Revealing the nature of magnetic shadows with numerical 3D-MHD simulations

We investigate the interaction of magneto-acoustic waves with magnetic network elements with the aim of finding possible signatures of the magnetic shadow phenomenon in the vicinity of network elements. We carried out three-dimensional numerical simulations of magneto-acoustic wave propagation in a model solar atmosphere that is threaded by a complexly structured magnetic field, resembling that of a typical magnetic network element and of internetwork regions. High-frequency waves of 10 mHz are excited at the bottom of the simulation domain. On their way through the upper convection zone and through the photosphere and the chromosphere they become perturbed, refracted, and converted into different mode types. We applied a standard Fourier analysis to produce oscillatory power-maps of the line-of-sight velocity. In the power maps of the upper photosphere and the lower chromosphere, we clearly see the magnetic shadow: a seam of suppressed power surrounding the magnetic network elements. We demonstrate that this shadow is linked to the mode conversion process and that power maps at these height levels show the signature of three different magneto-acoustic wave modes.Comment: Astronomy & Astrophysics Letters, in print 4 pages, 4 figure

Modelos de producción de madera sólida en plantaciones de Eucalyptus globulus de Galicia

El objetivo de este trabajo es establecer las interacciones existentes entre crecimiento diametral, crecimiento en altura y la dinámica de poda natural de Eucalyptus globulus con el objetivo de evaluar la eficiencia de distintas opciones de gestión forestal para producir madera de alta calidad. Para ello se ha desarrollado un modelo de crecimiento basados en el árbol individual. La toma de muestras comprende más de 900 Eucalyptus globulus medidos en diferentes localizaciones de Galicia (España). En esos árboles se ha encontrado una elevada correlación significativa entre el diámetro normal y el diámetro de la copa. El modelo basado en el ancho de copa permite calcular el espacio vital requerido por un árbol para alcanzar un diámetro predefinido a una cierta edad. Una vez conocido, es también posible calcular el número de árboles por hectárea así como el momento y la intensidad de las claras. Se ha podido comprobar que los eucaliptos orientados a la producción de madera aserrada en rotaciones cortas tienen que podarse y, dada la tendencia de los árboles a incluir el tocón de las ramas muertas, es recomendable realizar la poda mientras las ramas todavía están verdes. Para optimizar la producción de madera limpia, es esencial conocer el impacto de los tratamientos silvícolas y condiciones de crecimiento en el proceso de formación de la base de la copa. Se ha encontrado que la altura de la base de copa viva muestra una buena correlación con el diámetro normal, la altura del árbol y la edad. Empleando estas tres variables se ha desarrollado un modelo multivariable para predecir la altura de la primera rama viva, que puede ser de utilidad para optimizar el momento y la intensidad de las claras para producir madera de sierra de eucalipto en plantaciones de corta rotación._________________________________The study aims to point out the interactions between diameter and height growth and the self-pruning dynamics of Eucalyptus globulus in order to evaluate different management regimes according to their efficiency for high value timber production. Therefore a growth model for individual trees has been developed. The database contains over 900 trees of Eucalyptus globulus measured at different sites in Galicia / Spain. For these trees a significant relationship between diameter at breast height (d1,3) and crown diameter calculated from this area was found. The crown width model enables us to calculate the growing space needed by a tree to get a pre-defined diameter at a given age. Knowing this, it is also possible to calculate the number of trees/ha as well as time and intensity of thinning interventions. It could be shown that eucalypt grown for sawlogs in short rotations have to be pruned. Since the trees tend to include dead branches partly into the bole, it is recommendable to prune branches when still green. In order to optimize production of knot free timber, it is essential to know the impact of silvicultural treatments and growth conditions on the process of living crown base recession. It was found that height of living crown base shows good correlations with d1,3, tree height and age. Using these 3 variables a multivariate model for predicting height of the first living branch has been developed, which may be helpful to optimize time and intensity of pruning for eucalypt sawlog production in short rotation plantations

The UNSW Extrasolar Planet Search: Methods and First Results from a Field Centred on NGC 6633

