M. J. Martínez González

Internetwork magnetic field distribution from simultaneous 1.56

Aims. We study the contradictory magnetic field strength distributions retrieved from independent analyses of spectropolarimetric observations in the near-infrared (1.56 µm) and in the visible (630 nm) spectral ranges in internetwork regions. Methods. To solve this apparent controversy, we present simultaneous and co-spatial 1.56 µm and 630 nm observations of an internetwork area. The properties of the circular and linear polarization signals, as well as the Stokes V area and amplitude asymmetries, are discussed. As a complement, we also used inversion techniques to infer the physical parameters of the solar atmosphere. As a first step, the infrared and visible observations are analysed separately to check their compatibility. Finally, the simultaneous inversion of the two data sets is performed. Results. The magnetic flux densities retrieved from the individual analysis of the infrared and visible data sets are strongly correlated. The polarity of the Stokes V profiles is the same at co-spatial pixels in both wavelength ranges. This indicates that both 1.56 µm and 630 nm observations trace the same magnetic structures on the solar surface. The simultaneous inversion of the two pairs of lines reveals an internetwork full of sub-kG structures that fill only 2% of the resolution element. A correlation is found between the magnetic field strength and the continuum intensity: equipartition fields (B ∼ 500 G) tend to be located in dark intergranular lanes, whereas weaker field structures are found inside granules. The most probable unsigned magnetic flux density is 10 Mx/cm 2 .T he net magnetic flux density in the whole field of view is nearly zero. This means that both polarities cancel out almost exactly in our observed internetwork area.

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Aims. The purpose of this work is to analyze the reliability of the magnetic field strengths inferred from the 630 nm pair of Fe I lines in internetwork quiet Sun regions. Methods. Some numerical experiments have been performed that demonstrate the inability of these lines to recover the magnetic field strength in such low flux solar regions. Results. It is shown how different model atmospheres, with magnetic field strengths ranging from a few hundred Gauss to kiloGauss, give rise to Stokes profiles that cannot be distinguished. The reasons for this degeneracy are discussed.

m and 630 nm observations

We report a direct comparison of the amplitudes of Stokes spectra of the Fe  630 nm and 1.56 µm lines produced by realistic MHD simulations with simultaneous observations in the same spectral regions. The Stokes spectra were synthesized in snapshots with a mixed polarity magnetic field having a spatially averaged strength, � B� , between 10 and 30 G. The distribution of Stokes V amplitudes depends sensitively onB� . A quiet inter-network region was observed at the German VTT simultane- ously with TIP (1.56 µm) and POLIS (630 nm). We find that the Stokes V amplitudes of both infrared and visible observations are best reproduced by the simulation snapshot withB� = 20 G. In observations with 1 �� resolution, up to 2/3 of the magnetic flux can remain undetected.

On the validity of the 630 nm Fe I lines for magnetometry of the internetwork quiet Sun

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.

Magnetic flux in the internetwork quiet Sun

We present observational evidence for oscillations of magnetic flux density in the quiet areas of the Sun. The majority of magnetic fields on the solar surface have strengths of the order of or lower than the equipartition field (300-500?G). This results in a myriad of magnetic fields whose evolution is largely determined by the turbulent plasma motions. When granules evolve they squash the magnetic field lines together or pull them apart. Here, we report on the periodic deformation of the shapes of features in circular polarization observed at high resolution with SUNRISE. In particular, we note that the area of patches with a constant magnetic flux oscillates with time, which implies that the apparent magnetic field intensity oscillates in antiphase. The periods associated with this oscillatory pattern are compatible with the granular lifetime and change abruptly, which suggests that these oscillations might not correspond to characteristic oscillatory modes of magnetic structures, but to the forcing by granular motions. In one particular case, we find three patches around the same granule oscillating in phase, which means that the spatial coherence of these oscillations can reach 1600 km. Interestingly, the same kind of oscillatory phenomenon is also found in the upper photosphere.

MULTILINE SPECTROPOLARIMETRY OF THE QUIET SUN AT 5250 AND 6302 Å

The behavior of the observed polarization amplitudes with spatial resolution is a strong constraint on the nature and organization of solar magnetic fields below the resolution limit. We study the polarization of the very quiet Sun at different spatial resolutions using ground- and space-based observations. It is shown that 80% of the observed polarization signals do not change with spatial resolution, suggesting that, observationally, the very quiet Sun magnetism remains the same despite the high spatial resolution of space-based observations. Our analysis also reveals a cascade of spatial scales for the magnetic field within the resolution element. It is manifest that the Zeeman effect is sensitive to the microturbulent field usually associated to Hanle diagnostics. This demonstrates that Zeeman and Hanle studies show complementary perspectives of the same magnetism.

