3,475 research outputs found
Clumpy and fractal shocks, and the generation of a velocity dispersion in molecular clouds
We present an alternative explanation for the nature of turbulence in
molecular clouds. Often associated with classical models of turbulence, we
instead interpret the observed gas dynamics as random motions, induced when
clumpy gas is subject to a shock. From simulations of shocks, we show that a
supersonic velocity dispersion occurs in the shocked gas provided the initial
distribution of gas is sufficiently non-uniform. We investigate the velocity
size-scale relation for simulations of clumpy and
fractal gas, and show that clumpy shocks can produce realistic velocity
size-scale relations with mean . For a fractal
distribution, with a fractal dimension of 2.2 similar to what is observed in
the ISM, we find . The form of the velocity size-scale
relation can be understood as due to mass loading, i.e. the post-shock velocity
of the gas is determined by the amount of mass encountered as the gas enters
the shock. We support this hypothesis with analytical calculations of the
velocity dispersion relation for different initial distributions.
A prediction of this model is that the line-of sight velocity dispersion
should depend on the angle at which the shocked gas is viewed.Comment: 11 pages, 17 figures, accepted for publication in MNRA
Universal statistics of non-linear energy transfer in turbulent models
A class of shell models for turbulent energy transfer at varying the
inter-shell separation, , is investigated. Intermittent corrections in
the continuous limit of infinitely close shells () have
been measured. Although the model becomes, in this limit, non-intermittent, we
found universal aspects of the velocity statistics which can be interpreted in
the framework of log-poisson distributions, as proposed by She and Waymire
(1995, Phys. Rev. Lett. 74, 262). We suggest that non-universal aspects of
intermittency can be adsorbed in the parameters describing statistics and
properties of the most singular structure. On the other hand, universal aspects
can be found by looking at corrections to the monofractal scaling of the most
singular structure. Connections with similar results reported in other shell
models investigations and in real turbulent flows are discussed.Comment: 4 pages, 2 figures available upon request to [email protected]
Coupled attribute analysis on numerical data
The usual representation of quantitative data is to formalize it as an information table, which assumes the independence of attributes. In real-world data, attributes are more or less interacted and coupled via explicit or implicit relationships. Limited research has been conducted on analyzing such attribute interactions, which only describe a local picture of attribute couplings in an implicit way. This paper proposes a framework of the coupled attribute analysis to capture the global dependency of continuous attributes. Such global couplings integrate the intra-coupled interaction within an attribute (i.e. The correlations between attributes and their own powers) and inter-coupled interaction among different attributes (i.e. The correlations between attributes and the powers of others) to form a coupled representation for numerical objects by the Taylor-like expansion. This work makes one step forward towards explicitly addressing the global interactions of continuous attributes, verified by the applications in data structure analysis, data clustering, and data classification. Substantial experiments on 13 UCI data sets demonstrate that the coupled representation can effectively capture the global couplings of attributes and outperforms the traditional way, supported by statistical analysis
GRB970228 and the class of GRBs with an initial spikelike emission: do they follow the Amati relation?
On the basis of the recent understanding of GRB050315 and GRB060218, we
return to GRB970228, the first Gamma-Ray Burst (GRB) with detected afterglow.
