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Search Results: 1 - 10 of 599316 matches for " M. J. Martinez Gonzalez "
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The Magnetic Fields of the Quiet Sun
J. Sanchez Almeida,M. Martinez Gonzalez
Physics , 2011,
Abstract: This work reviews our understanding of the magnetic fields observed in the quiet Sun. The subject has undergone a major change during the last decade (quiet revolution), and it will remain changing since the techniques of diagnostic employed so far are known to be severely biased. Keeping these caveats in mind, our work covers the main observational properties of the quiet Sun magnetic fields: magnetic field strengths, unsigned magnetic flux densities, magnetic field inclinations, as well as the temporal evolution on short time-scales (loop emergence), and long time-scales (solar cycle). We also summarize the main theoretical ideas put forward to explain the origin of the quiet Sun magnetism. A final prospective section points out various areas of solar physics where the quiet Sun magnetism may have an important physical role to play (chromospheric and coronal structure, solar wind acceleration, and solar elemental abundances).
Emergence of small-scale magnetic loops through the quiet solar atmosphere
M. J. Martinez Gonzalez,L. R. Bellot Rubio
Physics , 2009, DOI: 10.1088/0004-637X/700/2/1391
Abstract: We investigate the emergence of magnetic flux in the quiet Sun at very small spatial scales, focusing on the magnetic connection between the photosphere and chromosphere. The observational data consist of spectropolarimetric measurements and filtergrams taken with the Hinode satellite and the Dutch Open Telescope. We find that a significant fraction of the magnetic flux present in internetwork regions appears in the form of Omega-shaped loops. The emergence rate is 0.02 loops per hour and arcsec^{-2}, which brings 1.1 x 10^12 Mx s^{-1} arcsec^{-2} of new flux to the solar surface. Initially, the loops are observed as small patches of linear polarization above a granular cell. Shortly afterwards, two footpoints of opposite polarity become visible in circular polarization within or at the edges of the granule and start to move toward the adjacent intergranular space. The orientation of the footpoints does not seem to obey Hale's polarity rules. The loops are continuously buffeted by convective motions, but they always retain a high degree of coherence. Interestingly, 23% of the loops that emerge in the photosphere reach the chromosphere (16 cases out of 69). They are first detected in Fe I 630 nm magnetograms and 5 minutes later in Mg I b 517.3 nm magnetograms. After about 8 minutes, some of them are also observed in Ca II H line-core images, where the footpoints produce small brightness enhancements.
Estimating the magnetic field strength from magnetograms
A. Asensio Ramos,M. J. Martinez Gonzalez,R. Manso Sainz
Physics , 2015, DOI: 10.1051/0004-6361/201423859
Abstract: A properly calibrated longitudinal magnetograph is an instrument that measures circular polarization and gives an estimation of the magnetic flux density in each observed resolution element. This usually constitutes a lower bound of the field strength in the resolution element, given that it can be made arbitrarily large as long as it occupies a proportionally smaller area of the resolution element and/or becomes more transversal to the observer and still produce the same magnetic signal. Yet, we know that arbitrarily stronger fields are less likely --hG fields are more probable than kG fields, with fields above several kG virtually absent-- and we may even have partial information about its angular distribution. Based on a set of sensible considerations, we derive simple formulae based on a Bayesian analysis to give an improved estimation of the magnetic field strength for magnetographs.
Partition function based analysis of CMB maps
J. M. Diego,E. Martinez-Gonzalez,J. L. Sanz,S. Mollerach,V. Martinez
Physics , 1999,
Abstract: We present an alternative method to analyse cosmic microwave background (CMB) maps. We base our analysis on the study of the partition function. This function is used to examine the CMB maps making use of the different information embedded at different scales and moments. Using the partition function in a likelihood analysis in two dimensions (Q_rms,n), we find the best-fitting model to the best data available at present the COBE--DMR 4 years data set. By means of this analysis we find a maximum in the likelihood function for n=1.8 (-0.65 +0.35) and Q_rms-PS = 10 (-2.5 +3) muK (95 % confidence level) in agreement with the results of other similar analyses (Smoot et al. 1994 (1 yr), Bennet et al. 1996 (4 yr)). Also making use of the partition function we perform a multifractal analysis and study the possible fractal nature of the CMB sky. We find that the measure used in the analysis is not a fractal. Finally, we use the partition function for testing the statistical distribution of the COBE--DMR data set. We conclude that no evidence of non-Gaussianity can be found by means of this method.
