D. Del Moro

MAGNETIC BRIGHT POINTS IN THE QUIET SUN

We present a visual determination of the number of bright points (BPs) existing in the quiet Sun, which are structures though to trace intense kG magnetic concentrations. The measurement is based on a 01 angular resolution G-band movie obtained with the Swedish Solar Telescope at the solar disk center. We find 0.97 BPs Mm?2, which is a factor 3 larger than any previous estimate. It corresponds to 1.2 BPs per solar granule. Depending on the details of the segmentation, the BPs cover between 0.9% and 2.2% of the solar surface. Assuming their field strength to be 1.5 kG, the detected BPs contribute to the solar magnetic flux with an unsigned flux density between 13 G and 33 G. If network and inter-network regions are counted separately, they contain 2.2 BPs Mm?2 and 0.85 BPs Mm?2, respectively.

Solar granulation properties derived from three different time series

We need automatic pattern recognition algorithms to extract a large statistical sample of granules from high spatial and temporal resolution images series of photospheric fields. In this paper, we present the new Two-level Structure Tracking (TST) algorithm, based on a two-level representation of granulation, which allows us to monitor the characteristics of identified photospheric structures during their duration. TST is also able to retrieve horizontal velocity maps from measured granule displacements. Direct comparison of the results from different works describing granular evolution is often not possible as discrepancies stemming from the use of different procedures or from different data cannot be distinguished. Here, three different solar granulation broadband time series, acquired at THEMIS in July 1999, at NSO-DST in October 1996, and at SVST in June 1995, with different spatial and temporal resolutions, are coherently analyzed via the same TST procedure, allowing direct comparison of the results. Among the obtained results, we confirm the dynamical heterogeneity of photospheric small scale structures pointed out by granular lifetime histograms, characterized by a stretched exponential function. Furthermore, by monitoring the breaking-up of a granule and the movements of its neighbours and of its fragments, we found evidence of how an exploding granule may produce a divergence signal on spatial and temporal mesogranular scales.

IMAGING SPECTROPOLARIMETRY WITH IBIS: EVOLUTION OF BRIGHT POINTS IN THE QUIET SUN

We present the results from first spectropolarimetric observations of the solar photosphere acquired at the Dunn Solar Telescope with the Interferometric Bidimensional Spectrometer. Full Stokes profiles were measured in the Fe I 630.15 nm and Fe I 630.25 nm lines with high spatial and spectral resolutions for 53 minutes, with a Stokes V noise of 3 × 10–3 the continuum intensity level. The data set allows us to study the evolution of several magnetic features associated with G-band bright points (BPs) in the quiet Sun. Here we focus on the analysis of three distinct processes, namely the coalescence, fragmentation, and cancellation of G-band BPs. Our analysis is based on an SIR inversion of the Stokes I and V profiles of both Fe I lines. The high spatial resolution of the G-band images combined with the inversion results helps to interpret the undergoing physical processes. The appearance (dissolution) of high-contrast G-band BPs is found to be related to the local increase (decrease) of the magnetic filling factor, without appreciable changes in the field strength. The cancellation of opposite-polarity BPs can be the signature of either magnetic reconnection or the emergence/submergence of magnetic loops.

Observation of bipolar moving magnetic features streaming out from a naked spot

Context. Mechanisms responsible for active-region formation, evolution, and decay have been investigated by many authors and several common features have been identified. In particular, a key element in the dispersal of the magnetic field seems to be the presence of magnetic elements, called moving magnetic features (MMFs). Aims. We analyze the short-lived sunspot group NOAA 10977, which appeared on the solar disk between 2 and 8 December 2007, to study the details of its emergence and decay phases. Methods. We performed a multi wavelength analysis of the region using images at visible (G band and Hα) and near-IR (Ca ii) wavelengths acquired by both the IBIS instrument and SOT/HINODE, EUV images (17.1 nm) acquired by TRACE, and MDI and SOT magnetograms. Results. The observed region exhibits some peculiarities. During the emergence phase the formation of the f-pore was initially observed, while the p-polarity later formed a naked spot, i.e., a sunspot without a penumbra. We measured a moat flow around this spot, and observed some MMFs streaming out from it during the decay phase. The characteristics of these MMFs allowed us to classify them as type I (U-shaped) MMFs. They were also cospatial with sites of increased brightness both in the photosphere and the chromosphere. Conclusions. The presence of bipolar MMFs in a naked spot indicates that current interpretation of bipolar MMFs, as extensions of the penumbral filaments beyond the sunspot outer boundaries, should be revised, to take into account this observational evidence. We believe that our results provide new insights into improving models of sunspot evolution.

