308 research outputs found
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
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
Modeling of Interstellar Scintillation Arcs from Pulsar B1133+16
The parabolic arc phenomenon visible in the Fourier analysis of the
scintillation spectra of pulsars provides a new method of investigating the
small scale structure in the ionized interstellar medium (ISM). We report
archival observations of the pulsar B1133+16 showing both forward and reverse
parabolic arcs sampled over 14 months. These features can be understood as the
mutual interference between an assembly of discrete features in the scattered
brightness distribution. By model-fitting to the observed arcs at one epoch we
obtain a ``snap-shot'' estimate of the scattered brightness, which we show to
be highly anisotropic (axial ratio >10:1), to be centered significantly off
axis and to have a small number of discrete maxima, which are coarser the
speckle expected from a Kolmogorov spectrum of interstellar plasma density. The
results suggest the effects of highly localized discrete scattering regions
which subtend 0.1-1 mas, but can scatter (or refract) the radiation by angles
that are five or more times larger.Comment: 14 pages, 4 figures, submitted to Astrophysical Journa
Quality Problems in Vocational Tourism Education
We consider the quality of vocational tourism education in terms of world trends of tourism industry development. We identify and describe the quality problems of vocational tourism education in detail from interested focus groups using empirical and theoretical methods. We also note that there is insufficient attention to the development and improvement of vocational tourism education in the world by UNESCO, a low level of scientific research in terms of assessment, and the absence of quality monitoring mechanisms of vocational tourism education level. We analyze the definition of “sustainable tourism” to ensure the quality of the tourism experience in three aspects: environmental, socio-cultural, and economic. As a result, we formulated the definition of vocational tourism education, considering modern business approaches to quality assessment. Besides, we offer innovative ways of partnership and cooperation in providing quality training for the tourism and hospitality industry at the international and national levels
PSR B0329+54: Statistics of Substructure Discovered within the Scattering Disk on RadioAstron Baselines of up to 235,000 km
We discovered fine-scale structure within the scattering disk of PSR B0329+54
in observations with the RadioAstron ground-space radio interferometer. Here,
we describe this phenomenon, characterize it with averages and correlation
functions, and interpret it as the result of decorrelation of the
impulse-response function of interstellar scattering between the
widely-separated antennas. This instrument included the 10-m Space Radio
Telescope, the 110-m Green Bank Telescope, the 14x25-m Westerbork Synthesis
Radio Telescope, and the 64-m Kalyazin Radio Telescope. The observations were
performed at 324 MHz, on baselines of up to 235,000 km in November 2012 and
January 2014. In the delay domain, on long baselines the interferometric
visibility consists of many discrete spikes within a limited range of delays.
On short baselines it consists of a sharp spike surrounded by lower spikes. The
average envelope of correlations of the visibility function show two
exponential scales, with characteristic delays of and , indicating the presence of two scales of
scattering in the interstellar medium. These two scales are present in the
pulse-broadening function. The longer scale contains 0.38 times the scattered
power of the shorter one. We suggest that the longer tail arises from
highly-scattered paths, possibly from anisotropic scattering or from
substructure at large angles.Comment: 15 pages, 6 figures, 3 tables; accepted by Astrophysical journa
Interstellar Plasma Turbulence Spectrum Toward the Pulsars PSR B0809+74 and B0950+08
Interstellar scintillations of pulsars PSR B0809+74 and B0950+08 have been
studied using observations at low frequencies (41, 62, 89, and 112 MHz).
Characteristic temporal and frequency scales of diffractive scintillations at
these frequencies have been determined. The comprehensive analysis of the
frequency and temporal structure functions reduced to the same frequency has
shown that the spectrum of interstellar plasma inhomogeneities toward both
pulsars is described by a power law. The exponent of the spectrum of
fluctuations of interstellar plasma inhomogeneities toward PSR B0950+08 (n =
3.00 +- 0.05) appreciably differs from the Kolmogorov exponent. Toward PSR
B0809+74 the spectrum is a power law with an exponent n = 3.7 +- 0.1. A strong
angular refraction has been detected toward PSR B0950+08. The distribution of
inhomogeneities along the line of sight has been analyzed; it has been shown
that the scintillations of PSR B0950+08 take place on a turbulent layer with
enhanced electron density, which is localized at approximately 10 pc from the
observer. For PSR B0809+74 the distribution of inhomogeneities is
quasi-uniform. Mean-square fluctuations of electron density on inhomogeneities
with a characteristic scale rho_0 = 10^7 m toward four pulsars have been
estimated. On this scale the local turbulence level in the 10-pc layer is 20
times higher than in an extended region responsible for the scintillations of
PSR B0809+74.Comment: 13 pages, 11 figure
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