Enhancing the spatiotemporal features of polar mesosphere summer echoes using coherent MIMO and radar imaging at MAARSY

Polar mesospheric summer echoes (PMSEs) are very strong radar echoes caused by the presence of ice particles, turbulence, and free electrons in the mesosphere over polar regions. For more than three decades, PMSEs have been used as natural tracers of the complicated atmospheric dynamics of this region. Neutral winds and turbulence parameters have been obtained assuming PMSE horizontal homogeneity on scales of tens of kilometers. Recent radar imaging studies have shown that PMSEs are not homogeneous on these scales and instead they are composed of kilometer-scale structures. In this paper, we present a technique that allows PMSE observations with unprecedented angular resolution (∼0.6). The technique combines the concept of coherent MIMO (Multiple Input Multiple Output) and two high-resolution imaging techniques, i.e., Capon and maximum entropy (MaxEnt). The resulting resolution is evaluated by imaging specular meteor echoes. The gain in angular resolution compared to previous approaches using SIMO (Single Input Multiple Output) and Capon is at least a factor of 2; i.e., at 85 km, we obtain a horizontal resolution of ∼900 m. The advantage of the new technique is evaluated with two events of 3-D PMSE structures showing: (1) horizontal wavelengths of 8-10 km and periods of 4-7 min, drifting with the background wind, and (2) horizontal wavelengths of 12-16 km and periods of 15-20 min, not drifting with the background wind. Besides the advantages of the implemented technique, we discuss its current challenges, like the use of reduced power aperture and processing time, as well as the future opportunities for improving the understanding of the complex small-scale atmospheric dynamics behind PMSEs. © 2019 Author(s)

Geomorphological control of fine sedimentation on the northern Portuguese shelf

Six cores sampled at two mud fields located on the northern Portuguese continental shelf have been studied in order to evaluate the control processes of fine sedimentation. The southern mud patch, offshore from the Douro River, is limited on the western side by some Cretaceous and Paleocene outcrops that constitute reliefs of 5-30 m in width; on the contrary, the northern mud patch, offshore from the Minho River, extends along a flat region. Granulometric distribution shows that the sediments from the Minho mud patch are coarser than those from the Douro mud patch, where the grain size diminishes towards the west. Carbonate content is also higher in the Minho mud field. The sediment organisation in fining-up sequences in the Minho mud patch and in the eastern part of the Douro mud patch indicates sediment remobilisation by storms. The Douro mud patch has higher sedimentation rates than the Minho mud patch. The highest values are probably related to the existence of reliefs, which may offer hydrodynamic protection or function as a fine sediment barrier. The elemental distribution for chemical elements was determined in different fractions of these cores. Geochemical patterns are generally quite similar for both mud fields, indicating a common continental origin for the sediments. The major differences in the chemical composition are mainly controlled by the grain-size distribution, either reflecting the presence of coarse quartz particles or a local enrichment in biogenic material.En este trabajo se han estudiado seis testigos de la zona norte de la plataforma continental portuguesa con el objetivo de evaluar los procesos que controlan la sedimentación de materiales finos en esta región. Los fangos acumulados más al sur, frente a la desembocadura del río Duero, están limitados al oeste por relieves cretácicos y paleocenos de 5 a 30 m de altura. Los fangos septentrionales, frente a la desembocadura del río Miño, se extienden sobre una región plana. La distribución granulométrica muestra que estos últimos son más gruesos que los depositados frente al Duero, en los que el tamaño de grano disminuye hacia el oeste. El contenido en carbonato es más elevado en la zona del Miño. Las secuencias granodecrecientes encontradas en los fangos del Miño y en los del sector oriental del Duero indican retrabajamiento del sedimento por tormentas. La tasa de sedimentación es mayor en los fangos situados frente al Duero, donde los valores más altos están relacionados probablemente con la existencia de relieves, los cuales ofrecerían una protección hidrodinámica o actuarían como una barrera para los sedimentos. La distribución de la concentración de diversos elementos químicos a lo largo de los testigos fue determinada para diferentes fracciones. El patrón geoquímico es bastante similar en ambas zonas y pone de manifiesto una procedencia continental de los sedimentos. Las principales diferencias en la composición química están fundamentalmente controladas por la distribución granulométrica, bien por la presencia de granos gruesos de cuarzo, bien por un enriquecimiento local en material biogénico.Instituto Español de Oceanografí

Exact Calculation of the Vortex-Antivortex Interaction Energy in the Anisotropic 3D XY-model

We have developed an exact method to calculate the vortex-antivortex interaction energy in the anisotropic 3D-XY model. For this calculation, dual transformation which is already known for the 2D XY-model was extended. We found an explicit form of this interaction energy as a function of the anisotropic ratio and the separation $r$ between the vortex and antivortex located on the same layer. The form of interaction energy is $\ln r$ at the small $r$ limi t but is proportional to $r$ at the opposite limit. This form of interaction energ y is consistent with the upper bound calculation using the variational method by Cataudella and Minnhagen.Comment: REVTeX 12 pages, In print for publication in Phys. Rev.

