What are we learning from the relative orientation between density structures and the magnetic field in molecular clouds?

We investigate the conditions of ideal magnetohydrodynamic (MHD) turbulence responsible for the relative orientation between density structures, characterized by their gradient, $\vec{\nabla}\rho$, and the magnetic field, $\vec{B}$, in molecular clouds (MCs). For that purpose, we construct an expression for the time evolution of the angle, $\phi$, between $\vec{\nabla}\rho$ and $\vec{B}$ based on the transport equations of MHD turbulence. Using this expression, we find that the configuration where $\vec{\nabla}\rho$ and $\vec{B}$ are mostly parallel, $\cos\phi=1$, and where $\vec{\nabla}\rho$ and $\vec{B}$ are mostly perpendicular, $\cos\phi=0$, constitute attractors, that is, the system tends to evolve towards either of these configurations and they are more represented than others. This fact would explain the predominant alignment or anti-alignment between column density, $N_H$, structures and the projected magnetic field orientation, $\hat{B}_\perp$, reported in observations. Additionally, we find that departures from the $\cos\phi=0$ configurations are related to convergent flows, quantified by the divergence of the velocity field, $\vec{\nabla}\cdot\vec{v}$, in the presence of a relatively strong magnetic field. This would explain the observed change in relative orientation between $N_H$-structures and $\hat{B}_\perp$ towards MCs, from mostly parallel at low $N_H$ to mostly perpendicular at the highest $N_H$, as the result of the gravitational collapse and/or convergence of flows. Finally, we show that the density threshold that marks the observed change in relative orientation towards MCs, from $N_H$ and $\hat{B}_\perp$ being mostly parallel at low $N_H$ to mostly perpendicular at the highest $N_H$, is related to the magnetic field strength and constitutes a crucial piece of information for determining the role of the magnetic field in the dynamics of MCs.Comment: 10 pages, 8 figures. Submitted to A&

Reactive nitrogen in the Spanish agri-food sector: environmental impact on atmosphere, soils, water and resources.

The presence of reactive species of nitrogen (N) in the biosphere might cause environmental impacts at local, regional and global scales. Nowadays the N flows generated by human activities greatly exceed natural processes, thus a necessity exists of identifying and quantifying the current state of environmental N loads. The aim of this work is to quantify the amount of reactive N used in the Spanish agri-food sector, assessing the related potential environmental impacts and potential uses of resources. Data from a previously calculated N flow analysis in the Spanish agricultural and food production system for the 1996-2000 time period were used. Total anthropogenic N inputs to the systems were calculated. Input and output flows were considered in each economic compartment in order to calculate use efficiency (ratio of useful outputs to total inputs), eco-efficiency (ratio of useful outputs to outputs to the environment) and recycling rate (ratio of flow recycled to an earlier life-cycle compartment divided by total outputs). Environmental impacts were assessed by quantifying the N balance between the economic and the environmental subsystems: water, atmosphere, ecosystems soils and other soils. In this case agricultural soils were also considered an environmental compartment, since they are an important intermediate path to the environment. The impact on resources was evaluated considering the net N imports into the system and legume fixing crops, pastures and forages versus feed and fertilizers within the system. Anthropogenic N inputs are relatively high in Spain, which is a net importer of nitrogen, mainly in fertilizers and food and feed commodities. Environmental compartments receive relative high amounts of reactive nitrogen, especially soils. Furthermore, there was a relative low use of domestic resources, with a low proportion of N recycled within the system

Non uniform rotating vortices and periodic orbits for the two-dimensional Euler Equations

This paper concerns the study of some special ordered structures in turbulent flows. In particular, a systematic and relevant methodology is proposed to construct non trivial and non radial rotating vortices with non necessarily uniform densities and with different $m$--fold symmetries, $m\ge 1$. In particular, a complete study is provided for the truncated quadratic density $(A|x|^2+B){\bf{1}}_{\mathbb{D}}(x)$, with $\mathbb{D}$ the unit disc. We exhibit different behaviors with respect to the coefficients $A$ and $B$ describing the rarefaction of bifurcating curves.Comment: 115 pages, 1 figur

The flow field in the slender combustion chambers of solid propellant rockets

We analyse the near inviscid flow field generated in a slender non-axisymmetric cylindrical cavity by the gasification and combustion reactions of a surrounding solid propellant grain. These reactions are confined to a thin layer adjacent to the surface of the solid propellant, so that the flow is non-reacting in most of the cavity, and of the same form as the flow in slender ducts due to fluid injection through lateral porous walls. The non-reacting flow can be described in terms of self-similar solutions of the Navier -Stokes equations that we calculate numerically for star-shaped grain configurations, with Reynolds numbers only moderately large for the flow to remain laminar and steady. The self-similar flows for non-axisymmetric configurations show strong axial vortices that we analyze using the Euler simplified form of the flow equations for large Reynolds numbers, and the general solution of the Burgers equation for strained vortices that describes their viscous cores; a logarithmic singularity could then be encountered at their center line

Detection of irrigation inhomogeneities in an olive grove using the NDRE vegetation index obtained from UAV images

We have developed a simple photogrammetric method to identify heterogeneous areas of irrigated olive groves and vineyard crops using a commercial multispectral camera mounted on an unmanned aerial vehicle (UAV). By comparing NDVI, GNDVI, SAVI, and NDRE vegetation indices, we find that the latter shows irrigation irregularities in an olive grove not discernible with the other indices. This may render the NDRE as particularly useful to identify growth inhomogeneities in crops. Given the fact that few satellite detectors are sensible in the red-edge (RE) band and none with the spatial resolution offered by UAVs, this finding has the potential of turning UAVs into a local farmer’s favourite aid tool.Peer ReviewedPostprint (published version

A soldier at the service of the Crown: Notes on the work of Diego Súarez Corvín

La guerra discontinua entre la Corona española y el Imperio Otomano localizada en el norte de África ha dejado numerosos testimonios literarios desperdigados por bibliotecas y archivos. Son, en su mayoría, obras de difícil clasificación, pues el hecho histórico y el dato inventado no siempre aparecen bien diferenciados. Retomamos esta cuestión, constante en el estudio del género autobiográfico, a propósito de una de estas obras sobre el conflicto: la Historia del Maestre último que fue de Montesa, escrita por el soldado Diego Suárez, el MontañésThe intermittent war between Spain and the Ottoman Empire developed in North Africa has left several testimonies scattered in libraries and archives. These works are difficult to classify since there is no remarkable difference between historical data and fiction. We return to the debate about one of these narrative works: Historia del Maestre último que fue de Montesa, written by Captain Diego Suárez Montañé