We report on the current status of the University of New South Wales Extrasolar Planet Search project, giving details of the methods we use to obtain millimagnitude precision photometry using the 0.5m Automated Patrol Telescope. We use a novel observing technique to optimally broaden the PSF and thus largely eliminate photometric noise due to intra-pixel sensitivity variations on the CCD. We have observed 8 crowded Galactic fields using this technique during 2003 and 2004. Our analysis of the first of these fields (centred on the open cluster NGC 6633) has yielded 49 variable stars and 4 shallow transit candidates. Follow-up observations of these candidates have identified them as eclipsing binary systems. We use a detailed simulation of our observations to estimate our sensitivity to short-period planets, and to select a new observing strategy to maximise the number of planets detected.Comment: 16 pages, 9 figures, version published in MNRAS Updated figures, references, and additional discussion in section

Detection of vortex tubes in solar granulation from observations with Sunrise

We have investigated a time series of continuum intensity maps and corresponding Dopplergrams of granulation in a very quiet solar region at the disk center, recorded with the Imaging Magnetograph eXperiment (IMaX) on board the balloon-borne solar observatory Sunrise. We find that granules frequently show substructure in the form of lanes composed of a leading bright rim and a trailing dark edge, which move together from the boundary of a granule into the granule itself. We find strikingly similar events in synthesized intensity maps from an ab initio numerical simulation of solar surface convection. From cross sections through the computational domain of the simulation, we conclude that these `granular lanes' are the visible signature of (horizontally oriented) vortex tubes. The characteristic optical appearance of vortex tubes at the solar surface is explained. We propose that the observed vortex tubes may represent only the large-scale end of a hierarchy of vortex tubes existing near the solar surface.Comment: Astrophysical Journal Letters: Sunrise Special Issue, reveived 2010 June 16; accepted 2010 August

Calculation of Spectral Darkening and Visibility Functions for Solar Oscillations

Calculations of spectral darkening and visibility functions for the brightness oscillations of the Sun resulting from global solar oscillations are presented. This has been done for a broad range of the visible and infrared continuum spectrum. The procedure for the calculations of these functions includes the numerical computation of depth-dependent derivatives of the opacity caused by p modes in the photosphere. A radiative-transport code was used for this purpose to get the disturbances of the opacities from temperature and density fluctuations. The visibility and darkening functions are obtained for adiabatic oscillations under the assumption that the temperature disturbances are proportional to the undisturbed temperature of the photosphere. The latter assumption is the only way to explore any opacity effects since the eigenfunctions of p-mode oscillations have not been obtained so far. This investigation reveals that opacity effects have to be taken into account because they dominate the violet and infrared part of the spectrum. Because of this dominance, the visibility functions are negative for those parts of the spectrum. Furthermore, the darkening functions show a wavelength-dependent change of sign for some wavelengths owing to these opacity effects. However, the visibility and darkening functions under the assumptions used contradict the observations of global p-mode oscillations, but it is beyond doubt that the opacity effects influence the brightness fluctuations of the Sun resulting from global oscillations

Observed Effect of Magnetic Fields on the Propagation of Magnetoacoustic Waves in the Lower Solar Atmosphere

We study Hinode/SOT-FG observations of intensity fluctuations in Ca II H-line and G-band image sequences and their relation to simultaneous and co-spatial magnetic field measurements. We explore the G-band and H-line intensity oscillation spectra both separately and comparatively via their relative phase differences, time delays and cross-coherences. In the non-magnetic situations, both sets of fluctuations show strong oscillatory power in the 3 - 7 mHz band centered at 4.5 mHz, but this is suppressed as magnetic field increases. A relative phase analysis gives a time delay of H-line after G-band of 20\pm1 s in non-magnetic situations implying a mean effective height difference of 140 km. The maximum coherence is at 4 - 7 mHz. Under strong magnetic influence the measured delay time shrinks to 11 s with the peak coherence near 4 mHz. A second coherence maximum appears between 7.5 - 10 mHz. Investigation of the locations of this doubled-frequency coherence locates it in diffuse rings outside photospheric magnetic structures. Some possible interpretations of these results are offered.Comment: 19 pages, 6 figure

