Nonlinear Instability of kink oscillations due to shear motions

First results from a high-resolution three-dimensional nonlinear numerical study of the kink oscillation are presented. We show in detail the development of a shear instability in an untwisted line-tied magnetic flux tube. The instability produces significant deformations of the tube boundary. An extended transition layer may naturally evolve as a result of the shear instability at a sharp transition between the flux tube and the external medium. We also discuss the possible effects of the instability on the process of resonant absorption when an inhomogeneous layer is included in the model. One of the implications of these results is that the azimuthal component of the magnetic field of a stable flux tube in the solar corona, needed to prevent the shear instability, is probably constrained to be in a very specific range

Three-Dimensional Propagation of Magnetohydrodynamic Waves in Solar Coronal Arcades

We numerically investigate the excitation and temporal evolution of oscillations in a two-dimensional coronal arcade by including the three-dimensional propagation of perturbations. The time evolution of impulsively generated perturbations is studied by solving the linear, ideal magnetohydrodynamic (MHD) equations in the zero-beta approximation. As we neglect gas pressure the slow mode is absent and therefore only coupled MHD fast and Alfven modes remain. Two types of numerical experiments are performed. First, the resonant wave energy transfer between a fast normal mode of the system and local Alfven waves is analyzed. It is seen how, because of resonant coupling, the fast wave with global character transfers its energy to Alfvenic oscillations localized around a particular magnetic surface within the arcade, thus producing the damping of the initial fast MHD mode. Second, the time evolution of a localized impulsive excitation, trying to mimic a nearby coronal disturbance, is considered. In this case, the generated fast wavefront leaves its energy on several magnetic surfaces within the arcade. The system is therefore able to trap energy in the form of Alfvenic oscillations, even in the absence of a density enhancement such as that of a coronal loop. These local oscillations are subsequently phase-mixed to smaller spatial scales. The amount of wave energy trapped by the system via wave energy conversion strongly depends on the wavelength of perturbations in the perpendicular direction, but is almost independent from the ratio of the magnetic to density scale heights.Comment: 27 pages, 11 figure

The effect of longitudinal flow on resonantly damped kink oscillations

The most promising mechanism acting towards damping the kink oscillations of coronal loops is resonant absorption. In this context most of previous studies neglected the effect of the obvious equilibrium flow along magnetic field lines. The flows are in general sub-Alfv\'enic and hence comparatively slow. Here we investigate the effect of an equilibrium flow on the resonant absorption of linear kink MHD waves in a cylindrical magnetic flux tube with the aim of determining the changes in the frequency of the forward and backward propagating waves and in the modification of the damping times due to the flow. A loop model with both the density and the longitudinal flow changing in the radial direction is considered. We use the thin tube thin boundary (TTTB) approximation in order to calculate the damping rates. The full resistive eigenvalue problem is also solved without assuming the TTTB approximation. Using the small ratio of flow and Alfv\'en speeds we derive simple analytical expressions to the damping rate. The analytical expressions are in good agreement with the resistive eigenmode calculations. Under typical coronal conditions the effect of the flow on the damped kink oscillations is small when the characteristic scale of the density layer is similar or smaller than the characteristic width of the velocity layer. However, in the opposite situation the damping rates can be significantly altered, specially for the backward propagating wave which is undamped while the forward wave is overdamped

La contradicción entre identidad vivida e identificación jurídico-política

Este artículo quiere llamar la atención de los antropólogos españoles sobre una contradicción implícita, por no decir sepultada, en los discursos sobre la identidad. Se trata de la contradicción entre la identidad como expresión de lo vivido y la identidad como quimera, y a la vez, estigma clasificatorio. Si bien, varias opiniones tratan de hacer pasar lo segundo por lo primero (el nacionalismo de clase, el racismo contra lenguas maternas, el poder político como expresión superior del sentimiento de vida, la identidad como certificado para un registro...), considero que el antropólogo debe poner de manifiesto que la identidad es, antes que nada, algo que se construye con la isma vida. Y, precisamente, la identidad más rica en siginificados para la comunicación social es la que expresa la vida más amenazada por las identificaciones económicas, jurídicas y políticas. En esta identidad se reconoce, adquiere sentido y valor, la cultura

Geometrical properties of the interaction between oblique incoming coronal waves and coronal holes

Observations of coronal waves (CWs) interacting with coronal holes (CHs) show the formation of typical wave-like features, such as reflected, refracted and transmitted waves (collectively, secondary waves). In accordance with these observations, numerical evidence for the wave characteristics of CWs is given by simulations which demonstrate effects of deflection and reflection when a CW interacts with regions exhibiting a sudden density drop, such as CHs. However, secondary waves are usually weak in their signal and simulations are limited in the way the according idealisations have to be chosen. Hence, several properties of the secondary waves during a CW-CH interaction are unclear or ambiguous and might lead to misinterpretations. In this study we follow a theoretical approach and focus in particular on the geometrical properties of secondary waves caused by the interaction between oblique incoming CWs and CHs. Based on a linear theory, we derive analytical expressions for reflection and transmission coefficients, which tell us how strongly the amplitudes of the secondary waves increase and decrease with respect to the incoming wave, respectively. Additionally, we provide analytical terms for crucial incidence angles that are capable of giving information about the energy flux, the phase and the reflection properties of the secondary waves. These novel expressions provide a supplementary tool for estimating CW properties in a fast and straightforward way and, therefore, might have relevant consequences for a possible new interpretation of already studied CW-CH interaction events and the clarification of ambiguous observational data.Comment: 15 pages, 10 figure