arXiv subjects

M. J. Martínez González

Publications and source records attributed to M. J. Martínez González.

25 records · Page 2Linked to original sources

arXiv2012

Hanle effect for stellar dipoles and quadrupoles

We derive exact expressions for the degree of lineal polarization over a resolved or integrated stellar disc due to resonance scattering and the Hanle effect from a dipolar or quadrupolar distribution of magnetic fields. We apply the theory of scattering polarization within the formalism of the spherical tensors representation for the density matrix and radiation field. The distribution of linear polarization over the stellar disk for different configurations of the magnetic field is studied and its topology discussed. For an unresolved dipole, the resulting polarization can be expressed in terms of just three functions (of the inclination angle and effective dipole strength), that are calculated numerically and their behaviour discussed. Dipolar and (aligned) quadrupoles are considered in some detail, but the techniques here ---in particular, the extensive use of the spherical tensor formalism for polarization---, can easily be applied to more general field configurations.

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arXiv2012

Anomalous circular polarization profiles in the He I 1083.0 nm multiplet from solar spicules

We report Stokes vector observations of solar spicules and a prominence in the He I 1083 nm multiplet carried out with the Tenerife Infrared Polarimeter. The observations show linear polarization profiles that are produced by scattering processes in the presence of a magnetic field. After a careful data reduction, we demonstrate the existence of extremely asymmetric Stokes V profiles in the spicular material that we are able to model with two magnetic components along the line of sight, and under the presence of atomic orientation in the energy levels that give rise to the multiplet. We discuss some possible scenarios that can generate the atomic orientation in spicules. We stress the importance of spectropolarimetric observations across the limb to distinguish such signals from observational artifacts.

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arXiv2012

Resolving the internal magnetic structure of the solar network

We analyze the spectral asymmetry of Stokes V (circularly polarized) profiles of an individual network patch in the quiet Sun observed by Sunrise/IMaX. At a spatial resolution of 0.15"-0.18", the network elements contain substructure which is revealed by the spatial distribution of Stokes V asymmetries. The area asymmetry between the red and blue lobes of Stokes V increases from nearly zero at the core of the structure to values close to unity at its edges (one-lobed profiles). Such a distribution of the area asymmetry is consistent with magnetic fields expanding with height, i.e., an expanding magnetic canopy (which is required to fulfill pressure balance and flux conservation in the solar atmosphere). Inversion of the Stokes I and V profiles of the patch confirms this picture, revealing a decreasing field strength and increasing height of the canopy base from the core to the periphery of the network patch. However, the non-roundish shape of the structure and the presence of negative area and amplitude asymmetries reveal that the scenario is more complex than a canonical flux tube expanding with height surrounded by downflows.

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arXiv2012

Dead calm areas in the very quiet Sun

We analyze two regions of the quiet Sun (35.6 x 35.6 Mm^2) observed at high spatial resolution (~100 km) in polarized light by the IMaX spectropolarimeter onboard the Sunrise balloon. We identify 497 small-scale (~400 km) magnetic loops, appearing at an effective rate of 0.25 loop h^{-1} arcsec^{-2}; further, we argue that this number and rate are underestimated by ~30%. However, we find that these small dipoles do not appear uniformly on the solar surface: their spatial distribution is rather filamentary and clumpy, creating dead calm areas, characterized by a very low magnetic signal and a lack of organized loop-like structures at the detection level of our instruments, that cannot be explained as just statistical fluctuations of a Poisson spatial process. We argue that this is an intrinsic characteristic of the mechanism that generates the magnetic fields in the very quiet Sun. The spatio-temporal coherences and the clumpy structure of the phenomenon suggest a recurrent, intermittent mechanism for the generation of magnetic fields in the quietest areas of the Sun.

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arXiv2012

Analytical calculation of Stokes profiles of rotating stellar magnetic dipole

The observation of the polarization emerging from a rotating star at different phases opens up the possibility to map the magnetic field in the stellar surface thanks to the well-known Zeeman Doppler Imaging. When the magnetic field is sufficiently weak, the circular and linear polarization profiles locally in each point of the star are proportional to the first and second derivatives of the unperturbed intensity profile, respectively. We show that the weak-field approximation (for weak lines in the case of linear polarization) can be generalized to the case of a rotating star including the Doppler effect and taking into account the integration on the stellar surface. The Stokes profiles are written as a linear combination of wavelength-dependent terms expressed as series expansions in terms of Hermite polynomials. These terms contain the surface integrated magnetic field and velocity components. The direct numerical evaluation of these quantities is limited to rotation velocities not larger than 8 times the Doppler width of the local absorption profiles. Additionally, we demonstrate that, in a rotating star, the circular polarization flux depends on the derivative of the intensity flux with respect to the wavelength and also on the profile itself. Likewise, the linear polarization depends on the profile and on its first and second derivative with respect to the wavelength. We particularize the general expressions to a rotating dipole.

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arXiv2011

Analytical maximum likelihood estimation of stellar magnetic fields

The polarised spectrum of stellar radiation encodes valuable information on the conditions of stellar atmospheres and the magnetic fields that permeate them. In this paper, we give explicit expressions to estimate the magnetic field vector and its associated error from the observed Stokes parameters. We study the solar case where specific intensities are observed and then the stellar case, where we receive the polarised flux. In this second case, we concentrate on the explicit expression for the case of a slow rotator with a dipolar magnetic field geometry. Moreover, we also give explicit formulae to retrieve the magnetic field vector from the LSD profiles without assuming mean values for the LSD artificial spectral line. The formulae have been obtained assuming that the spectral lines can be described in the weak field regime and using a maximum likelihood approach. The errors are recovered by means of the hermitian matrix. The bias of the estimators are analysed in depth.

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arXiv2007

Multi-Line Quiet Sun Spectro-Polarimetry at 5250 and 6302 Å

The reliability of quiet Sun magnetic field diagnostics based on the \ion{Fe}{1} lines at 6302 Åhas been questioned by recent work. We present here the results of a thorough study of high-resolution multi-line observations taken with the new spectro-polarimeter SPINOR, comprising the 5250 and 6302 Åspectral domains. The observations were analyzed using several inversion algorithms, including Milne-Eddington, LTE with 1 and 2 components, and MISMA codes. We find that the line-ratio technique applied to the 5250 Ålines is not sufficiently reliable to provide a direct magnetic diagnostic in the presence of thermal fluctuations and variable line broadening. In general, one needs to resort to inversion algorithms, ideally with realistic magneto-hydrodynamical constrains. When this is done, the 5250 Ålines do not seem to provide any significant advantage over those at 6302 Å. In fact, our results point towards a better performance with the latter (in the presence of turbulent line broadening). In any case, for very weak flux concentrations, neither spectral region alone provides sufficient constraints to fully disentangle the intrinsic field strengths. Instead, we advocate for a combined analysis of both spectral ranges, which yields a better determination of the quiet Sun magnetic properties. Finally, we propose the use of two other \ion{Fe}{1} lines (at 4122 and 9000 Å) with identical line opacities that seem to work much better than the others.

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