Interpretation of HINODE SOT/SP asymmetric Stokes profiles observed in the quiet Sun network and internetwork

Aims. Stokes profiles emerging from the magnetized solar photosphere and observed by SOT/SP aboard the HINODE satellite present a variety of complex shapes. These are indicative of unresolved magnetic structures that have been overlooked in the inversion analyses performed so far. Here we present the first interpretation of the Stokes profile asymmetries measured in the Fei 630 nm lines by SOT/SP, in both quiet Sun internetwork (IN) and network regions. Methods. The inversion is carried out under the hypothesis of MIcro-Structured Magnetized Atmosphere (MISMA), where the unresolved structure is assumed to be optically thin. We analyze a 29.52 00 � 31.70 00 subfield carefully selected to be representative of the properties of a 302 00 � 162 00 quiet Sun field-of-view at disk center. Results. The inversion code is able to reproduce the observed asymmetries in a very satisfactory way, including the 35 % of inverted profiles presenting large asymmetries. The inversion code interprets 25 % of inverted profiles as emerging from pixels in which both positive and negative polarities coexist. These pixels are located either in frontiers between opposite polarity patches or in very quiet regions. kG field strengths are found at the base of the photosphere in both network and IN; in the case of the latter, both kG fields and hG fields are admixed. When considering the magnetic properties at the mid photosphere most kG fields are gone, and the statistics is dominated by hG fields. According to the magnetic filling factors derived from the inversion, we constrain the magnetic field of only 4.5 % of the analyzed photosphere (and this percentage reduces to 1.3 % when referred to all pixels, including those with low polarization not analyzed). The rest of the plasma is consistent with the presence of weak fields not contributing to the detected polarization signals. The average flux densities derived in the full subfield and in IN regions are higher than the ones derived from the same dataset by Milne-Eddington inversion. Conclusions. The existence of large asymmetries in HINODE SOT/SP polarization profiles is uncovered. These are not negligible in quiet Sun data. The MISMA inversion code reproduces them in a satisfactory way, and provides a statistical description of the magnetized IN and network which partly differs and complements the results obtained so far. From this it follows the importance of having a complete interpretation of the line profile shapes.

MHD wave transmission in the Sun's atmosphere

MHD wave propagation inside the Suns atmosphere is closely related to the magnetic field topology. For example, magneti c fields are able to lower the cutoff frequency for acoustic waves thus allowing waves which would otherwise be trapped below the photosphere to propagate into the upper atmosphere. Another example is that MHD waves can be transmitted or converted into other forms of waves at altitudes where the sound speed equals the Alfven speed. We take advantage of the large field-of-view provided b y the IBIS experiment to study the wave propagation at two heights in the solar atmosphere, as sampled by the photospheric Fe 617.3 nm spectral line and the chromospheric Ca 854.2 nm spectral line, and its relationship to the local magneti c field. Among other things, we find substantial leakage of waves with 5-minute periods in the chromosphere at the edges of a pore and in the diffused magnetic field surrounding it. By using spectro-polarimetric inversions of Hinode SOT/SP data, we also find a relationship between the photospheric power spectrum and the magnetic field inclination angle; in p articular, well-defined transmission peaks around 25 ◦ for 5 minutes waves and around 15 ◦ for 3 minutes waves. We propose a very simple model based upon wave transmission theory to explain this behavior. Finally, an analysis of both the power spectra and chromospheric amplification spectra suggests the presence of longitudinal acoustic waves along the magnetic field lines.