Influence of phase-diversity image reconstruction techniques on circular polarization asymmetries
A. Asensio Ramos,M. J. Martinez Gonzalez,E. Khomenko,V. Martinez Pillet
Physics , 2011, DOI: 10.1051/0004-6361/201117729
Abstract: Full Stokes filter-polarimeters are key instruments for investigating the rapid evolution of magnetic structures on the solar surface. To this end, the image quality is routinely improved using a-posteriori image reconstruction methods. We analyze the robustness of circular polarization asymmetries to phase-diversity image reconstruction techniques. We use snapshots of magneto-hydrodynamical simulations carried out with different initial conditions to synthesize spectra of the magnetically sensitive Fe I line at 5250.2 A. We degrade the synthetic profiles spatially and spectrally to simulate observations with the IMaX full Stokes filter-polarimeter. We also simulate the focused/defocused pairs of images used by the phase-diversity algorithm for reconstruction and the polarimetric modulation scheme. We assume that standard optimization methods are able to infer the projection of the wavefront on the Zernike polynomials with 10% precision. We also consider the less favorable case of 25% precision. We obtain reconstructed monochromatic modulated images that are later demodulated and compared with the original maps. Although asymmetries are often difficult to define in the quiet Sun due to the complexity of the Stokes V profiles, we show how asymmetries are degraded with spatial and spectral smearing. The results indicate that, although image reconstruction techniques reduce the spatial smearing, they can modify the asymmetries of the profiles, mainly caused by the appearance of spatially-correlated noise.
Quadratic differentials and asymptotics of Laguerre polynomials with varying complex parameters
M. J. Atia,A. Martinez-Finkelshtein,P. Martinez-Gonzalez,F. Thabet
Mathematics , 2013,
Abstract: In this paper we study the asymptotics (as $n\to \infty$) of the sequences of Laguerre polynomials with varying complex parameters $\alpha$ depending on the degree $n$. More precisely, we assume that $\alpha_n = n A_n, $ and $ \lim_n A_n=A \in \mathbb{C}$. This study has been carried out previously only for $\alpha_n\in \mathbb{R}$, but complex values of $A$ introduce an asymmetry that makes the problem more difficult. The main ingredient of the asymptotic analysis is the right choice of the contour of orthogonality, which requires the analysis of the global structure of trajectories of an associated quadratic differential on the complex plane, which may have an independent interest. While the weak asymptotics is obtained by reduction to the theorem of Gonchar--Rakhmanov--Stahl, the strong asymptotic results are derived via the non-commutative steepest descent analysis based on the Riemann-Hilbert characterization of the Laguerre polynomials.
Predictions on high frequency polarization properties of extragalactic radio sources and implications for CMB polarization measurements
M. Tucci,E. Martinez-Gonzalez,L. Toffolatti,J. Gonzalez-Nuevo,G. De Zotti
Physics , 2003, DOI: 10.1111/j.1365-2966.2004.07593.x
Abstract: We present a method to simulate the polarization properties of extragalactic radio sources at microwave frequencies. Polarization measurements of nearly 2x10^6 sources at 1.4 GHz are provided by the NVSS survey. Using this catalogue and the GB6 survey, we study the distribution of the polarization degree of both steep- and flat-spectrum sources. We find that the polarization degree is anti-correlated with the flux density for the former population, while no correlation is detected for the latter. The available high-frequency data are exploited to determine the frequency dependence of the distribution of polarization degrees. Using such information and the evolutionary model by Toffolatti et al. (1998), we estimate the polarization power spectrum of extragalactic radio sources at \geq 30 GHz and their contamination of CMB polarization maps. Two distinct methods to compute point-source polarization spectra are presented, extending and improving the one generally used in previous analyses. While extragalactic radio sources can significantly contaminate the CMB E-mode power spectrum only at low frequencies (<30 GHz), they can severely constrain the detectability of the CMB B-mode up to ~100 GHz.
