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 $E^u(k)/E^b(k)$, i.e., fluid to magnetic energy ratios when the normalized cross helicity is zero. We find that $K$ increases from 1.47 to 4.12 as we go from fully fluid case $(E^b=0)$ to a situation when $E^u/E^b=0.5$, then it decreases to 3.55 in a fully magnetic limit $(E^u=0)$. When $E^u/E^b=1$, we find that $K=3.43$.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 $D^0\to(\bar K^0, \bar K^{*0})\eta,\eta'$ and $(D^+,D_s^+)\to(\pi^+,\rho^+)\eta,\eta'$ are studied in the generalized factorization approach. Form factors for $(D,D_s^+)\to(\eta,\eta')$ 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 $D^0\to\bar K^0\eta', D^+\to\pi^+\eta$ and $D_s^+\to\rho^+\eta'$; the calculated decay rates for the first two decay modes are too small by an order of magnitude. We show that the weak decays $D^0\to K^-\pi^+$ and $D^+\to K^+\bar K^0$ followed by resonance-induced final-state interactions (FSI), which are amenable technically, are able to enhance the branching ratios of $D^0\to\bar K^0\eta'$ and $D^+\to\pi^+\eta$ dramatically without affecting the agreement between theory and experiment for $D^0\to\bar K^0\eta$ and $D^+\to\pi^+\eta'$. We argue that it is difficult to understand the observed large decay rates of $D_s^+\to \rho^+\eta'$ and $\rho^+\eta$ 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 $D_s^+\to\rho^+ \eta'$ and $D_s^+\to\eta' e^+\nu$ 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