Properties of high-frequency wave power halos around active regions: an analysis of multi-height data from HMI and AIA onboard SDO

We study properties of waves of frequencies above the photospheric acoustic cut-off of $\approx$5.3 mHz, around four active regions, through spatial maps of their power estimated using data from Helioseismic and Magnetic Imager (HMI) and Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO). The wavelength channels 1600 {\AA} and 1700 {\AA} from AIA are now known to capture clear oscillation signals due to helioseismic p modes as well as waves propagating up through to the chromosphere. Here we study in detail, in comparison with HMI Doppler data, properties of the power maps, especially the so called 'acoustic halos' seen around active regions, as a function of wave frequencies, inclination and strength of magnetic field (derived from the vector field observations by HMI) and observation height. We infer possible signatures of (magneto-)acoustic wave refraction from the observation height dependent changes, and hence due to changing magnetic strength and geometry, in the dependences of power maps on the photospheric magnetic quantities. We discuss the implications for theories of p mode absorption and mode conversions by the magnetic field.Comment: 22 pages, 12 figures, Accepted by journal Solar Physic

The energy of waves in the photosphere and lower chromosphere: III. Inversion setup for Ca II H spectra in local thermal equilibrium

The Ca II H line is one of the strongest lines in the solar spectrum and provides continuous information on the solar atmosphere from the photosphere to the lower chromosphere. We describe an inversion approach that reproduces observed Ca II H spectra assuming LTE. We developed an inversion strategy based on the SIR code. The approach uses a two-step procedure with an archive of pre-calculated spectra to fit the line core and a subsequent iterative modification to improve the fit in the line wing. Simultaneous spectra in the 630nm range can optionally be used to fix the continuum temperature. The method retrieves 1D temperature stratifications neglecting lateral radiative transport. LOS velocities are included by an empirical approach. An archive of about 300.000 pre-calculated spectra is more than sufficient to reproduce the line core of observed Ca II H spectra both in quiet Sun and in active regions. The final thermodynamical stratifications match observed and best-fit spectra to a level of about 0.5 (1) % of Ic in the line wing (core). Inversion schemes based on pre-calculated spectra allow one a reliable and relatively fast retrieval of solar properties from observed chromospheric spectra. The approach can be easily extended to an 1D NLTE case by a simple exchange of the pre-calculated archive spectra.Comment: 15 pages, 15 figures, accepted for publication in A&A. The animation will only be provided in the A&A online sectio

The energy of waves in the photosphere and lower chromosphere: IV. Inversion results of Ca II H spectra

Most static 1D atmosphere models in the quiet Sun predict a rise of the gas temperature at chromospheric layers, but numerical simulations only yield an increase in the brightness temperature. We investigate the thermal structure in the solar chromosphere as derived from an LTE inversion of Ca II H spectra in QS and active regions. We investigate the temperature stratifications on differences between magnetic and field-free regions in the QS, and between QS and ARs. We determine the energy content of individual calcium bright grains (BGs). The rms temperature fluctuations are below 100 K in the photosphere and 200-300 K in the chromosphere. The average temperature stratification in the QS does not exhibit a clear chromospheric temperature rise, opposite to the AR case. We find an energy content of about 7*10E18 J for BGs that repeat with a cadence of about 160 secs. The precursors of BGs have a vertical extent of about 200 km and a horizontal extent of about 1 Mm. The comparison of observed with synthetic NLTE profiles confirms that the solar chromosphere in the QS oscillates between an atmosphere in radiative equilibrium and one with a moderate chromospheric temperature rise. Two-dimensional x-z temperature maps exhibit nearly horizontal canopy-like structures with a few Mm extent around photospheric magnetic field concentrations at a height of about 600 km. The large difference between QS regions and ARs, and the better match of AR and non-LTE reference spectra suggest that magnetic heating processes are more important than commonly assumed. The temperature fluctuations in QS derived by the LTE inversion do not suffice on average to maintain a stationary chromospheric temperature rise. The spatially and vertically resolved information on the temperature structure allows one to investigate in detail the topology and evolution of the thermal structure in the lower solar atmosphere.Comment: 16 pages, 16 figures + 1 page Appendix, accepted by A&