I. Domínguez Cerdeña

Quiet-Sun Magnetic Fields at High Spatial Resolution

We present spectropolarimetric observations of internetwork magnetic fields at the solar disk center. A Fabry-Perot spectrometer was used to scan the two Fe I lines at 6301.5 and 6302.5 A. High spatial resolution (05) magnetograms were obtained after speckle reconstruction. The patches with magnetic fields above noise cover approximately 45% of the observed area. Such large coverage renders a mean unsigned magnetic flux density of some 20 G (or 20 Mx cm-2), which exceeds all previous measurements. Magnetic signals occur predominantly in intergranular spaces. The systematic difference between the flux densities measured in the two iron lines leads to the conclusion that, typically, we detect structures with intrinsic field strengths larger than 1 kG occupying only 2% of the surface.

Inter-network magnetic fields observed with sub-arcsec resolution

We analyze a time sequence of Inter-Network (IN) magnetograms observed at the solar disk center. Speckle recon- struction techniques provide a good spatial resolution (0: 00 5c uto frequency) yet maintaining a fair sensitivity (some 20 G). Patches with signal above noise cover 60% of the observed area, most of which corresponds to intergranular lanes. The large surface covered by signal renders a mean unsigned magnetic flux density between 17 G and 21 G (1 G 1M x cm 2 ). The dierence depends on the spectral line used to generate the magnetograms (Fei6302.5 A or Fei6301.5 A). Such systematic dierence can be understood if the magnetic structures producing the polarization have intrinsic field strengths exceeding 1 kG, and consequently, occupying only a very small fraction of the surface (some 2%). We observe both, magnetic signals changing in time scales smaller than 1 min, and a persistent pattern lasting longer than the duration of the sequence (17 min). The pattern resembles a network with a spatial scale between 5 and 10 arcsec, which we identify as the mesogranulation. The strong depen- dence of the polarization signals on spatial resolution and sensitivity suggests that much quiet Sun magnetic flux still remains undetected.

The Distribution of Quiet Sun Magnetic Field Strengths from 0 to 1800 G

The quiet-Sun photospheric plasma has a variety of magnetic field strengths going from zero to 1800 G. The empirical characterization of these field strengths requires a probability density function (PDF), i.e., a function P(B) describing the fraction of quiet Sun occupied by each field strength B. We show how to combine magnetic field strength measurements based on the Zeeman effect and the Hanle effect to estimate an unbiased P(B). The application of the method to real observations renders a set of possible PDFs, which outline the general characteristics of the quiet-Sun magnetic fields. Their most probable field strength differs from zero. The magnetic energy density is a significant fraction of the kinetic energy of the granular motions at the base of the photosphere (larger than 15% or larger than 2 × 103 ergs cm-3). The unsigned flux density (or mean magnetic field strength) has to be between 130 and 190 G. A significant part of the unsigned flux (between 10% and 50%) and of the magnetic energy (between 45% and 85%) are provided by the field strengths larger than 500 G, which, however, occupy only a small fraction of the surface (between 1% and 10%). The fraction of kG fields in the quiet Sun is even smaller, but they are important for a number of reasons. The kG fields still trace a significant fraction of the total magnetic energy, they reach the high photosphere, and they appear in unpolarized light images. The quiet-Sun photosphere has far more unsigned magnetic flux and magnetic energy than the active regions and the network combined.

Quiet Sun Magnetic Fields from Simultaneous Inversions of Visible and Infrared Spectropolarimetric Observations

We study the quiet Sun magnetic fields using spectropolarimetric observations of the infrared and visible Fe I lines at 6301.5, 6302.5, 15648, and 15653 A. Magnetic field strengths and filling factors are inferred by the simultaneous fit of the observed Stokes profiles under the MISMA hypothesis. The observations cover an intranetwork region at the solar disk center. We analyze 2280 Stokes profiles whose polarization signals are above noise in the two spectral ranges, which correspond to 40% of the field of view. Most of these profiles can be reproduced only with a model atmosphere including three magnetic components with very different field strengths, which indicates the coexistence of kG and sub-kG fields in our 15 resolution elements. We measure an unsigned magnetic flux density of 9.6 G considering the full field of view. Half of the pixels present magnetic fields with mixed polarities in the resolution element. The fraction of mixed polarities increases as the polarization weakens. We compute the probability density function of finding each magnetic field strength. It has a significant contribution of kG field strengths, which concentrates most of the observed magnetic flux and energy. This kG contribution has a preferred magnetic polarity, while the polarity of the weak fields is balanced.

Bright Points in the Internetwork Quiet Sun

High-resolution G-band images of the interior of a supergranulation cell show ubiquitous bright points (BPs; some 0.3 BPs per Mm2). They are located in intergranular lanes and often form chains of elongated blobs whose smallest dimension is at the resolution limit (135 km on the Sun). Most of them live for a few minutes, having peak intensities from 0.8 to 1.8 times the mean photospheric intensity. These BPs are probably tracing intense magnetic concentrations, whose existence has been inferred in spectropolarimetric measurements. Our finding provides a new convenient tool for the study of the internetwork magnetism, so far restricted to the interpretation of weak polarimetric signals.

Simultaneous Visible and Infrared Spectropolarimetry of a Solar Internetwork Region

We present the first simultaneous infrared (IR) and visible spectropolarimetric observations of a solar internetwork region. The Fe I lines at 6301.5, 6302.5, 15648, and 15652 A were observed, with a lag of only 1 minute, using highly sensitive spectropolarimeters operated in two different telescopes (Vacuum Tower Telescope and THEMIS at the Observatorio del Teide). Some 30% of the observed region shows IR and visible Stokes V signals above noise. These polarization signals indicate the presence of kilogauss (kG) magnetic field strengths (traced by the visible lines) coexisting with sub-kG fields (traced by the infrared lines). In addition, one-quarter of the pixels with signal have visible and IR Stokes V profiles with opposite polarity. We estimate the probability density function of finding each longitudinal magnetic field strength in the region. It has a tail of kG field strengths that accounts for most of the (unsigned) magnetic flux of the region.

MULTILINE SPECTROPOLARIMETRY OF THE QUIET SUN AT 5250 AND 6302 Å

The reliability of quiet-Sun magnetic field diagnostics based on the Fe I lines at 6302 A has been questioned by recent work. Here we present the results of a thorough study of high-resolution multiline observations taken with the new spectropolarimeter SPINOR, comprising the 5250 and 6302 A spectral 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 A lines 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 magnetohydrodynamic constrains. When this is done, the 5250 A lines do not seem to provide any significant advantage over those at 6302 A. In fact, our results point toward 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 Fe I lines (at 4122 and 9000 A) with identical line opacities that seem to work much better than the others.

Simultaneous Visible and IR Spectropolarimetry of the Quiet Sun

This work presents the first quiet Sun spectropolarimetric observations carried out in the visible and the infrared (IR) simultaneously. The Fe i lines at 6301.5, 6302.5, 15648, and 15652 A were observed co-spatially, and at the same time (with a time lag of only 1 minute), with high sensitive spectropolarimeters operated in two di erent telescopes (VTT and THEMIS at the Observatorio del Teide). We find Stokes V profiles above noise in 30% of the observed area, showing intrinsic magnetic fields of kG (traced by visible lines) co-existing with sub-kG fields (traced by infrared lines). We also found V profiles with opposite polarity in the visible and the IR in 25% of the pixels under study (8% of the area).