Unnoticed Magnetic Field Oscillations in the Very Quiet Sun Revealed by SUNRISE/IMaX

Received 11 May 2006 Accepted 1 December 2006 Context. The magnetism in the quiet regions of the solar photosphere carries information on the dynamo processes and its interaction with the convection of the outer layers of the sun. Unfortunately, the scales of the magnetic structures on these regions are mostly unresolved. It is therefore instrumental to tell apart the intrinsic field strengths in those regions from the flux through the resolution element. This disentanglement has been far from obvious, leading to opposing views of the magnetic topology in the unresolved structures of the quiet Sun. Aims. Our study contributes to the disentanglement of field strength from flux in the quiet Sun, at least, through the use of new observational constrains in the form of spectropolarimetry of Mn I lines observed in the solar spectrum. Methods. The chosen Mn lines present a strong coupling with hyperfine structure resulting in spectral features, present or absent as a function of field strength alone. We observe one of those lines simultaneously and co-spatially with the Fe I lines at 630 nm, at the core of the previous measurements. Results. The inversion of the observed Fe lines results in either strong or weak fields depending on the initializations of the inversion algorithm. All the solutions show nevertheless equally good values for the σ parameter and are therefore equally valuable as solutions. The Mn however selects unambiguously strong or weak fields, sometimes agreeing with the inversions of the Fe lines, but half the time disagreeing with them. Conclusions. The Fe lines at 630 nm, in the conditions found in the quiet Sun, carry no binding information on field strength. A proper analysis of quiet Sun magnetism should necessarily pass through its simultaneous and co-spatial observation with other lines imposing constraints on field strength, as the Mn I lines here analyzed. Ultimately, the magnetic topology of the quiet Sun shall arise from the coherent analysis of all these lines, sensitive to the Zeeman effect.

Statistical Analysis of the very Quiet Sun Magnetism

Context. It is plausible that at least part of the water ice of cometary nuclei is initially in an amorphous phase, doped with other volatiles. As the nuclei are heated, the amorphous ice would then transform irreversibly into cubic ice. The net energy liberated in this transformation may be affected by the presence of any impurities because part of the energy liberated during crystallization may be expended in the desorption of dopant elements. Aims. Our goal is to study the evolution of the crystallization front of the amorphous ice in a simulated nucleus, providing quantitative results. In particular, the influence of the net energy released during crystallization on the thermophysical evolution will be analyzed. Methods. We use a simplified thermophysical model to simulate a cometary nucleus, where the ice is assumed initially to be in an amorphous phase. The model allows us to estimate the instantaneous rate of crystallization and the time spent in crystallization, for a fixed volume of amorphous ice, as a function of the net energy released. Simulations are performed for different characterizations of the nucleus interior such as dust-to-ice ratio, density, or thermal inertia. Results. As expected, the evolution of the crystallization front depends strongly on the characteristics of the nucleus interior. If the nucleus interior has, however, a dust-to-ice ratio smaller than 1, and a low thermal inertia, approximately of 20 W K −1 m −2 s 1/2 ,t he crystallization front evolves discontinuously, with quasi-periodic increases in the crystallization rate. Those increases have a period that ranges from 1 to 40 days, if the energy released by crystallization is unaffected by impurities. These surges of crystallization could be responsible for the periodic outbursts observed for comet 9P/Tempel 1 shortly before the Deep Impact experiment. The evolution of the crystallization front becomes continuous and almost steady, if the net energy released is half that of the pure, exothermic case, regardless of the characteristics of the nucleus interior. On the other hand, if the dust-to-ice ratio is high (larger than 1) and/or the thermal inertia is high (larger than 100), the crystallization front evolves in a continuous and smooth manner, even for pure, exothermic crystallization. Other quantitative results, including a comparison with plausible erosion rates, are described.

Determination of field strengths in the quiet Sun

Context. Titan’s stratosphere contains oxygen compounds (CO, CO2, and H2O), implying an external source of oxygen whose nature is still uncertain. Recent observations from the Herschel Space Observatory using the HIFI and PACS instruments and the Cassini/CIRS, as well as steady state photochemical modeling indicate that the amounts of CO2 and H2O in Titan’s stratosphere may imply inconsistent values of the OH/H2O input flux, and that the oxygen source is time variable. Aims. We attempt to reconcile the H2O and CO2 observed profiles in Titan’s atmosphere by using an updated photochemical scheme and developing several time dependent scenarios for the influx/evolution of oxygen species. Methods. We use a time dependent photochemical model of Titan’s atmosphere to calculate e ective lifetimes and the response of Titan’s oxygen compounds to changes in the oxygen input flux. Two variants for the C H O chemical network are considered. We investigate a time variable Enceladus source and the evolution of material delivered by a cometary impact. Results. We find that the e ective lifetime of H2O in Titan’s atmosphere is only a factor of six shorter than that of CO2 and exceeds 10 yr below 200 km. A time variable Enceladus source, involving a decrease by a factor of 5–20 in the OH/H2O flux over the last few centuries, shows promise in explaining the relative CO2/H2O profiles. However, if the measurements from the Herschel Space Observatory are representative of Titan’s atmospheric water, an additional H2O loss to the haze term is needed to bring the model in full agreement with the data. In an alternate situation, CO2 production following a cometary impact that occurred at least 220 300 yr ago can in principle explain the CO2 “excess” in Titan’s stratosphere, but this scenario is highly unlikely, given the estimates of the impact rate at Titan.

Evolution of the crystallization front in cometary models Effect of the net energy released during crystallization

Aims. 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. Methods. 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 PCAZDI approach proposed recently by Semel et al. (2006, ASPC, 358, 355) for the detection of polarized signals, combines principle components analysis (PCA) and the Zeeman Doppler imaging technique (ZDI). This new method has a main advantage: the polarized signature is extracted using cross correlations between the stellar spectra and 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 called multi Zeeman signatures. Results. 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. We have shown in particular using inversion codes that the magnetic field vector can be properly inferred with both approaches despite the magnetic field regime.