Optimal Biological Level of Total Lysine for Finishing Pigs Fed Sorghum-Soybean Meal Diets
M. Martinez,J.L. Figueroa,M.J. Gonzalez,J.L. Landero,R. Medina
Journal of Animal and Veterinary Advances , 2012,
Abstract: Dietary lysine concentration determines growth performance of pigs when all other nutrients fulfill the requirements. The optimal biological level sets the maximum pig response. An experiment was conducted with 60 crossbred (Yorkshire Landrace sows, Yorkshire Duroc boars; 30 barrows and 30 gilts) finishing (47.47 kg of initial weight) pigs to estimate the Optimal Biological Level (OBL) of total dietary lysine in sorghum-soybean meal diets. Total dietary lysine levels were: 0.56, 0.66, 0.76, 0.85 and 0.96%. The analyzed variables were: Average Daily Gain (ADG), Average Daily Feed Intake (ADFI), Feed Gain Ratio (FGR), Backfat Thickness (BFT), Longissimus Muscle Area (LMA), Fat Free Lean Gain (FFLG) and Plasma Urea Nitrogen concentration (PUN). The global data showed that just FFLG was affected by dietary lysine level. There was no effect of the lysine concentration on any other variable in barrows; but there PUN was affected by the dietary lysine level in gilts. The OBL to maximize FFLG (in all pigs), calculated by regression analysis, was 0.89% total lysine and the OBL to minimize PUN was 0.72% total lysine for gilts. These results confirmed that the OBL for finishing pigs to use more efficiently the dietary protein concentration will be different, upon the pig sex and the optimization criterion used.
Spectropolarimetric multi line analysis of stellar magnetic fields
J. C. Ramirez Velez,M. Semel,M. Stift,M. J. Martinez Gonzalez,P. Petit,N. Dunstone
Physics , 2010, DOI: 10.1051/0004-6361/200810484
Abstract: In this paper we study the feasibility of inferring the magnetic field from polarized multi-line spectra using two methods: The pseudo line approach and The PCA-ZDI approach. We use multi-line techniques, meaning that all the lines of a stellar spectrum contribute to obtain a polarization signature. The use of multiple lines dramatically increases the signal to noise ratio of these polarizations signatures. Using one technique, the pseudo-line approach, we construct the pseudo-line as the mean profile of all the individual lines. The other technique, the PCA-ZDI approach proposed recently by Semel et al. (2006) for the detection of polarized signals, combines Principle Components Analysis (PCA) and the Zeeman Do ppler Imaging technique (ZDI). This new method has a main advantage: the polarized signature is extracted using cross correlations between the stellar spectra nd functions containing the polarization properties of each line. These functions are the principal components of a database of synthetic spectra. The synthesis of the spectra of the database are obtained using the radiative transfer equations in LTE. The profiles built with the PCA-ZDI technique are denominated Multi-Zeeman-Signatures. The construction of the pseudo line as well as the Multi-Zeeman-Signatures is a powerful tool in the study of stellar and solar magnetic fields. The information of the physical parameters that governs the line formation is contained in the final polarized profiles. In particular, using inversion codes, we have shown that the magnetic field vector can be properly inferred with both approaches despite the magnetic field regime.
Thermal frequency noise in low oscillation amplitude Dynamic Scanning Force Microscopy
J. Colchero,M. Cuenca,J. F. Gonzalez Martinez,J. Abad,E. Palacios-Lidon,J. Abellan
Physics , 2010,
Abstract: Thermal fluctuation of the cantilever position sets a fundamental limit for the precision of any Scanning Force Microscope. In the present work we analyse how these fluctuations limit the determination of the resonance frequency of the tip-sample system. The basic principles of frequency detection in Dynamic Scanning Force Microscopy are revised and the precise response of a typical frequency detection unit to thermal fluctuation of the cantilever is analysed in detail. A general relation for thermal frequency noise is found as a function of measurement bandwidth and cantilever oscillation. For large oscillation amplitude and low bandwidth, this relation converges to the result known from the literature, while for low oscillation amplitude and large bandwidth we find that the thermal frequency noise is equal to the width of the resonance curve and therefore stays finite, contrary to what is predicted by the relation known so far. The results presented in this work fundamentally determine the ultimate limits of Dynamic Scanning Force Microscopy.
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