Topology and Strings: Topics in N=2N=2

A review on topological strings and the geometry of the space of two dimensional theories. (Lectures given by C. Gomez at the Enrico Fermi Summer School, Varenna, July 1994)Comment: 61 pages, late

A Note on the String Analog of N=2N=2 Super-Symmetric Yang-Mills

A connection between the conifold locus of the type II string on the $W\:P_{11226}^4$ Calabi-Yau manifold and the geometry of the quantum moduli of $N = 2$ $SU(2)$ super Yang-Mills is presented. This relation is obtained from the anomalous behaviour of the $SU(2)$ super Yang-Mills special coordinates under $S$-duality transformation in $Sl(2;Z) / \Gamma_2$.Comment: 7 pages, Late

DMSP F7 observations of a substorm field‐aligned current

In this paper we present observations of a substorm field-aligned current (FAC) system that DMSP F7 traversed just after 0300 UT on April 25, 1985. Ground magnetometer data show that a major substorm was in progress at that time and that DMSP F7 flew through a region of predominantly upward FAC. The DMSP F7 magnetic field data are consistent with this interpretation. The precipitating particle data suggest that there were three distinct large-scale FAC systems. In ascending latitude these were a downward current, an upward current, and a paired upward/downward current system. We identify the first current, which was coincident with the diffuse aurora, as region 2. The next (upward) FAC was coincident with a spatially unstructured region of energetic (∼12 keV) electron precipitation. This was the substorm-associated FAC that made up part of the current wedge. The upward/downward current pair was coincident with a region of highly structured precipitation. We suggest that these currents may have been the duskside region 1 and, poleward of that, the extension of the dawnside region 1. The particle data show that the upward substorm current lay well equatorward of the boundary between open and closed field lines. In fact, using a model field, the equatorward boundary of the substorm FAC maps to the neutral sheet at 6.9 RE. While one should be cautious in stressing results obtained by mapping model field lines, our result is consistent with scenarios for substorms which postulate a disruption and diversion of the near-Earth cross-tail current

Closed time path approach to the Casimir energy in real media

The closed time path formalism is applied, in the framework of open quantum systems, to study the time evolution of the expectation value of the energy-momentum tensor of a scalar field in the presence of real materials. We analyze quantum fluctuations in a fully non-equilibrium scenario, when the scalar field is interacting with the polarization degrees of freedom of matter, described as quantum Brownian particles. A generalized analysis was done for two types of couplings between the field and the material. On the one hand, we considered a bilinear coupling, and on the other hand, a (more realistic) current-type coupling as in the case of the electromagnetic field interacting with matter. We considered the high temperature limit for the field, keeping arbitrary temperatures for each part of the volume elements of the material. We obtained a closed form for the Hadamard propagator, which let us study the dynamical evolution of the expectations values of the energy-momentum tensor components from the initial time. We showed that two contributions always take place in the transient evolution: one of these is associated to the material and the other one is only associated to the field. Transient features were studied and the long-time limit was derived in several cases. We proved that in the steady situation of a field in n + 1 dimensions, the material always contribute unless is non-dissipative. Conversely, the proper field contribution vanishes unless the material is non-dissipative or, moreover, at least for the 1 + 1 case, if there are regions without material. We conclude that any steady quantization scheme in 1 + 1 dimensions must consider both contributions and we argue why these results are physically expected from a dynamical point of view, and also could be valid for higher dimensions based on the expected continuity between the non-dissipative and real material cases.Comment: 28 pages, no figures. Version to appear in Phys. Rev.

Plankton blooms in vortices: The role of biological and hydrodynamic time scales

We study the interplay of hydrodynamic mesoscale structures and the growth of plankton in the wake of an island, and its interaction with a coastal upwelling. Our focus is on a mechanism for the emergence of localized plankton blooms in vortices. Using a coupled system of a kinematic flow mimicking the mesoscale structures behind the island and a simple three component model for the marine ecosystem, we show that the long residence times of nutrients and plankton in the vicinity of the island and the confinement of plankton within vortices are key factors for the appearance of localized plankton bloomsComment: 29 pages, 9 figure

Biological activity in the wake of an island close to a coastal upwelling

Hydrodynamic forcing plays an important role in shaping the dynamics of marine organisms, in particular of plankton. In this work we study the planktonic biological activity in the wake of an island which is close to an upwelling region. Our research is based on numerical analysis of a kinematic flow mimicking the hydrodynamics in the wake, coupled to a three-component plankton model. Depending on model parameters different phenomena are described: a) The lack of transport of nutrients and plankton across the wake, so that the influence of upwelling on primary production on the other side of the wake is blocked. b) For sufficiently high vorticity, the role of the wake in facilitating this transport and leading to an enhancement of primary production. Finally c) we show that under certain conditions the interplay between wake structures and biological growth leads to plankton blooms inside mesoscale hydrodynamic vortices that act as incubators of primary production.Comment: 42 pages, 9 figure

Probabilistic Inference from Arbitrary Uncertainty using Mixtures of Factorized Generalized Gaussians

This paper presents a general and efficient framework for probabilistic inference and learning from arbitrary uncertain information. It exploits the calculation properties of finite mixture models, conjugate families and factorization. Both the joint probability density of the variables and the likelihood function of the (objective or subjective) observation are approximated by a special mixture model, in such a way that any desired conditional distribution can be directly obtained without numerical integration. We have developed an extended version of the expectation maximization (EM) algorithm to estimate the parameters of mixture models from uncertain training examples (indirect observations). As a consequence, any piece of exact or uncertain information about both input and output values is consistently handled in the inference and learning stages. This ability, extremely useful in certain situations, is not found in most alternative methods. The proposed framework is formally justified from standard probabilistic principles and illustrative examples are provided in the fields of nonparametric pattern classification, nonlinear regression and pattern completion. Finally, experiments on a real application and comparative results over standard databases provide empirical evidence of the utility of the method in a wide range of applications

High Resolution Imaging of the Magnetic Field in the central parsec of the Galaxy

We discuss a high resolution (FWHM~ 0.45 arcsec) image of the emissive polarization from warm dust in the minispiral in the Galactic Centre and discuss the implications for the magnetic field in the dusty filaments. The image was obtained at a wavelength of 12.5 microns with the CanariCam multimode mid-infrared imager on the Gran Telescopio Canarias. It confirms the results obtained from previous observations but also reveals new details of the polarization structures. In particular, we identify regions of coherent magnetic field emission at position angles of ~45 deg to the predominantly north--south run of field lines in the Northern Arm which may be related to orbital motions inclined to the general flow of the Northern Arm. The luminous stars that have been identified as bow-shock sources in the Northern Arm do not disrupt or dilute the field but are linked by a coherent field structure, implying that the winds from these objects may push and compress the field but do not overwhelm it. The magnetic field in the the low surface brightness regions in the East-West Bar to the south of SgrA* lies along the Bar, but the brighter regions generally have different polarization position angles, suggesting that they are distinct structures. In the region of the Northern Arm sampled here, there is only a weak correlation between the intensity of the emission and the degree of polarization. This is consistent with saturated grain alignment where the degree of polarization depends on geometric effects, including the angle of inclination of the field to the line of sight and superposition of filaments with different field directions, rather than the alignment efficiency.Comment: 9 pages, 3 figures, Proceedings of Cosmic Dust X, held in Mitaka, Japan in August 201