Dependence of the MHD shock thickness on the finite electrical conductivity

The results of MHD plane shock waves with infinite electrical conductivity are generalized for a plasma with a finite conductivity. We derive the adiabatic curves that describe the evolution of the shocked gas as well as the change in the entropy density. For a parallel shock (i.e., in which the magnetic field is parallel to the normal to the shock front) we find an expression for the shock thickness which is a function of the ambient magnetic field and of the finite electrical conductivity of the plasma. We give numerical estimates of the physical parameters for which the shock thickness is of the order of, or greater than, the mean free path of the plasma particles in a strongly magnetized plasma.Comment: 8 pages, uses standard revtex, to appear in Journal of Plasma Physic

Cosmological Magnetic Fields from Gauge-Mediated Supersymmetry-Breaking Models

We study the generation of primordial magnetic fields, coherent over cosmologically interesting scales, by gravitational creation of charged scalar particles during the reheating period. We show that magnetic fields consistent with those detected by observation may obtained if the particle mean life \tau_s is in the range 10^{-14} sec \leq \tau_s \leq 10{-7} sec. We apply this mechanism to minimal gauge mediated supersymmetry-breaking models, in the case in which the lightest stau \tilde\tau_1 is the next-to-lightest supersymmetric particle. We show that, for a large range of phenomenologically acceptable values of the supersymmetry-breaking scale \sqrt{F}, the generated primordial magnetic field can be strong enough to seed the galactic dynamo.Comment: 12 pages, Latex. Final version accepted for publication in Phys. Lett.

On the mean field dynamo with Hall effect

We study in the present paper how Hall effect modifies the quenching process of the electromotive force (e.m.f.) in Mean Field Dynamo (MFD) theories. We write down the evolution equations for the e.m.f. and for the large and small scale magnetic helicity, treat Hall effect as a perturbation and integrate the resulting equations assuming boundary conditions such that the total divergencies vanish. For force-free large scale magnetic fields, Hall effect acts by coupling the small scale velocity and magnetic fields. For the range of parameters considered, the overall effect is a stronger quenching of the e.m.f. than in standard MHD and a damping of the inverse cascade of magnetic helicity. In astrophysical environments characterized by the parameters considered here, Hall effect would produce an earlier quenching of the e.m.f. and consequently a weaker large scale magnetic field.Comment: 8 pages, 4 figures. Accepted by A&

On the thickness of a mildly relativistic collisional shock wave

We consider an imperfect relativistic fluid which develops a shock wave and discuss its structure and thickness, taking into account the effects of viscosity and heat conduction in the form of sound absorption. The junction conditions and the non linear equations describing the evolution of the shock are derived with the corresponding Newtonian limit discussed in detail. As happens in the non relativistic regime, the thickness is inversely proportional to the discontinuity in the pressure, but new terms of purely relativistic origin are present. Particularizing for a polytropic gas, it is found that the pure viscous relativistic shock is thicker than its nonrelativistic counterpart, while the opposite holds for pure heat conduction.Comment: 11 pages, no figures, title changed, improved introduction and discussion. New author adde

Ohm's Law for Plasma in General Relativity and Cowling's Theorem

The general-relativistic Ohm's law for a two-component plasma which includes the gravitomagnetic force terms even in the case of quasi-neutrality has been derived. The equations that describe the electromagnetic processes in a plasma surrounding a neutron star are obtained by using the general relativistic form of Maxwell equations in a geometry of slow rotating gravitational object. In addition to the general-relativistic effect first discussed by Khanna \& Camenzind (1996) we predict a mechanism of the generation of azimuthal current under the general relativistic effect of dragging of inertial frames on radial current in a plasma around neutron star. The azimuthal current being proportional to the angular velocity $\omega$ of the dragging of inertial frames can give valuable contribution on the evolution of the stellar magnetic field if $\omega$ exceeds $2.7\times 10^{17} (n/\sigma) \textrm{s}^{-1}$ ($n$ is the number density of the charged particles, $\sigma$ is the conductivity of plasma). Thus in general relativity a rotating neutron star, embedded in plasma, can in principle generate axial-symmetric magnetic fields even in axisymmetry. However, classical Cowling's antidynamo theorem, according to which a stationary axial-symmetric magnetic field can not be sustained against ohmic diffusion, has to be hold in the general-relativistic case for the typical plasma being responsible for the rotating neutron star.Comment: Accepted for publication in Astrophysics & Space Scienc

Self consistent estimates of magnetic fields from reheating

We investigate the generation of primordial magnetic fields from stochastic currents created by the cosmological transition from inflation to reheating. We consider N charged scalar fields coupled to the electromagnetic field in a curved background and derive self-consistent equations for the evolution of the two point functions of the fields, which in the large N limit give a decoupled set for the scalar and the electromagnetic functions. The main contribution to the electric current comes from the infrared portion of the spectrum of created particles, and in this limit the damping of the magnetic field is not due to normal conductivity but to London currents in the scalar field. For a given set of the physical parameters of the problem, we solved this equation numerically and found that, due to the fact that the London currents are oscillating, the field actually grows exponentially during the time interval in which our large-N limit equations are valid. Although for the chosen parameters the induced field is weak, the present uncertainties on their actual values leave open the possibility for higher intensities.Comment: 25 pages, three figures. One figure added, discussion on non adiabatic induction of the magnetic field improved. References added. Final version accepted for publication in Phys. Rev.

