10,174 research outputs found
Critical Cooperation Range to Improve Spatial Network Robustness
A robust worldwide air-transportation network (WAN) is one that minimizes the
number of stranded passengers under a sequence of airport closures. Building on
top of this realistic example, here we address how spatial network robustness
can profit from cooperation between local actors. We swap a series of links
within a certain distance, a cooperation range, while following typical
constraints of spatially embedded networks. We find that the network robustness
is only improved above a critical cooperation range. Such improvement can be
described in the framework of a continuum transition, where the critical
exponents depend on the spatial correlation of connected nodes. For the WAN we
show that, except for Australia, all continental networks fall into the same
universality class. Practical implications of this result are also discussed
Computation of Kolmogorov's Constant in Magnetohydrodynamic Turbulence
In this paper we calculate Kolmogorov's constant for magnetohydrodynamic
turbulence to one loop order in perturbation theory using the direct
interaction approximation technique of Kraichnan. We have computed the
constants for various , i.e., fluid to magnetic energy ratios
when the normalized cross helicity is zero. We find that increases from
1.47 to 4.12 as we go from fully fluid case to a situation when , then it decreases to 3.55 in a fully magnetic limit .
When , we find that .Comment: Latex, 10 pages, no figures, To appear in Euro. Phys. Lett., 199
Algebraic Aspects of Abelian Sandpile Models
The abelian sandpile models feature a finite abelian group G generated by the
operators corresponding to particle addition at various sites. We study the
canonical decomposition of G as a product of cyclic groups G = Z_{d_1} X
Z_{d_2} X Z_{d_3}...X Z_{d_g}, where g is the least number of generators of G,
and d_i is a multiple of d_{i+1}. The structure of G is determined in terms of
toppling matrix. We construct scalar functions, linear in height variables of
the pile, that are invariant toppling at any site. These invariants provide
convenient coordinates to label the recurrent configurations of the sandpile.
For an L X L square lattice, we show that g = L. In this case, we observe that
the system has nontrivial symmetries coming from the action of the cyclotomic
Galois group of the (2L+2)th roots of unity which operates on the set of
eigenvalues of the toppling matrix. These eigenvalues are algebraic integers,
whose product is the order |G|. With the help of this Galois group, we obtain
an explicit factorizaration of |G|. We also use it to define other simpler,
though under-complete, sets of toppling invariants.Comment: 39 pages, TIFR/TH/94-3
Mapping of Large Scale 158 micron [CII] Line Emission: Orion A
We present the first results of an observational programme undertaken to map
the fine structure line emission of singly ionized carbon ([CII] 157.7409
micron) over extended regions using a Fabry Perot spectrometer newly installed
at the focal plane of a 100cm balloon-borne far-infrared telescope. This new
combination of instruments has a velocity resolution of ~200 km/s and an
angular resolution of 1.5'. During the first flight, an area of 30'x15' in
Orion A was mapped. The observed [CII] intensity distribution has been compared
with the velocity-integrated intensity distributions of 13CO(1-0), CI(1-0) and
CO(3-2) from the literature. The observed line intensities and ratios have been
analyzed using the PDR models by Kaufman et al. 1999 to derive the incident UV
flux and volume density at a few selected positions.Comment: To appear in Astronomy & Astrophysic
High-resolution imaging and near-infrared spectroscopy of penumbral decay
Combining high-resolution spectropolarimetric and imaging data is key to
understanding the decay process of sunspots as it allows us scrutinizing the
velocity and magnetic fields of sunspots and their surroundings. Active region
NOAA 12597 was observed on 24/09/2016 with the 1.5-m GREGOR solar telescope
using high-spatial resolution imaging as well as imaging spectroscopy and
near-infrared (NIR) spectropolarimetry. Horizontal proper motions were
estimated with LCT, whereas LOS velocities were computed with spectral line
fitting methods. The magnetic field properties were inferred with the SIR code
for the Si I and Ca I NIR lines. At the time of the GREGOR observations, the
leading sunspot had two light-bridges indicating the onset of its decay. One of
the light-bridges disappeared, and an elongated, dark umbral core at its edge
appeared in a decaying penumbral sector facing the newly emerging flux. The
flow and magnetic field properties of this penumbral sector exhibited weak
Evershed flow, moat flow, and horizontal magnetic field. The penumbral gap
adjacent to the elongated umbral core and the penumbra in that penumbral sector
displayed LOS velocities similar to granulation. The separating polarities of a
new flux system interacted with the leading and central part of the already
established active region. As a consequence, the leading spot rotated 55-degree
in clockwise direction over 12 hours. In the high-resolution observations of a
decaying sunspot, the penumbral filaments facing flux emergence site contained
a darkened area resembling an umbral core filled with umbral dots. This umbral
core had velocity and magnetic field properties similar to the sunspot umbra.
This implies that the horizontal magnetic fields in the decaying penumbra
became vertical as observed in flare-induced rapid penumbral decay, but on a
very different time-scale.Comment: 14 pages, 11 figures, Accepted to be published in Astronomy and
Astrophysic
Wide-Area Mapping of 155 Micron Continuum Emission from the Orion Molecular Cloud Complex
We present the results of a wide-area mapping of the far-infrared continuum
emission toward the Orion complex by using a Japanese balloon-borne telescope.
The 155-um continuum emission was detected over a region of 1.5 deg^2 around
the KL nebula with 3' resolution similar to that of the IRAS 100-um map.
Assuming a single-temperature model of the thermal equilibrium dust, maps of
the temperature and the optical depth were derived from the 155 um intensity
and the IRAS 100 um intensity. The derived dust temperature is 5 - 15 K lower
and the derived dust optical thickness were derived from the 155-um intensity
and the IRAS 100-um intensity. The derived dust temperature is 5 - 15 K lower
and the derived dust optical depth is 5 - 300 times larger than those derived
from the IRAS 60 and 100-um intensities due to the significant contribution of
the statistically heated very small grains to the IRAS 60-um intensity. The
optical-thickness distribution shows a filamentary dust ridge that has a 1.5
degrees extent in the north - south direction and well resembles the
Integral-Shaped Filament (ISF) molecular gas distribution. The gas-to-dust
ratio derived from the CO molecular gas distribution along the ISF is in the
range 30 - 200, which may be interpreted as being an effect of CO depletion due
to the photodissociation and/or the freezing on dust grains.Comment: 23 pages, 7 figures, 1 table, to appear in PASJ, Vol. 56, No.
Exclusive Hadronic D Decays to eta' and eta
Hadronic decay modes and
are studied in the generalized
factorization approach. Form factors for transitions
are carefully evaluated by taking into account the wave function normalization
of the eta and eta'. The predicted branching ratios are generally in agreement
with experiment except for and
; the calculated decay rates for the first two decay modes
are too small by an order of magnitude. We show that the weak decays and followed by resonance-induced final-state
interactions (FSI), which are amenable technically, are able to enhance the
branching ratios of and dramatically
without affecting the agreement between theory and experiment for and . We argue that it is difficult to understand
the observed large decay rates of and
simultaneously; FSI, W-annihilation and the production of excess eta' from
gluons are not helpful in this regard. The large discrepancy between the
factorization hypothesis and experiment for the ratio of
and remains as an enigma.Comment: 15 pages, 1 figure, to appear in Phys. Rev. D. Form factors for D to
eta and eta' transitions are slightly change
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