the
experimental resolution. Under the selections based on a simulation of K^+Xe
collisions, the statistical significance of the signal reaches 5.5\sigma. We
interpret this observation as strong evidence for formation of a pentaquark
baryon with positive strangeness, \Theta^+(uudd\bar{s}), in the charge-exchange
reaction K^+ n --> K^0 p on a bound neutron. The mass of the \Theta^+ baryon is
measured as m(\Theta^+) = 1538+-2 MeV. Using the ratio between the numbers of
resonant and non-resonant charge-exchange events in the peak region, the
intrinsic width of this baryon resonance is determined as \Gamma(\Theta^+) =
0.34+-0.10 MeV.Comment: 19 pages, 8 figure
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