Relationship between environmental heterogeneity and birds distribution patterns and richness in two landscapes units of the Paraná River Delta, Argentine

Los factores implicados en determinar la diversidad y la abundancia en las comunidades de aves son variados. Entre ellos, la estructura del hábitat, referida tanto a la fisonomía de la vegetación como a su variación espacial, ha sido reconocida como un factor de gran importancia en la determinación de la distribución y diversidad de aves por influir tanto en la selección de hábitat como en la disponibilidad de recursos críticos. En este contexto, el objetivo general de este estudio fue analizar la influencia de la heterogeneidad ambiental como condicionante de la diversidad de aves en dos unidades de paisaje de la región del Delta del Río Paraná. Se realizaron censos de aves a partir de los cuales se calcularon parámetros comunitarios. Se estimaron, además, variables relacionadas con la estructura de la vegetación. Se correlacionaron distintas medidas de heterogeneidad de la vegetación con la riqueza de especies de aves. Los resultados obtenidos muestran un patrón diferencial del ensamble de aves de cada unidad y esto se corresponde con lo observado en la vegetación a nivel fisonómico y de composición de especies. Las variables ambientales permitieron diferenciar los ensambles presentes en los distintos ambientes de cada unidad de paisaje.Different factors are involved in determining the diversity and abundance of bird communities. Habitat structure including both the vegetation physiognomy and its spatial variation has been recognized as a major factor in determining distribution and diversity of birds because it influences habitat selection processes and the availability of critical resources. In this context, the objective of this study was to analyze how environmental heterogeneity influences bird diversity in two landscape units located in the Paraná River Delta region. Bird censuses were conducted in order to estimate different community parameters. In addition, in the same census stations we estimated some variables of vegetation structure. Habitat heterogeneity variables were correlated with birds diversity. Differences in environmental variables between both studied landscapes were associated to different birds assemblages. The results show the importance of the spatial variation of the vegetation physiognomy in structuring bird communities in each of the studied areas. In each landscape unit, bird assemblages were differentiated with the considered environmental variables.Fil: De Stefano, Karina. Universidad de Buenos Aires. Facultad de Cs.exactas y Naturales. Departamento de Ecología, Genética y Evolución. Laboratorio de Ecología Regional; ArgentinaFil: Merler, Jorge A.. Universidad de Buenos Aires. Facultad de Cs.exactas y Naturales. Departamento de Ecología, Genética y Evolución. Laboratorio de Ecología Regional; ArgentinaFil: Magnano, Andrea Laura. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Martín. Instituto de Investigación e Ingeniería Ambiental. Laboratorio de Biodiversidad, Limnología y Conservación; ArgentinaFil: Nanni, Analía Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Martín. Instituto de Investigación e Ingeniería Ambiental. Laboratorio de Biodiversidad, Limnología y Conservación; ArgentinaFil: Kandus, Patricia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Martín. Instituto de Investigación e Ingeniería Ambiental. Laboratorio de Ecología, Teledetección y Ecoinformática; ArgentinaFil: Quintana, Ruben Dario. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Cs.exactas y Naturales. Departamento de Ecología, Genética y Evolución. Laboratorio de Ecología Regional; Argentina. Universidad Nacional de San Martín. Instituto de Investigación e Ingeniería Ambiental. Laboratorio de Biodiversidad, Limnología y Conservación; Argentin

Seminal magnetic fields from Inflato-electromagnetic Inflation

We extend some previous attempts to explain the origin and evolution of primordial magnetic fields during inflation induced from a 5D vacuum. We show that the usual quantum fluctuations of a generalized 5D electromagnetic field cannot provide us with the desired magnetic seeds. We show that special fields without propagation on the extra non-compact dimension are needed to arrive to appreciable magnetic strengths. We also identify a new magnetic tensor field $B_{ij}$ in this kind of extra dimensional theories. Our results are in very good agreement with observational requirements, in particular from TeV Blazars and CMB radiation limits we obtain that primordial cosmological magnetic fields should be close scale invariance.Comment: Improved version. arXiv admin note: text overlap with arXiv:1007.3891 by other author