Spatial Clustering of Photospheric Structures

The study of large-scale structures existing in solar photospheric flows provides an essential tool for constraining the models of solar and stellar convection and for understanding physical processes that are at the basis of solar variability. Recent convection models and N-body dynamic simulations predict the evolution of small-scale features (i.e., granules) into large-scale ones. In this work we address the question of the emergence of large spatial scale patterns as a direct consequence of the organization of small-scale plasma flows on time periods longer than the mean granular lifetime. Our analysis reveals that the photospheric dynamics plays a key role in structuring stable intensity features and suggests that surface flows organize small-scale plasma structures, sweeping them up to form clusters of recurrent and stable granular features. Using a quite novel statistical method, the hexagonal normalized information entropy, we establish that sites where recurrent and stable granular features are observed exhibit a clustering spatial scale of about 8 Mm and timescale around 10 minutes.

Diffusion of magnetic elements in a supergranular cell

Small scale magnetic fields (magnetic elements) are ubiquitous in the solar photosphere. Their interaction can provide energy to the upper atmospheric layers, and contribute to heat the solar corona. In this work, the dynamic properties of magnetic elements in the quiet Sun are investigated. The high number of magnetic elements detected in a supergranular cell allowed us to compute their displacement spectrum ((Δr){sup 2})∝τ{sup γ} (with γ > 0, and τ the time since the first detection), separating the contribution of the network (NW) and the internetwork (IN) regions. In particular, we found γ = 1.27 ± 0.05 and γ = 1.08 ± 0.11 in NW (at smaller and larger scales, respectively), and γ = 1.44 ± 0.08 in IN. These results are discussed in light of the literature on the topic, as well as the implications for the build-up of the magnetic network.

Dynamics of the solar atmosphere above a pore with a light bridge

Context. Solar pores are small sunspots lacking a penumbra that have a prevailing vertical magnetic-field component. They can include light bridges at places with locally reduced magnetic field. Like sunspots, they exhibit a wide range of oscillatory phenomena. Aims. A large isolated pore with a light bridge (NOAA 11005) is studied to obtain characteristics of a chromospheric filamentary structure around the pore, to analyse oscillations and waves in and around the pore, and to understand the structure and brightness of the light bridge. Methods. Spectral imaging observations in the line Ca II 854.2 nm and complementary spectropolarimetry in Fe I lines, obtained with the DST/IBIS spectrometer and HINODE/SOT spectropolarimeter, were used to measure photospheric and chromospheric velocity fields, oscillations, waves, the magnetic field in the photosphere, and acoustic energy flux and radiative losses in the chromosphere. Results. The chromospheric filamentary structure around the pore has all important characteristics of a superpenumbra: it shows an inverse Evershed effect and running waves, and has a similar morphology and oscillation character. The granular structure of the light bridge in the upper photosphere can be explained by radiative heating. Acoustic waves leaking up from the photosphere along the inclined magnetic field in the light bridge transfer enough energy flux to balance the entire radiative losses of the light-bridge chromosphere. Conclusions. A penumbra is not a necessary condition for the formation of a superpenumbra. The light bridge is heated by radiation in the photosphere and by acoustic waves in the chromosphere.

Diffusion of solar magnetic elements up to supergranular spatial and temporal scales

The study of spatial and temporal scales on which small magnetic structures (magnetic elements) are organized in the quiet Sun may be approached by determining how they are transported on the solar photosphere by convective motions. The process involved is diffusion. Taking advantage of Hinode high spatial resolution magnetograms of a quiet-Sun region at the disk center, we tracked 20,145 magnetic elements. The large field of view (~50?Mm) and the long duration of the observations (over 25?hr without interruption at a cadence of 90?s) allowed us to investigate the turbulent flows at unprecedented large spatial and temporal scales. In the field of view an entire supergranule is clearly recognizable. The magnetic element displacement spectrum shows a double-regime behavior: superdiffusive (? = 1.34 ? 0.02) up to granular spatial scales (~1500?km) and slightly superdiffusive (? = 1.20 ? 0.05) up to supergranular scales.