We proposed it as the prototype for a new class of GRBs with "an occasional
softer extended emission lasting tenths of seconds after an initial spikelike
emission". Detailed theoretical computation of the GRB970228 light curves in
selected energy bands for the prompt emission are presented and compared with
observational BeppoSAX data. From our analysis we conclude that GRB970228 and
likely the ones of the above mentioned new class of GRBs are "canonical GRBs"
have only one peculiarity: they exploded in a galactic environment, possibly
the halo, with a very low value of CBM density. Here we investigate how
GRB970228 unveils another peculiarity of this class of GRBs: they do not
fulfill the "Amati relation". We provide a theoretical explanation within the
fireshell model for the apparent absence of such correlation for the GRBs
belonging to this new class.Comment: 5 pages, 3 figures, in the Proceedings of the "4th Italian-Sino
Workshop on Relativistic Astrophysics", held in Pescara, Italy, July 20-28,
2007, C.L. Bianco, S.-S. Xue, Editor
Vortex Reconnection as the Dissipative Scattering of Dipoles
We propose a phenomenological model of vortex tube reconnection at high
Reynolds numbers. The basic picture is that squeezed vortex lines, formed by
stretching in the region of closest approach between filaments, interact like
dipoles (monopole-antimonopole pairs) of a confining electrostatic theory. The
probability of dipole creation is found from a canonical ensemble spanned by
foldings of the vortex tubes, with temperature parameter estimated from the
typical energy variation taking place in the reconnection process. Vortex line
reshuffling by viscous diffusion is described in terms of directional
transitions of the dipoles. The model is used to fit with reasonable accuracy
experimental data established long ago on the symmetric collision of vortex
rings. We also study along similar lines the asymmetric case, related to the
reconnection of non-parallel vortex tubes.Comment: 8 pages, 3 postscript figure
Emergence of a filamentary structure in the fireball from GRB spectra
It is shown that the concept of a fireball with a definite filamentary
structure naturally emerges from the analysis of the spectra of Gamma-Ray
Bursts (GRBs). These results, made possible by the recently obtained analytic
expressions of the equitemporal surfaces in the GRB afterglow, depend crucially
on the single parameter R describing the effective area of the fireball
emitting the X- and gamma ray radiation. The X- and gamma ray components of the
afterglow radiation are shown to have a thermal spectrum in the co-moving frame
of the fireball and originate from a stable shock front described
self-consistently by the Rankine-Hugoniot equations. Precise predictions are
presented on a correlations between spectral changes and intensity variations
in the prompt radiation verifiable, e.g., by the Swift and future missions. The
highly variable optical and radio emission depends instead on the parameters of
the surrounding medium. The GRB 991216 is used as a prototype for this model.Comment: 9 pages, 3 figures, to appear on International Journal of Modern
Physics
Developed turbulence: From full simulations to full mode reductions
Developed Navier-Stokes turbulence is simulated with varying wavevector mode
reductions. The flatness and the skewness of the velocity derivative depend on
the degree of mode reduction. They show a crossover towards the value of the
full numerical simulation when the viscous subrange starts to be resolved. The
intermittency corrections of the scaling exponents of the pth order velocity
structure functions seem to depend mainly on the proper resolution of the
inertial subrange. Universal scaling properties (i.e., independent of the
degree of mode reduction) are found for the relative scaling exponents rho
which were recently defined by Benzi et al.Comment: 4 pages, 5 eps-figures, replaces version from August 5th, 199
Towards a comprehensive view of dust events from multiple satellite and ground measurements: exemplified by the May 2017 East Asian dust storm
One or several aspects of the
source, distribution, transport, and optical properties of airborne dust have
been characterized using different types of satellite and ground
measurements, each with unique advantages. In this study, a dust event that occurred over the
East Asia area in May 2017 was exemplified to demonstrate how all the above-mentioned aspects of a dust event can be pictured by combining the advantages
of different satellite and ground measurements. The data used included the
Himawari-8 satellite Advanced Himawari Imager (AHI) true-colour images, the
Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)
Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aerosol vertical
profiles, the Aura satellite Ozone Monitoring Instrument (OMI) aerosol index
images, and the ground-based Aerosol Robotic Network (AERONET) aerosol
properties and the ground station particulate matter (PM) measurements. From
the multi-satellite/sensor (AHI, CALIOP, and OMI) time series observations,
the dust storm was found to originate from the Gobi Desert on the morning of
3 May 2017 and transport north-eastward to the Bering Sea, eastward to the
Korean Peninsula and Japan, and southward to south-central China. The air
quality in China deteriorated drastically: the PM10 (PM < 10 ”m in aerodynamic diameter) concentrations measured at some air quality
stations located in northern China reached 4333 ”g mâ3. At the
AOE_Baotou, Beijing, Xuzhou-CUMT, and Ussuriysk AERONET sites,
the maximum aerosol optical depth values reached 2.96, 2.13, 2.87, and 0.65
and the extinction Ă
ngström exponent dropped to 0.023, 0.068, 0.03,
and 0.097, respectively. The dust storm also induced unusual aerosol spectral
single-scattering albedo and volume size distribution.</p
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