Aspect-ratio dependence of the spin stiffness of a two-dimensional XY model

We calculate the superfluid stiffness of 2D lattice hard-core bosons at half-filling (equivalent to the S=1/2 XY-model) using the squared winding number quantum Monte Carlo estimator. For L_x x L_y lattices with aspect ratio L_x/L_y=R, and L_x,L_y -> infinity, we confirm the recent prediction [N. Prokof'ev and B.V. Svistunov, Phys. Rev. B 61, 11282 (1999)] that the finite-temperature stiffness parameters \rho^W_x and \rho^W_y determined from the winding number differ from each other and from the true superfluid density \rho_s. Formally, \rho^W_y -> \rho_s in the limit in which L_x -> infinity first and then L_y -> infinity. In practice we find that \rho^W_y converges exponentially to \rho_s for R>1. We also confirm that for 3D systems, \rho^W_x = \rho^W_y = \rho^W_z = \rho_s for any R. In addition, we determine the Kosterlitz-Thouless transition temperature to be T_KT/J=0.34303(8) for the 2D model.Comment: 7 pages, 8 figures, 1 table. Minor changes to published versio

Boundary Effects on Dynamic Behavior of Josephson-Junction Arrays

The boundary effects on the current-voltage characteristics in two-dimensional arrays of resistively shunted Josephson junctions are examined. In particular, we consider both the conventional boundary conditions (CBC) and the fluctuating twist boundary conditions (FTBC), and make comparison of the obtained results. It is observed that the CBC, which have been widely adopted in existing simulations, may give a problem in scaling, arising from rather large boundary effects; the FTBC in general turn out to be effective in reducing the finite-size effects, yielding results with good scaling behavior. To resolve the discrepancy between the two boundary conditions, we propose that the proper scaling in the CBC should be performed with the boundary data discarded: This is shown to give results which indeed scale well and are the same as those from the FTBC.Comment: RevTex, Final version to appear in Phys. Rev.

Localization criteria for Anderson models on locally finite graphs

We prove spectral and dynamical localization for Anderson models on locally finite graphs using the fractional moment method. Our theorems extend earlier results on localization for the Anderson model on \ZZ^d. We establish geometric assumptions for the underlying graph such that localization can be proven in the case of sufficiently large disorder

Effective T-odd P-even hadronic interactions from quark models

Tests of time reversal symmetry at low and medium energies may be analyzed in the framework of effective hadronic interactions. Here, we consider the quark structure of hadrons to make a connection to the more fundamental degrees of freedom. It turns out that for P-even T-odd interactions hadronic matrix elements evaluated in terms of quark models give rise to factors of 2 to 5. Also, it is possible to relate the strength of the anomalous part of the effective rho-type T-odd P-even tensor coupling to quark structure effects.Comment: 6 pages, 1 figure, RevTe

Scaling determination of the nonlinear I-V characteristics for 2D superconducting networks

It is shown from computer simulations that the current-voltage ($I$-$V$) characteristics for the two-dimensional XY model with resistively-shunted Josephson junction dynamics and Monte Carlo dynamics obeys a finite-size scaling form from which the nonlinear $I$-$V$ exponent $a$ can be determined to good precision. This determination supports the conclusion $a=z+1$, where $z$ is the dynamic critical exponent. The results are discussed in the light of the contrary conclusion reached by Tang and Chen [Phys. Rev. B {\bf 67}, 024508 (2003)] and the possibility of a breakdown of scaling suggested by Bormann [Phys. Rev. Lett. {\bf 78}, 4324 (1997)].Comment: 6 pages, to appear in PR

Lattice effects on the current-voltage characteristics of superconducting arrays

The lattice effects on the current-voltage characteristics of two-dimensional arrays of resistively shunted Josephson junctions are investigated. The lattice potential energies due to the discrete lattice structure are calculated for several geometries and directions of current injection. We compare the energy barrier for vortex-pair unbinding with the lattice pinning potential, which shows that lattice effects are negligible in the low-current limit as well as in the high-current limit. At intermediate currents, on the other hand, the lattice potential becomes comparable to the barrier height and the lattice effects may be observed in the current-voltage characteristics.Comment: 5 pages including 5 figures in two columns, to appear in Phys. Rev.